Find an updated version of this guide at https://community.rti.com/documentation.
RTIConnext DDS
Core Libraries
Getting Started Guide
Version 6.0.1
© 2020 Real-Time Innovations, Inc.
All rights reserved.
Printed in U.S.A. First printing.
March 2020.
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Contents
Chapter 1 Welcome to RTIConnext DDS!
1.1 A Guide to the Provided Documentation
2
1.2 Paths Mentioned in Documentation
4
1.3 Why Choose Connext DDS?
5
1.3.1 Reduce Risk Through Performance and Availability
5
1.3.2 Reduce Cost through Ease of Use and Simplified Deployment
6
1.3.3 Ensure Success with Unmatched Flexibility
6
1.3.4 Connect Heterogeneous Systems
7
1.3.5 Interoperate with Databases, Event Engines, and JMS Systems
7
1.4 What Can Connext DDS Do?
7
1.5 Am I Better Off Building My Own Middleware?
9
Chapter 2 Navigating the Directories
10
Chapter 3 Selecting a Development Environment
3.1 Using the Command-Line Tools
12
3.2 Using Microsoft Visual Studio
13
Chapter 4 AQuick Overview
4.1 Building and Running “Hello, World”
14
4.1.1 Step 1: Set Up the Environment
14
4.1.1.1 Set Up the Environment on Your Development Machine
14
4.1.1.2 Set Up the Environment on Your Deployment Machine
17
4.1.2 Step 2: Compile the Hello World Program
18
4.1.3 Step 3: Start the Subscriber
21
4.1.4 Step 4: Start the Publisher
22
4.2 Building and Running a Request-Reply Example
24
4.3 An Introduction to DDS
25
4.3.1 An Overview of DDS Objects
26
iii
iv
4.3.2 DomainParticipants
26
4.3.2.1 What is QoS?
29
4.3.3 Publishers and DataWriters
29
4.3.4 Subscribers and DataReaders
31
4.3.5 Topics
35
4.3.6 Keys and DDS Samples
36
4.3.7 Requesters and Repliers
37
Chapter 5 Capabilities and Performance
5.1 Automatic Application Discovery
40
5.1.1 When to Set the Discovery Peers
40
5.1.2 How to Set Your Discovery Peers
41
5.2 Customizing Behavior: QoS Configuration
42
5.3 Compact, Type-Safe Data Programming with DDSData Types
45
5.3.1 Using Built-in DDS Types
48
5.3.2 Using DDS Types Defined at Compile Time
48
5.3.3 Generating Code with RTI Code Generator
49
5.3.3.1 Building the Generated Code
52
5.3.3.2 Running the Example Applications
54
5.3.4 Running with Dynamic DDS Types
56
Chapter 6 Design Patterns for Rapid Development
6.1 Building and Running the News Examples
58
6.2 Subscribing Only to Relevant Data
60
6.2.1 Content-Based Filtering
61
6.2.1.1 Implementation
61
6.2.1.2 Running & Verifying
62
6.2.2 Lifespan and History Depth
63
6.2.2.1 Implementation
63
6.2.2.2 Running & Verifying
65
6.2.3 Time-Based Filtering
67
6.2.3.1 Implementation
68
6.2.3.2 Running & Verifying
69
6.3 Accessing Historical Data when Joining the Network
69
6.3.1 Implementation
70
6.3.2 Running & Verifying
71
6.4 Caching Data within the Middleware
71
6.4.1 Implementation
72
6.4.2 Running & Verifying
75
6.5 Receiving Notifications When Data Delivery Is Late
75
6.5.1 Implementation
75
6.5.1.1 Offered Deadlines
75
6.5.1.2 Requested Deadlines
76
6.5.2 Running & Verifying
78
Chapter 7 Design Patterns for High Performance
7.1 Building and Running the Code Examples
81
7.1.1 Understanding the Performance Results
82
7.1.2 Is this the Best Possible Performance?
84
7.2 Reliable Messaging
84
7.2.1 Implementation
85
7.2.1.1 Enable Reliable Communication
85
7.2.1.2 Set History To KEEP_ALL
86
7.2.1.3 Controlling Middleware Resources
87
7.3 High Throughput for Streaming Data
88
7.3.1 Implementation
90
7.4 Sending Large Data
91
7.4.1 FlatData Language Binding
91
7.4.2 Zero Copy Transfer Over Shared Memory
93
7.4.3 Choosing between FlatData Language Binding and Zero Copy Transfer over Shared Memory
94
7.5 Streaming Data over Unreliable Network Connections
95
7.5.1 Implementation
96
7.5.1.1 Managing Your Sample Size
96
7.5.1.2 Acknowledge and Repair Efficiently
98
7.5.1.3 Make Sure Repair Packets Don’t Exceed Bandwidth Limitation
99
7.5.1.4 Use Batching to Maximize Throughput for Small Samples
100
v
Chapter 1 Welcome to RTIConnext DDS!
RTI® Connext® DDS solutions provide a flexible connectivity software framework for integrating
data sources of all types. At its core is the world's leading ultra-high performance, distributed net-
working databus. It connects data within applications as well as across devices, systems and net-
works. Connext DDS also delivers large data sets with microsecond performance and granular
quality-of-service control. Connext DDS is a standards-based, open architecture that connects
devices from deeply embedded real-time platforms to enterprise servers across a variety of net-
works. Connext DDS provides:
l Ultra-low latency, extremely-high throughput messaging
l Industry-leading reliability and determinism
l Connectivity for heterogeneous systems spanning thousands of applications
Connext DDS is flexible; extensive quality-of-service (QoS) parameters adapt to your application,
assuring that you meet your real-time, reliability, and resource usage requirements.
This document introduces basic concepts and summarizes how Connext DDS addresses your high-
performance needs. After this introduction, we'll jump right into building distributed systems. The
rest of this guide covers:
l First steps: Creating your first simple application.
l Learning more: An overview of the APIs and programming model with a special focus on
the communication model, data types and qualities of service.
l Towards real-world applications: An introduction to meeting common real-world require-
ments.
To install Connext DDS, see the separate RTIConnext DDS Installation Guide.
1
1.1 A Guide to the Provided Documentation
2
1.1 A Guide to the Provided Documentation
We invite you to explore further by referring to the wealth of available information, examples, and
resources.
We refer to the main installation directory as <NDDSHOME>. See 1.2 Paths Mentioned in
Documentation on page4.
After installing Connext DDS, you'll find these in <NDDSHOME>/doc/manuals/connext_dds:
l Installation Guide—Describes how to install Connext DDS and license management.
l Getting Started Guide—Introduces you to the benefits and concepts behind the product and takes
you step-by-step through the creation of a simple example application.
If you want to use the Connext DDS Extensible Types feature, please also read:
l Addendum for Extensible Types —Extensible Types allow you to define DDS data types
in a more flexible way. Your DDS data types can evolve over time—without giving up port-
ability, interoperability, or the expressiveness of the DDS type system.
If you are using Connext DDS on an Android®, iOS®, or embedded platform, or with a database,
you will find additional documents that specifically address these configurations:
l Addendum for Android Systems
l Addendum for iOSSystems
l Addendum for Embedded Systems
l Addendum for Database Setup
l What’s New—Provides an overview of new features and enhancements in the current version of
Connext DDS.
l Release Notes—Describes system requirements, what's new, what's fixed, and known issues.
l User’s Manual—Describes the features of the product and how to use them. It is organized around
the structure of the Connext DDS APIs and certain common high-level tasks.
l Platform Notes—Provides platform-specific information about the product, including specific
information required to build your applications using Connext DDS, such as compiler flags and lib-
raries.
l API Reference HTML Documentation (<NDDSHOME>/README.html)—This extensively
cross-referenced documentation, available for all supported programming languages, is your in-
depth reference to every operation and configuration parameter in Connext DDS. Even experienced
Connext DDS developers will often consult this information.
l The Programming How To's provide a good place to begin learning the APIs. These are hyper-
linked code snippets to the full API documentation.
1.1 A Guide to the Provided Documentation
From the README.html file, select one of the supported programming languages, then scroll
down to the Programming How To’s. Start by reviewing the Publication Example and Subscription
Example, which provide step-by step examples of how to send and receive data with Connext DDS.
Many readers will also want to look at additional documentation available online. In particular, RTI recom-
mends the following:
l The Migration Guide describes how to migrate to the current release from a previous Connext
DDS release, including what compatibility issues you may need to account for during your upgrade.
It is provided on the RTI Community Portal (https://community.rti.com/documentation) and is
updated as needed.
l The RTI Customer Portal (http://support.rti.com/) Use this portal to download RTI software,
access documentation and contact RTI Support. The RTI Customer Portal requires a username and
password. You will receive this in the email confirming your purchase. If you do not have this
email, please contact license@rti.com. Resetting your login password can be done directly at the
RTI Customer Portal.
l The RTI Community website (https://community.rti.com) provides a wealth of knowledge to help
you use Connext DDS, including:
l Documentation, at https://community.rti.com/documentation
l Best Practices
l Example code for specific features, as well as more complete use-case examples,
l Solutions to common questions,
l A glossary,
l Downloads of experimental software,
l And more.
l Whitepapers and other articles are available from http://www.rti.com/resources
Of course, RTI also offers excellent technical support and professional services. To contact RTI Support,
simply log into the Customer Portal, send email to support@rti.com, or call the telephone number provided
for your region.
3
1.2 Paths Mentioned in Documentation
4
1.2 Paths Mentioned in Documentation
The documentation refers to:
l <NDDSHOME>
This refers to the installation directory for RTI® Connext® DDS. The default installation paths are:
l macOS® systems:
/Applications/rti_connext_dds-6.0.1
l Linux systems, non-root user:
/home/<your user name>/rti_connext_dds-6.0.1
l Linux systems, root user:
/opt/rti_connext_dds-6.0.1
l Windows® systems, user without Administrator privileges:
<your home directory>\rti_connext_dds-6.0.1
l Windows systems, user with Administrator privileges:
C:\Program Files\rti_connext_dds-6.0.1
You may also see $NDDSHOME or %NDDSHOME%, which refers to an environment variable
set to the installation path.
Wherever you see <NDDSHOME> used in a path, replace it with your installation path.
Note for Windows Users: When using a command prompt to enter a command that includes the
path C:\Program Files (or any directory name that has a space), enclose the path in quotation
marks. For example:
“C:\Program Files\rti_connext_dds-6.0.1\bin\rtiddsgen”
Or if you have defined the NDDSHOME environment variable:
“%NDDSHOME%\bin\rtiddsgen”
l <path to examples>
By default, examples are copied into your home directory the first time you run RTI Launcher or
any script in <NDDSHOME>/bin. This document refers to the location of the copied examples as
<path to examples>.
Wherever you see <path to examples>, replace it with the appropriate path.
Default path to the examples:
l macOS systems: /Users/<your user name>/rti_workspace/6.0.1/examples
l Linux systems: /home/<your user name>/rti_workspace/6.0.1/examples
1.3 Why Choose Connext DDS?
l Windows systems: <your Windows documents folder>\rti_workspace\6.0.1\examples
Where 'your Windows documents folder' depends on your version of Windows. For example,
on Windows 10, the folder is C:\Users\<your user name>\Documents.
Note: You can specify a different location for rti_workspace. You can also specify that you do not
want the examples copied to the workspace. For details, see Controlling Location for
RTIWorkspace and Copying of Examples in the RTIConnext DDS Installation Guide.
1.3 Why Choose Connext DDS?
Connext DDS implements publish/subscribe networking for high-performance distributed applications. It
complies with the Data Distribution Service (DDS) standard from the Object Management Group (OMG).
Developers familiar with JMS and other middleware will see similarities, but will also find that DDS is not
just another MOM (message-oriented middleware) standard! Its unique peer-to-peer architecture and unpre-
cedented flexibility delivers much higher performance and adaptation to challenging applications. DDS is
the first truly real-time networking technology standard. Connext DDS is by far the market leading imple-
mentation of DDS.
1.3.1 Reduce Risk Through Performance and Availability
Connext DDS provides top performance, whether measured in terms of latency, throughput, or real-time
determinism
1
. One reason is its elegant peer-to-peer architecture.
Traditional messaging middleware requires dedicated servers to broker messages—introducing throughput
and latency bottlenecks and timing variations. Brokers also increase system administration costs and rep-
resent single points of failure within a distributed application, putting data reliability and availability at risk.
Connext DDS doesn't use brokers. Messages flow directly from publishers to subscribers with minimal
overhead. All the functions of the broker, including discovery (finding data sources and sinks), routing (dir-
ecting data flow), and naming (identifying DDS data types and Topics) are handled in a fully-distributed,
reliable fashion behind the scenes. It requires no special server software or hardware.
1
You can review the data from several performance benchmarks here: http://www.rti.com/products/dds/benchmarks.html.
Updated results for new releases are typically published within two months after general availability of that release.
5
1.3.2 Reduce Cost through Ease of Use and Simplified Deployment
6
Traditional message-oriented middleware implementations require a broker to forward every message, increasing
latency and decreasing determinism and fault tolerance. RTI's unique peer-to-peer architecture eliminates bottlenecks
and single points of failure.
The design also delivers high reliability and availability, with automatic failover, configurable retries, and
support for redundant publishers, subscribers, networks, and more. Publishers and subscribers can start in
any order, and enter and leave the network at any time; Connext DDS will connect and disconnect them
automatically. Connext DDS provides fine-grained control over failure behavior and recovery, as well as
detailed status notifications to allow applications to react to situations such as missed delivery deadlines,
dropped connections, and slow or unresponsive nodes.
The RTIConnext DDS Core Libraries User's Manual has details on these and all other capabilities. This
guide only provides an overview.
1.3.2 Reduce Cost through Ease of Use and Simplified Deployment
Connext DDS helps keep development and deployment costs low by:
l Increasing developer productivity—Easy-to-use, standards-compliant DDS APIs get your code
running quickly. DDS is the established connectivity framework standard for real-time pub-
lish/subscribe communication in the defense industry and is expanding rapidly in utilities, trans-
portation, intelligence, finance, and other commercial industries.
l Simplifying deployment—Connext DDS automatically discovers connections, so you don't need to
configure or manage server machines or processes. This translates into faster turnaround and lower
overhead for your deployment and administration needs.
l Reducing hardware costs—Traditional messaging products require dedicated servers or accel-
eration hardware in order to host brokers. The extreme efficiency and reduced overhead of RTI's
implementation, on the other hand, allows you to achieve the same performance using standard off-
the-shelf hardware, with fewer machines than the competition.
1.3.3 Ensure Success with Unmatched Flexibility
Out of the box, Connext DDS is configured to achieve simple data communications. However, when you
need it, RTI provides a high degree of fine-grained, low-level control over the middleware, including:
l The volume of meta-traffic sent to assure reliability.
l The frequencies and time-outs associated with all events within the middleware.
l The amount of memory consumed, including the policies under which additional memory may be
allocated by the middleware.
1.3.4 Connect Heterogeneous Systems
RTI’s unique and powerful Quality-of-Service (QoS) policies can be specified in configuration files so that
they can be tested and validated independently of the application logic. When not specified, Connext DDS
will use default values chosen to provide good performance for a wide range of applications.
The result is simple-to-use networking that can expand and adapt to challenging applications, both current
and future. RTI eliminates what is perhaps the greatest risk of commercial middleware: outgrowing the cap-
ability or flexibility of the design.
1.3.4 Connect Heterogeneous Systems
Connext DDS provides complete functionality and industry-leading performance for a wide variety of pro-
gramming languages and platforms, including:
l C, C++, .NET
1
, Java, and Ada
2
development platforms
l Windows, Linux, Solaris, Android, AIX, and other enterprise-class systems
l VxWorks, INTEGRITY, LynxOS, and other real-time and/or embedded systems
Applications written in different programming languages, running on different hardware under different
operating systems, can interoperate seamlessly over Connext DDS, allowing disparate applications to work
together in even very complex systems.
1.3.5 Interoperate with Databases, Event Engines, and JMS Systems
Connext DDS provides connections between its middleware core and many types of enterprise software.
Simple-but-powerful integrations with databases, Complex Event Processing (CEP) engines, and other
middlewares ensure that Connext DDS can bind together your real-time and enterprise systems.
For more information about interoperability with other middleware implementations, please consult your
RTI account representative.
1.4 What Can Connext DDS Do?
Under the hood, Connext DDS goes beyond basic publish-subscribe communication to target the needs of
applications with high-performance, real-time, and/or low-overhead requirements. It features:
l Peer-to-peer, publish-subscribe communications—The most elegant, flexible data com-
munications model.
1
The Connext DDS .NET language binding is currently supported for C# and C++/CLI.
2
Ada support requires a separate add-on product, Ada Language Support.
7
1.4 What Can Connext DDS Do?
8
l Simplified distributed application programming
l Time-critical data flow with minimal latency
l Clear semantics for managing multiple sources of the same data.
l Customizable Quality of Service and error notification.
l Guaranteed periodic messages, with minimum and maximum rates set by subscriptions
l Notifications when applications fail to meet their deadlines.
l Synchronous or asynchronous message delivery to give applications control over the degree
of concurrency.
l Ability to send the same message to multiple subscribers efficiently, including support for reli-
able multicast with customizable levels of positive and negative message acknowledgement.
l Request-Reply Communications—As applications become more complex, it often becomes neces-
sary to use other communication patterns in addition to publish-subscribe. Sometimes an application
needs to get a one-time snapshot of information; for example, to make a query into a database or
retrieve configuration parameters that never change. Other times an application needs to ask a
remote application to perform an action on its behalf. To support these scenarios, Connext DDS
includes support for the request-reply communication pattern. The Requester (service consumer or
client) sends a request message and waits for a reply message. The Replier (service provider)
receives the request message and responds with a reply message.
l Reliable messaging—Enables subscribing applications to customize the degree of reliability
required. Reliability can be tuned; you can guarantee delivery no matter how many retries are
needed or try messages only once for fast and deterministic performance. You can also specify any
settings in between. No other middleware lets you make this critical trade off on a per-message
stream basis.
l Multiple communication networks—Multiple independent communication networks (DDS
domains), each using Connext DDS, can be used over the same physical network to isolate unre-
lated systems or subsystems. Individual applications can participate in one or multiple DDS
domains.
l Symmetric architecture—Makes your application robust. No central server or privileged
nodes means your system is robust to application and/or node failures.
l Dynamic—Topics, subscriptions, and publications can be added or removed from the system
at any time.
l Multiple network transports—Connext DDS includes support for UDP/IP (IPv4 and IPv6)—
including, for example, Ethernet, wireless, and Infiniband networks—and shared memory
transports. It also includes the ability to dynamically plug in additional network transports and
route messages over them. It can be configured to operate over a variety of transport mech-
anisms, including backplanes, switched fabrics, and other networking technologies.
1.5 Am I Better Off Building My Own Middleware?
l Multi-platform and heterogeneous system support—Applications based on Connext DDS can
communicate transparently with each other regardless of the underlying operating system or
hardware. Consult the Release Notes to see which platforms are supported in this release.
l Vendor neutrality and standards compliance—The Connext DDS API complies with the DDS
specification. On the network, it supports the open DDS Interoperability Protocol, Real-Time
Publish Subscribe (RTPS), which is also an open standard from the OMG.
1.5 Am I Better Off Building My Own Middleware?
Sometimes application projects start with minimal networking needs. So it’s natural to consider whether
building a simplified middleware in-house is a better alternative to purchasing a more-complex commercial
middleware. While doing a complete Return on Investment (ROI) analysis is outside the scope of this doc-
ument, with Connext DDS, you get a rich set of high-performance networking features by just turning on
configuration parameters, often without writing a single line of additional code.
RTI has decades of experience with hundreds of applications. This effort created an integrated and time-
tested architecture that seamlessly provides the flexibility you need to succeed now and grow into the
future. Many features require fundamental support in the core of the middleware and cannot just be
cobbled onto an existing framework. It would take many man-years of effort to duplicate even a small sub-
set of the functionality to the same level of stability and reliability as delivered by RTI.
For example, some of the middleware functionality that your application can get by just enabling con-
figuration parameters include:
l Tuning reliable behavior for multicast, lossy and high-latency networks.
l Supporting high data-availability by enabling redundant data sources (for example, “keep receiving
data from source A. If source A stops publishing, then start receiving data from source B”), and
providing notifications when applications enter or leave the network.
l Optimizing network and system resources for transmission of periodic data by supporting time-
based filtering of data (example: “receive a DDS sample no more than once per second”) and dead-
lines (example: “expect to receive a DDS sample at least once per second”).
Writing network code to connect a few nodes is deceptively easy. Making that code scale, handle all the
error scenarios you will eventually face, work across platforms, keep current with technology, and adapt to
unexpected future requirements is another matter entirely. The initial cost of a custom design may seem
tempting, but beware; robust architectures take years to evolve. Going with a fire-tested, widely used
design assures you the best performance and functionality that will scale with your distributed application
as it matures. And it will minimize the profound cost of going down the wrong path.
9
Chapter 2 Navigating the Directories
To install Connext DDS, see the separate RTIConnext DDS Installation Guide.
After installing Connext DDS, your installation directory will include:
l /bin — Batch files for running the target package installer, utilities, code generator, etc.
l /doc/manuals - PDF documentation.
l /doc/api - API Reference HTML documentation.
l /include — Header files for C and C++ APIs, specification files for Ada
l /lib — Library files
l /resource — Document-format definitions, template files used by rtiddsgen, as well as the
run-time components used by the RTI tools and services, including the RTI libraries and
JRE.
l /uninstall — Uninstaller
l /README.html — Starting page for accessing the APIReference HTML documentation
The doc directory contains the Connext DDS library information in PDF and HTML formats. You
may want to bookmark the doc directory since you will be referring to this page a lot as you
explore the RTI technology platform.
Examples are also available, see 1.2 Paths Mentioned in Documentation on page4.
See the instructions in each example’s README_ME.txt file. These examples include:
l hello_world: This example demonstrates a simple "hello world" built with Connext DDS: it
does nothing but publish and subscribe to short strings of text. This example is described in
detail in 4.1 Building and Running “Hello, World” on page14.
10
Chapter 2 Navigating the Directories
11
l hello_builtin, hello_idl, hello_dynamic
a
: These examples demonstrate some more of the unique
capabilities of Connext DDS: strongly typed data, QoS-based customization of behavior, and
industry-leading performance. These examples are described in Chapter 5 Capabilities and Per-
formance on page39 and Chapter 7 Design Patterns for High Performance on page80.
l hello_world_request_reply: This example demonstrates how to use 4.3.7 Requesters and Repliers
on page37. The Replier is capable of computing the prime numbers below a certain positive integer;
the Requester will request these numbers. The Replier provides the prime numbers as they are being
calculated, sending multiple replies. See 4.2 Building and Running a Request-Reply Example on
page24.
l news: This example demonstrates a subset of the rich functionality Connext DDS offers, including
flexible support for historical and durable data, built-in data caching, powerful filtering capabilities,
and tools for working with periodic data. This example is described in Chapter 6 Design Patterns for
Rapid Development on page57.
You can find more examples at http://www.rti.com/examples. This page contains example code snippets
on how to use individual features, examples illustrating specific use cases, as well as performance test
examples.
Connext DDS supports the C, C++, C++/CLI, C#, Java, and Ada
b
programming languages. There are two
versions of the C++API, the traditional C++API and the modern C++PSMAPI. While we will examine
the C++, Java, and Ada examples in the following sections, you can access similar code in the language of
your choice.
a
hello_dynamic is not provided for Ada.
b
Ada support requires a separate component, Ada Language Support.
Chapter 3 Selecting a Development
Environment
You can develop applications with Connext DDS either by building and executing commands
from a shell window, or by using a development environment like Microsoft® Visual Studio®,
Eclipse™, or GPS from AdaCore
1
.
The following discusses building and running the provided examples from the command line or
Visual Studio. Then we'll step through the details in 4.1 Building and Running “Hello, World” on
page14.
The <path to examples>is described in 1.2 Paths Mentioned in Documentation on page4.
3.1 Using the Command-Line Tools
For most Java applications, you can use the following script files to build and execute.
l <path to examples>/connext_dds/java/<example>/build.sh (or build.cmd on Windows
systems) —Builds Java source files; no parameters are needed.
l <path to examples>/connext_dds/java/<example>/run.sh (or run.cmd on Windows sys-
tems)
—Runs the main program, Hello, in either Publisher or Subscriber mode. It accepts the fol-
lowing parameters:
l [pub|sub] —Runs as publisher or subscriber
l –verbose —Increases output verbosity
(This script accepts other options, which we will discuss later.)
1
Ada support requires a separate add-on product, Ada Language Support.
12
3.2 Using Microsoft Visual Studio
13
The hello_world example uses the rtiddsgen tool to generate code, instead of these scripts.The steps to
build and run hello_world are described in 4.1 Building and Running “Hello, World” on page14.
For C, C++, and Ada applications:
l You can use the make command with this makefile:
<path to examples>/connext_dds/<language>/<example>/makefile_<example>_<architecture>
For Java users:
l The native libraries used by the RTI Java API require the Visual Studio redistributable libraries on
the target machine. You can obtain this package from Microsoft or RTI.
3.2 Using Microsoft Visual Studio
For C, C++, and C# users: Please use a supported version of Microsoft Visual Studio to build and run the
examples. Supported versions of Visual Studio are listed in the Platform Notes.
Connext DDS includes solutions and project files for Microsoft Visual Studio in
<path to examples>\connext_dds\[c|c++|cs]\<example>.
To use these solution files:
1. Start Visual Studio.
2. Select File, Open, Project/Solution.
3. In the File dialog, select the solution file for your architecture. For example, a solution file for Visual
Studio 2012 for 32-bit platforms is in
<path to examples>\connext_dds\[c|c++|cs]\<example>\win32\Hello-i86Win32VS2012.sln.
Chapter 4 AQuick Overview
Before continuing, follow the instructions in the RTIConnext DDS Installation Guide.
This chapter gets you up and running with Connext DDS. First you will build and run your first
Connext DDS-based application. Then we'll take a step back to learn about the general concepts in
Connext DDS and show how they are applied in the example application you ran.
4.1 Building and Running “Hello, World”
Let’s start by compiling and running Hello World, a basic program that publishes information over
the network.
For now, do not worry about understanding the code (we start covering it in Capabilities and Per-
formance (Chapter 5 on page39). Use the following instructions to run your first middleware pro-
gram using Connext DDS.
Find a new version of this exercise, in an updated getting started guide, at
https://community.rti.com/documentation.
4.1.1 Step 1: Set Up the Environment
There are a few things to take care of before you start working with the example code.
4.1.1.1 Set Up the Environment on Your Development Machine
1. Set the NDDSHOME environment variable.
Set the environment variable NDDSHOME to the Connext DDS install directory. (Connext
DDS itself does not require that you set this environment variable. It is used in the scripts
used to build and run the examples code because it is a simple way to locate the install dir-
ectory. You may or may not choose to use the same mechanism when you create scripts to
14
4.1.1.1 Set Up the Environment on Your Development Machine
15
build and/or run your own applications.)
The default installation paths are described in 1.2 Paths Mentioned in Documentation on page4:
If you have multiple versions of Connext DDS installed:
Connext DDS does not require that you have the environment variable NDDSHOME set at run
time. However, if it is set, Connext DDS will use it to load certain configuration files. Additionally,
you may have previously set your path based on the value of that variable. Therefore, if you have
NDDSHOME set, be sure it is pointing to the right copy of Connext DDS.
2. Update your path.
Add Connext DDS's bin directory to your path. This will allow you to run some of the simple com-
mand-line utilities included in your distribution without typing the full path to the executable.
On UNIX-based systems:
Add the directory to your PATH environment variable.
On Windows systems:
Add the directory to your Path environment variable.
3. If you will be using the separate add-on product, Ada Language Support:
Add $NDDSHOME/lib/gnat to your ADA_PROJECT_PATH environment variable. This dir-
ectory contains Ada project files that will be used in the generated example project file.
Make sure the Ada compiler, gprbuild, is in your path. The makefile used by the example assumes
that gprbuild is in your path.
On UNIX-based systems:
Add the path to gprbuild to your PATH environment variable.
4. Make sure Java is available (only needed if you will be developing in Java).
Ensure that appropriate java and javac executables are in your path. They can be found within the
bin directory of your JDK installation. The Release Notes list the Java versions that are supported.
On Linux systems:
Note that GNU java (from the GNU Classpath project) is not supported—and will typically not
work—but is in the path by default on many Linux systems.
4.1.1.1 Set Up the Environment on Your Development Machine
5. Make sure the preprocessor is available.
Check whether the C preprocessor (e.g., cpp) is in your search path. This step is optional, but makes
code generation with the rtiddsgen utility more convenient.Capabilities and Performance (Chapter 5
on page39) describes how to use rtiddsgen.
On Windows systems, if Microsoft Visual Studio is installed:
Running the script vcvars32.bat, vsvars32.bat, or vcvarsall.bat (depending on your version
of Visual Studio) will update the path for a given command prompt. If the Visual Studio
installer did not add cl to your path already, you can run this script before running rtiddsgen.
On Windows systems, if Microsoft Visual Studio is not installed:
This is often the case with Java users. You can either choose not to use a preprocessor or to
obtain a no-cost version of Visual Studio from Microsoft's web site.
6. Get your project files ready.
On Windows systems:
If you installed Connext DDS in C:\Program Files or another system directory, Microsoft Visual
Studio may present you with a warning message when you open a solution file from the installation
directory. If you see this dialog, you may want to copy the example directory somewhere else, as
described above.
On UNIX-based systems:
The makefiles that RTI provides with the example code are intended to be used with the GNU dis-
tribution of the make utility. On modern Linux systems, the make binary typically is GNU make.
16
4.1.1.2 Set Up the Environment on Your Deployment Machine
17
On other systems, GNU make is called gmake. For the sake of clarity, the name gmake is used
below. Make sure that the GNU make binary is on your path before continuing.
4.1.1.2 Set Up the Environment on Your Deployment Machine
Some configuration has to be done for the machine(s) on which you run your application; the RTI installer
can’t do that for you, because those machines may or may not be the same as where you created and built
the application.
1. Make sure Java is available (only needed if you will be developing in Java).
Ensure that appropriate java and javac executables are in your path. They can be found within the
bin directory of your JDK installation. The Release Notes list the Java versions that are supported.
On Linux systems: Note that GNU java (from the GNU Classpath project) is not supported—and
will typically not work—but is in the path by default on many Linux systems.
2. Make sure the dynamic libraries are available.
Make sure that your application can load the Connext DDS dynamic libraries. If you use C, C++, or
Ada with static libraries (the default configuration in the examples covered in this document), you
can skip this step. However, if you plan to use dynamic libraries, or Java or .NET
a
(which always
use dynamic libraries), you will need to modify your environment as described here.
To see if dynamic libraries are supported for your machine’s architecture, see the Connext DDS
Core Libraries Platform Notes
b
.For more information about where the Windows OS looks for
dynamic libraries, see: http://msdn.microsoft.com/en-us/library/ms682586.aspx.
C/C++:
The dynamic libraries needed by C or C++ applications are in the directory
${NDDSHOME}/lib/<architecture>. The dynamic libraries needed by Ada applications are
in the directory ${NDDSHOME}/lib/GNATgcc/relocatable.
On UNIX-based systems: Add this directory to your LD_LIBRARY_PATH environment vari-
able.
On macOS systems: Add this directory to your DYLD_LIBRARY_PATH environment vari-
able.
a
Connext DDS .NET language binding is currently supported for C# and C++/CLI.
b
In the Platform Notes, see the “Building Instructions...” table for your target architecture.
4.1.2 Step 2: Compile the Hello World Program
On Windows systems: Add this directory to your Path environment variable or copy the DLLs
into the directory containing your executable.
On AIX systems: Add this directory to your LIBPATH environment variable.
Java:
The native dynamic libraries needed by Java applications are in the directory
${NDDSHOME}/lib/<architecture>. The native dynamic libraries needed by Ada applic-
ations are in the directory $NDDSHOME/lib/<architecture>.
l On UNIX-based systems: Add this directory to your LD_LIBRARY_PATH envir-
onment variable.
l On macOS systems: Add this directory to your DYLD_LIBRARY_PATH environment
variable.
l On Windows systems: Add this directory to your Path environment variable.
l On AIX systems: Add this directory to your LIBPATH environment variable.
Java .jar files are in the directory ${NDDSHOME}/lib/java. They will need to be on your
application’s class path.
.NET:
On Windows systems: The dynamic libraries needed by .NET applications are in the directory
%NDDSHOME%\lib\i86Win32VSnnnn, where nnnn represents the Visual Studio version
number. You will need to either copy the DLLs from that directory to the directory containing
your executable, or add the directory containing the DLLs to your Path environment variable
a
.
(If the .NET framework is unable to load the dynamic libraries at run time, it will throw a Sys-
tem.IO.FileNotFoundException and your application may fail to launch.)
4.1.2 Step 2: Compile the Hello World Program
The same example code is provided in C, Traditional C++, C#, Java, and Ada
b
. The following instructions
cover C++, Java, and Ada in detail; the procedures for C and C# are very similar. The same source code
can be built and run on different architectures. Examples for the Modern C++ API are provided in the
RTICommunity portal.
a
The file nddsdotnet.dll (or nddsdotnetd.dll for debug) must be in the executable directory. Visual Studio
will, by default, do this automatically.
b
Ada support requires a separate add-on product, Ada Language Support.
18
4.1.2 Step 2: Compile the Hello World Program
19
By default, examples are copied into your home directory the first time you run RTI Launcher or any script
in <NDDSHOME>/bin. This document refers to the location of the copied examples as <path to
examples>.
Wherever you see <path to examples>, replace it with the appropriate path.
Default path to the examples:
l macOS systems: /Users/<your user name>/rti_workspace/6.0.1/examples
l Linux systems: /home/<your user name>/rti_workspace/6.0.1/examples
l Windows systems: <your Windows documents folder>\rti_workspace\6.0.1\examples
Where 'your Windows documents folder' depends on your version of Windows. For example, on
Windows 10, the folder is C:\Users\<your user name>\Documents.
Note: You can specify a different location for rti_workspace. You can also specify that you do not want
the examples copied to the workspace. For details, see Controlling Location for RTIWorkspace and Copy-
ing of Examples in the RTIConnext DDS Installation Guide.
The instructions also focus on Windows and UNIX-based systems. If you will be using an embedded plat-
form, see the Embedded Systems Addendum for more instructions.
C++ on Windows Systems:
1. In the Windows Explorer, go to <path to examples>\connext_dds\c++\hello_world\win32 and
open the Visual Studio solution file for your architecture. For example, the file for Visual Studio
2013 for 32-bit platforms is HelloWorld-i86Win32VS2013.sln.
Note: If your Windows SDK Version is not 10.0.15063.0, you may be prompted to retarget the file.
If this happens, in the Retarget Projects window that appears, select an installed version of Windows
SDK and click OK.
2. The Solution Configuration combo box in the toolbar indicates whether you are building debug or
release executables; select Release. Select Build Solution from the Build menu.
4.1.2 Step 2: Compile the Hello World Program
C++ on UNIX-based Systems:
From your command shell, go to <path to examples>/connext_dds/c++/hello_world/ and type:
> gmake -f make/makefile_HelloWorld_<architecture>
where <architecture> is one of the supported architectures; see the contents of the examples directory for
a list of available architectures. (If you do not see a makefile for your architecture, please refer to 5.3.3
Generating Code with RTI Code Generator on page49 to learn how to generate a makefile or project files
for your platform). This command will build a release executable.
To build a debug version instead:
> gmake -f make/makefile_HelloWorld_<architecture> DEBUG=1
Java on Windows systems:
From your command shell, go to <path to examples>\connext_dds\java\hello_world and type:
> build
20
4.1.3 Step 3: Start the Subscriber
21
Java on UNIX-based systems:
From your command shell, go to <path to examples>/connext_dds/java/hello_world and type:
> ./build.sh
ADA on UNIX-based systems:
From your command shell, go to <path to examples>/connext_dds/ada/hello_world/ and type:
> gmake -f make/makefile_HelloWorld_<architecture>
where <architecture> is one of the supported architectures; see the contents of the examples directory for
a list of available architectures. (If you do not see a makefile for your architecture, please refer to 5.3.3
Generating Code with RTI Code Generator on page49 to learn how to generate a makefile or project files
for your platform). This command will build a release executable.
To build a debug version instead:
> gmake -f make/makefile_HelloWorld_<architecture> DEBUG=1
4.1.3 Step 3: Start the Subscriber
C++ on Windows systems:
From your command shell, go to examples\connext_dds\c++\hello_world and type:
> objs\<architecture>\HelloWorld_subscriber.exe
where <architecture> is one of the supported architectures; see the contents of the examples directory for a
list of available architectures. For example, the Windows architecture name corresponding to 32-bit Visual
Studio 2013 is i86Win32VS2013.
C++ on UNIX-based systems:
From your command shell, go to /examples/connext_dds/c++/hello_world and type:
> objs/<architecture>/HelloWorld_subscriber
where <architecture> is one of the supported architectures; see the contents of the examples directory for
a list of available architectures. For example, a Red Hat® Enterprise Linux architecture is i86Linux3gc-
c4.8.2.
Java:
(As described in 4.1.1 Step 1: Set Up the Environment on page14, you should have already set your path
appropriately so that the example application can load the native libraries on which Connext DDS
depends. If you have not, you can set the variable RTI_EXAMPLE_ARCH in your command shell—e.g.,
to i86Win32VS2013 or i86Linux3gcc4.8.2—and the example launch scripts will use it to set your path for
you.)
4.1.4 Step 4: Start the Publisher
Java on Windows systems:
From your command shell, go to examples\connext_dds\java\hello_world and type:
> runSub
Java on UNIX-based systems:
From your command shell, go to examples/connext_dds/java/hello_world and type:
> ./runSub.sh
Ada on UNIX-based systems:
From your command shell, go to /examples/connext_dds/ada/hello_world and type:
> objs/<architecture>/hellosubscriber
Where <architecture> is one of the supported architectures; see the content of the examples directory for a
list of available architectures. For example, an architecture name corresponding to a Red Hat Enterprise
Linux system is x64Linux2.6gcc4.4.5.
4.1.4 Step 4: Start the Publisher
Connext DDS interoperates across all of the programming languages it supports, so you can choose
whether to run the publisher in the same language you chose for the subscriber or a different language.
C++ on Windows systems:
From a different command shell, go to examples\connext_dds\c++\hello_world and type:
> objs\<architecture>\HelloWorld_publisher.exe
where <architecture> is one of the supported architectures; see the contents of the examples directory for
a list of available architectures. For example, the Windows architecture name corresponding to 32-bit
Visual Studio 2013 is i86Win32VS2013.
C++ on UNIX-based systems:
From a different command shell, go to examples/connext_dds/c++/hello_world and type:
> objs/<architecture>/HelloWorld_publisher
where <architecture> is one of the supported architectures; see the contents of the example directory for a
list of available architectures. For example, a Red Hat Enterprise Linux architecture name is i86Linux3gc-
c4.8.2.
Java:
(As described above, you should have already set your path appropriately so that the example application
can load the native libraries on which Connext DDS depends. If you have not, you can set the variable
RTI_EXAMPLE_ARCH in your command shell—e.g., to i86Win32VS2013 or i86Linux3gcc4.8.2.—
22
4.1.4 Step 4: Start the Publisher
23
and the example launch scripts will use it to set your path for you.)
Java on Windows systems:
From a different command shell, go to <path to examples>\connext_dds\java\hello_world and type:
> runPub
Java on UNIX-based systems:
From a different command shell, go to <path to examples>/connext_dds/java/hello_world and type:
> ./runPub.sh
Ada on UNIX-based systems:
From a different command shell, go to <path to examples>/connext_dds/ada/hello_world and type:
> objs/<architecture>/hellopublisher
Where <architecture> is one of the supported architectures; see the contents of the example directory for a
list of available architectures. For example, an architecture name corresponding to a Red Hat Enterprise
Linux system is i86Linux3gcc4.8.2.
When you run the publishing and subscribing applications, you should see output similar to the following:
Congratulations! You’ve run your first Connext DDS program!
4.2 Building and Running a Request-Reply Example
4.2 Building and Running a Request-Reply Example
This section describes the Request-Reply communication pattern, which is only available with the
RTI Connext DDS Professional, Secure, Basic, and Evaluation package types.
The section on 4.3.7 Requesters and Repliers on page37 contains an introduction to the Connext DDS
Request-Reply API. More detailed information is available in the RTIConnext DDS Core Libraries User's
Manual (see Part 4: Request-Reply Communication) and in the API Reference HTML documentation
(open <NDDSHOME>/README.html and select a programming language; then select Modules, RTI
Connext DDS API Reference, RTI Connext Messaging API Reference). See 1.1 A Guide to the
Provided Documentation on page2.
Connext DDS provides the libraries that you will need when compiling an application that uses the
Request-Reply API.
l In C, you need the additional rticonnextmsgc libraries and you will use a set of macros that instan-
tiate DDS type-specific code. You will see how to do this in the code example.
l In C++, you need the additional rticonnextmsgcpp libraries and the header file ndds/ndds_
requestreply_cpp.h.
l In Java, you need an additional JAR file: rticonnextmsg.jar.
l In .NET (C# and C++/CLI), you need an additional assembly:
l For .NET2.0:rticonnextmsgdotnet.dll
l For .NET4.0:rticonnextmsgdotnet40.dll
l For .NET4.5: rticonnextmsgdotnet45.dll
l For .NET4.5.1:rticonnextmsgdotnet451.dll
l For .NET4.6:rticonnextmsgdotnet46.dll
To set up your environment follow the same instructions in 4.1.1 Step 1: Set Up the Environment on
page14.
By default, examples are copied into your home directory the first time you run RTI Launcher or any script
in <NDDSHOME>/bin. This document refers to the location of the copied examples as <path to
examples>.
Wherever you see <path to examples>, replace it with the appropriate path.
Default path to the examples:
l macOS systems: /Users/<your user name>/rti_workspace/6.0.1/examples
l Linux systems: /home/<your user name>/rti_workspace/6.0.1/examples
24
4.3 An Introduction to DDS
25
l Windows systems: <your Windows documents folder>\rti_workspace\6.0.1\examples
Where 'your Windows documents folder' depends on your version of Windows. For example, on
Windows 10, the folder is C:\Users\<your user name>\Documents.
Note: You can specify a different location for rti_workspace. You can also specify that you do not want
the examples copied to the workspace. For details, see Controlling Location for RTIWorkspace and Copy-
ing of Examples in the RTIConnext DDS Installation Guide.
The Request-Reply examples are:
l examples/connext_dds/c/hello_world_request_reply
l examples/connext_dds/c++/ hello_world_request_reply
l examples/connext_dds/cs/hello_world_request_reply
l examples/connext_dds/java/hello_world_request_reply
To compile the examples, follow the instructions in 4.1.2 Step 2: Compile the Hello World Program on
page18. Similar makefiles for UNIX-based systems and scripts for Java and Visual Studio projects are
provided in these examples. Instructions for running the examples are in READ_ME.txt in the example
directories. See the instructions in each example’s README_ME.txt file.
4.3 An Introduction to DDS
Connext DDS is a software connectivity framework for real-time distributed applications. It provides the
middleware communications service that programmers need to distribute time-critical data between embed-
ded and/or enterprise devices or nodes. Connext DDS uses a publish-subscribe communications model to
make data-distribution efficient and robust. Connext DDS also supports the Request-Reply communication
pattern
a
.
Connext DDS implements the Data-Centric Publish-Subscribe (DCPS) API of the OMG’s specification,
Data Distribution Service (DDS) for Real-Time Systems. DDS is the first standard developed for the needs
of real-time systems, providing an efficient way to transfer data in a distributed system. With Connext
DDS, you begin your development with a fault-tolerant and flexible communications infrastructure that
will work over a wide variety of computer hardware, operating systems, languages, and networking trans-
port protocols. Connext DDS is highly configurable, so programmers can adapt it to meet an application’s
specific communication requirements.
a
Request-reply communication is available only when using the Connext DDS Professional, Secure,
Basic, and Evaluation package types.
4.3.1 An Overview of DDS Objects
4.3.1 An Overview of DDS Objects
The primary objects in DDS are:
l 4.3.2 DomainParticipants below
l 4.3.3 Publishers and DataWriters on page29
l 4.3.4 Subscribers and DataReaders on page31
l 4.3.5 Topics on page35
l 4.3.6 Keys and DDS Samples on page36
l 4.3.7 Requesters and Repliers on page37
Figure 4.1:
Connext DDS
Components
4.3.2 DomainParticipants
A DDS domain is a concept used to bind individual applications together for communication. To com-
municate with each other, DataWriters and DataReaders must have the same Topic of the same DDS data
type and be members of the same DDS domain. Each DDS domain has a unique integer domain ID.
Applications in one DDS domain cannot subscribe to data published in a different DDS domain. Multiple
DDS domains allow you to have multiple virtual distributed systems on the same physical network. This
26
4.3.2 DomainParticipants
27
can be very useful if you want to run multiple independent tests of the same applications. You can run
them at the same time on the same network as long as each test runs in a different DDS domain. Another
typical configuration is to isolate your development team from your test and production teams: you can
assign each team or even each developer a unique DDS domain.
DomainParticipant objects enable an application to exchange messages within DDS domains. An applic-
ation must have a DomainParticipant for every DDS domain in which the application will communicate.
(Unless your application is a bridging application, it will typically participate in only one DDS domain and
have only one DomainParticipant.)
A DomainParticipant is analogous to a JMS Connection.
DomainParticipants are used to create Topics, Publishers, DataWriters, Subscribers, and DataReaders in
the corresponding DDS domain.
Figure 4.2: Segregating Applications with Domains
The following code shows how to instantiate a DomainParticipant. You can find more information about
all of the APIs in the API Reference HTML documentation.
To create a DomainParticipant:
4.3.2 DomainParticipants
In Traditional C++:
participant =
DDSDomainParticipantFactory::get_instance()->create_participant(
0, /* Domain ID */
DDS_PARTICIPANT_QOS_DEFAULT, /* QoS */
NULL, /* Listener */
DDS_STATUS_MASK_NONE);
In Modern C++:
dds::domain::DomainParticipant participant(0);
In Java:
participant =
DomainParticipantFactory.get_instance().create_participant(
0, // Domain ID
DomainParticipantFactory.PARTICIPANT_QOS_DEFAULT,
null, // Listener
StatusKind.STATUS_MASK_NONE);
In Ada
a
:
participant :=
DDS.DomainParticipantFactory.Get_Instance.Create_Participant(
0, -- Domain ID
DDS.DomainParticipantFactory.PARTICIPANT_QOS_DEFAULT, -- QoS
null, -- Listener
DDS.STATUS_MASK_NONE);
As you can see, there are four pieces of information you supply when creating a new DomainParticipant:
l The ID of the DDS domain to which it belongs.
l Its qualities of service (QoS). The discussion on 4.3.2.1 What is QoS? on the next page gives a
brief introduction to the concept of QoS; you will learn more in Chapter 5 Capabilities and Per-
formance on page39.
l Its listener and listener mask, which indicate the events that will generate callbacks to the
DomainParticipant. You will see a brief example of a listener callback when we discuss DataRead-
ers below. For a more comprehensive discussion of the Connext DDS status and notification system,
see Chapter 4 in the RTIConnext DDS Core Libraries User's Manual.
(In the Modern C++ API the QoSand listener and mask are optional parameters.)
a
Ada support requires a separate add-on product, Ada Language Support.
28
4.3.2.1 What is QoS?
29
4.3.2.1 What is QoS?
Fine control over Quality of Service (QoS) is perhaps the most important feature of Connext DDS. Each
data producer-consumer pair can establish independent quality of service agreements—even in many-to-
many topologies. This allows applications to support extremely complex and flexible data-flow require-
ments.
QoS policies control virtually every aspect of Connext DDS and the underlying communications mech-
anisms. Many QoS policies are implemented as "contracts" between data producers (DataWriters) and con-
sumers (DataReaders); producers offer and consumers request levels of service. Connext DDS is
responsible for determining if the offer can satisfy the request, thereby establishing the communication or
indicating an incompatibility error. Ensuring that participants meet the level-of-service contracts guarantees
predictable operation. For example:
Periodic producers can indicate the speed at which they can publish by offering guaranteed update dead-
lines. By setting a deadline, a producer promises to send updates at a minimum rate. Consumers may then
request data at that or any slower rate. If a consumer requests a higher data rate than the producer offers,
Connext DDS will flag that pair as incompatible and notify both the publishing and subscribing applic-
ations.
Producers may offer different levels of reliability, characterized in part by the number of past DDS data
samples they store for retransmission. Consumers may then request differing levels of reliable delivery, ran-
ging from fast-but-unreliable "best effort" to highly reliable in-order delivery. This provides per-data-
stream reliability control. A single producer can support consumers with different levels of reliability sim-
ultaneously.
Other QoS policies control when Connext DDS detects nodes that have failed, set delivery order, attach
user data, prioritize messages, set resource utilization limits, partition the system into namespaces, control
durability (for fault tolerance) and much more. The Connext DDS QoS policies offer unprecedented flex-
ible communications control. The RTIConnext DDS Core Libraries User's Manual contains details about
all available QoS policies.
4.3.3 Publishers and DataWriters
An application uses a DataWriter to publish data into a DDS domain. Once a DataWriter is created and
configured with the correct QoS settings, an application only needs to use the DataWriter’s “write” oper-
ation to publish data.
A DataWriter is analogous to a JMS TopicPublisher. A Publisher is analogous to the producing aspect of
a JMS TopicSession.
A Publisher is used to group individual DataWriters. You can specify default QoS behavior for a Pub-
lisher and have it apply to all the DataWriters in that Publisher’s group.
4.3.3 Publishers and DataWriters
Figure 4.3: Entities Associated with Publications
To create a DataWriter:
In Traditional C++:
data_writer = participant->create_datawriter(
topic,
DDS_DATAWRITER_QOS_DEFAULT, /* QoS */
NULL, /* Listener */
DDS_STATUS_MASK_NONE);
In Modern C++:
dds::pub::DataWriter<dds::core::StringTopicType> writer(
rti::pub::implicit_publisher(participant), topic);
In Ada:
data_writer := participant.Create_DataWriter(
topic,
DDS.Publisher.DATAWRITER_QOS_DEFAULT, -- QoS
null, -- Listener
DDS.STATUS_MASK_NONE);
As you can see, each DataWriter is tied to a single topic. All data published by that DataWriter will be dir-
ected to that Topic.
30
4.3.4 Subscribers and DataReaders
31
As you will learn in Chapter 5 Capabilities and Performance on page39, each Topic—and therefore all
DataWriters for that Topic—is associated with a particular concrete DDS data type. The write operation,
which publishes data, is type safe, which means that before you can write data, you must perform a type
cast, like this (in Modern C++ the DataWriter is already typed and there is no cast to perform):
In Traditional C++:
string_writer = DDSStringDataWriter::narrow(data_writer);
In Java:
StringDataWriter dataWriter =
(StringDataWriter) participant.create_datawriter(
topic,
Publisher.DATAWRITER_QOS_DEFAULT,
null, // listener
StatusKind.STATUS_MASK_NONE);
In Ada:
string_writer := DDS.Builtin_String_DataWriter.Narrow(data_writer);
Note that in this particular code example, you will not find any reference to the Publisher class. In fact, cre-
ating the Publisher object explicitly is optional, because many applications do not have the need to cus-
tomize any behavior at that level. If you choose not to create a Publisher, Connext DDS will implicitly
choose an internal Publisher object. If you do want to create a Publisher explicitly, create it with a call to
participant.create_publisher() (you can find more about this method in the API Reference HTML doc-
umentation) and then simply replace the call to participant.create_datawriter() with a call to pub-
lisher.create_datawriter().
4.3.4 Subscribers and DataReaders
A DataReader is the point through which a subscribing application accesses the data that it has received
over the network.
A DataReader is analogous to a JMS TopicSubscriber. A Subscriber is analogous to the consuming
aspect of a JMS TopicSession.
Just as Publishers are used to group DataWriters, Subscribers are used to group DataReaders. Again, this
allows you to configure a default set of QoS parameters and event handling routines that will apply to all
DataReaders in the Subscriber's group.
4.3.4 Subscribers and DataReaders
Figure 4.4: Entities Associated with Subscriptions
Each DataReader is tied to a single topic. A DataReader will only receive data that was published on its
Topic.
To create a DataReader:
In Traditional C++:
data_reader = participant->create_datareader(
topic,
DDS_DATAREADER_QOS_DEFAULT, /* QoS */
&listener, /* Listener */
DDS_DATA_AVAILABLE_STATUS);
32
4.3.4 Subscribers and DataReaders
33
In Modern C++:
// Without a listener:
dds::sub::DataReader<dds::core::StringTopicType>data_reader(
rti::sub::implicit_subscriber(participant), topic);
// With a listener
dds::sub::DataReader<dds::core::StringTopicType>data_reader(
rti::sub::implicit_subscriber(participant),
topic,
participant->default_datareader_qos(),
&listener,
dds::core::status::StatusMask::data_available());
In Java:
StringDataReader dataReader =
(StringDataReader) participant.create_datareader(
topic,
Subscriber.DATAREADER_QOS_DEFAULT, // QoS
new HelloSubscriber(), // Listener
StatusKind.DATA_AVAILABLE_STATUS);
In Ada:
dataReader := DDS. Builtin_String_DataReader.Narrow(
participant.Create_DataReader(
topic.As_TopicDescription,
DDS.Subscriber.DATAREADER_QOS_DEFAULT, -- QoS
readerListener'Unchecked_Access, -- Listener
DDS.DATA_AVAILABLE_STATUS));
Connext DDS provides multiple ways for you to access your data: you can receive it asynchronously in a
listener, you can block your own thread waiting for it to arrive using a helper object called a WaitSet, or
you can poll in a non-blocking fashion. This example uses the former mechanism, and you will see that it
passes a non-NULL listener to the create_datareader() method. The listener mask (DATA_
AVAILABLE_STATUS) indicates that the application is only interested in receiving notifications of
newly arriving data.
Let’s look at the callback implementation in the following example code.
In Traditional C++:
retcode = string_reader->take_next_sample(ptr_sample,info);
if (retcode == DDS_RETCODE_NO_DATA) {
/* No more samples */
break;
} else if (retcode != DDS_RETCODE_OK) {
cerr << "Unable to take data from data reader, error "
<< retcode << endl;
return;
}
4.3.4 Subscribers and DataReaders
In Modern C++:
dds::sub::LoanedSamples<dds::core::StringTopicType>samples = reader.take();
if (samples.length() == 0){
std::cout <<"No samples\n"
}
// In case of error an exception is thrown
In Ada:
begin
data_reader.Read_Next_Sample (ptr_sample, sample_info'Access);
if sample_info.Valid_Data then
Ada.Text_IO.Put_Line (DDS.To_Standard_String (ptr_sample));
end if;
exception
when DDS.NO_DATA =>
-- No more samples
exit;
when others =>
Self.receiving := FALSE;
exit;
end;
The take_next_sample() method retrieves a single DDS data sample (i.e., a message) from the
DataReader, one at a time without blocking. If it was able to retrieve a DDS sample, it will return DDS_
RETCODE_OK. If there was no data to take, it will return DDS_RETCODE_NO_DATA. Finally, if it
tried to take data but failed to do so because it encountered a problem, it will return DDS_RETCODE_
ERROR or another DDS_ReturnCode_t value (see the API Reference HTML documentation for a full
list of error codes).
Connext DDS can publish not only actual data to a Topic, but also meta-data indicating, for example, that
an object about which the DataReader has been receiving data no longer exists. In the latter case, the info
argument to take_next_sample() will have its valid_data flag set to false.
This simple example is interested only in DDS data samples, not meta-data, so it only processes “valid”
data.
In Modern C++ we are retrieving all the samples at once.
In Traditional C++:
if (info.valid_data) {
// Valid (this isn't just a lifecycle sample): print it
cout << ptr_sample << endl;
}
In Modern C++:
for (auto sample :samples) {
if (sample.info().valid()) {
std::cout << sample.data() <<std::endl;
}
}
34
4.3.5 Topics
35
In Java:
try {
String sample = stringReader.take_next_sample(info);
if (info.valid_data) {
System.out.println(sample);
}
} catch (RETCODE_NO_DATA noData) {
// No more data to read
break;
} catch (RETCODE_ERROR e) {
// An error occurred
e.printStackTrace();
}
In Ada:
if sample_info.Valid_Data then
-- Valid, print it
Ada.Text_IO.Put_Line (DDS.To_Standard_String (ptr_sample));
end if;
Note that in this particular code example, you will not find any reference to the Subscriber class. In fact, as
with Publishers, creating the Subscriber object explicitly is optional, because many applications do not
have the need to customize any behavior at that level. If you, like this example, choose not to create a Sub-
scriber, the middleware will implicitly choose an internal Subscriber object. If you do want to create a Sub-
scriber explicitly, create it with a call to participant.create_subscriber (you can find more about this
method in the API Reference HTML documentation) and then simply replace the call to par-
ticipant.create_datareader with a call to subscriber.create_datareader.
4.3.5 Topics
Topics provide the basic connection points between DataWriters and DataReaders. To communicate, the
Topic of a DataWriter on one node must match the Topic of a DataReader on any other node.
A Topic is comprised of a name and a DDS type. The name is a string that uniquely identifies the Topic
within a DDS domain. The DDS type is the structural definition of the data contained within the Topic;
this capability is described in Chapter 5 Capabilities and Performance on page39.
You can create a Topic with the following code:
In Traditional C++:
topic = participant->create_topic(
"Hello, World", /* Topic name*/
DDSStringTypeSupport::get_type_name(), /* Type name */
DDS_TOPIC_QOS_DEFAULT, /* Topic QoS */
NULL, /* Listener */
DDS_STATUS_MASK_NONE);
4.3.6 Keys and DDS Samples
In Modern C++:
dds::topic::Topic<dds::core::StringTopicType> topic(
participant, "Hello, World");
In Java:
Topic topic = participant.create_topic(
"Hello, World", // Topic name
StringTypeSupport.get_type_name(), // Type name
DomainParticipant.TOPIC_QOS_DEFAULT, // QoS
null, // Listener
StatusKind.STATUS_MASK_NONE);
In Ada:
topic := participant.Create_Topic(
DDS.To_DDS_Builtin_String ("Hello, World"), -- Topic name
DDS.Builtin_String_TypeSupport.Get_Type_Name, -- Type name
DDS.DomainParticipant.TOPIC_QOS_DEFAULT, -- Topic QoS
null, -- Listener
DDS.STATUS_MASK_NONE);
Besides the new Topic’s name and DDS type, an application specifies three things:
l A suggested set of QoS for DataReaders and DataWriters for this Topic.
l A listener and listener mask that indicate which events the application wishes to be notified of, if
any.
In this case, the Topic’s DDS type is a simple string, a type that is built into the middleware.
4.3.6 Keys and DDS Samples
The data values associated with a Topic can change over time. The different values of the Topic passed
between applications are called DDS samples. A DDS sample is analogous to a message in other publish-
subscribe middleware.
An application may use a single Topic to carry data about many objects. For example, a stock-trading
application may have a single topic, "Stock Price," that it uses to communicate information about Apple,
Google, Microsoft, and many other companies. Similarly, a radar track management application may have
a single topic, "Aircraft Position," that carries data about many different airplanes and other vehicles.
These objects within a Topic are called instances. For a specific DDS data type, you can select one or
more fields within the DDS data type to form a key. A key is used to uniquely identify one instance of a
Topic from another instance of the same Topic, very much like how the primary key in a database table
identifies one record or another. DDS samples of different instances have different values for the key.
DDS samples of the same instance of a Topic have the same key. Note that not all Topics have keys. For
Topics without keys, there is only a single instance of that Topic.
36
4.3.7 Requesters and Repliers
37
4.3.7 Requesters and Repliers
This section describes the Request-Reply communication pattern, which is only available with the
RTI Connext DDSProfessional, Evaluation, and Basic package types.
Requesters and Repliers provide a way to use the Request-Reply communication pattern on top of the pre-
viously described Connext DDS entities.
An application uses a Requester to send requests to a Replier; another application using a Replier receives
a request and can send one or more replies for that request. The Requester that sent the request (and only
that one) will receive the reply (or replies).
A Requester uses an existing DomainParticipant to communicate through a domain. It owns a DataWriter
for writing requests and a DataReader for receiving replies.
Similarly, a Replier uses an existing DomainParticipant to communicate through a domain and owns a
DataReader for receiving requests and a DataWriter for writing replies.
Figure 4.5: Request-Reply Overview
4.3.7 Requesters and Repliers
The Reply Topic filters samples so replies are received by exactly one Requester—the one that wrote the
related request sample.
You can specify the QoS for the DataWriters and DataReaders that Requesters and Repliers create.
The following code shows how to create a Requester:
In C++:
Requester<Foo, Bar> * requester =
new Requester<Foo, Bar>(participant, "MyService");
In Java:
Requester<Foo, Bar> requester = new Requester<Foo, Bar>(
participant, "MyService",
FooTypeSupport.get_instance(),
BarTypeSupport.get_instance());
As you can see, we are passing an existing DomainParticipant to the constructor.
Foo is the request type and Bar is the reply type. In 5.3 Compact, Type-Safe Data Programming with
DDSData Types on page45, you will learn what types you can use and how to create them.
The constructor also receives a string "MyService." This is the service name, and is used to create the
Request Topic and the Reply Topic. In this example, the Requester will create a Request Topic called
"MyServiceRequest" and a Reply Topic called "MyServiceReply."
Creating a Replier is very similar. The following code shows how to create a Replier:
In C++:
Replier<Foo, Bar> * replier =
new Replier<Foo, Bar>(participant, "MyService");
In Java:
Replier<Foo, Bar> replier = new Replier<Foo, Bar>(
participant, "MyService",
FooTypeSupport.get_instance(),
BarTypeSupport.get_instance());
This Replier will communicate with the Requester we created before, because they use the same service
name (hence the topics are the same) and they use compatible QoS (the default).
More example code is available for C++, C, Java and C# as part of the API Reference HTML doc-
umentation, under Modules, Programming How-To’s, Request-Reply Examples.
38
Chapter 5 Capabilities and Performance
In AQuick Overview (Chapter 4 on page14), you learned the basic concepts in Connext DDS
and applied them to a simple "Hello, World" application. In this section, you will learn more about
some of the powerful and unique benefits of Connext DDS:
l A rich set of functionality, implemented for you by the middleware so that you don't have to
build it into your application. Most of this functionality—including sophisticated data fil-
tering and expiration, support for durable historical data, and built-in support for periodic
data and deadline enforcement—can be defined partially or even completely in declarative
quality-of-service (QoS) policies specified in an XML file, allowing you to examine and
update your application's configuration without rebuilding or redeploying it. See 5.2 Cus-
tomizing Behavior: QoS Configuration on page42 for more information about how to con-
figure QoS policies. Design Patterns for High Performance (Chapter 7 on page80) describes
how to reduce, filter, and cache data as well as other common functional design patterns.
l Compact, type-safe data. The unique and expressive data-typing system built into Connext
DDS supports not only opaque payloads but also highly structured data. It provides both
static and dynamic type safety—without the network overhead of the "self-describing" mes-
sages of other networking middleware implementations. See 5.3 Compact, Type-Safe Data
Programming with DDSData Types on page45 for more information.
l Industry-leading performance. Connext DDS provides industry-leading latency, throughput,
and jitter performance. Design Patterns for High Performance (Chapter 7 on page80)
provides specific QoS configuration examples to help you get started. You can quickly see
the effects of the changes you make using the code examples described in that section. You
can benchmark the performance of Connext DDS on your own systems with the RTI
Example Performance Test. You can download the Example Performance Test from
http://www.rti.com/examples/.
l You can also review the data from several performance benchmarks here: http://www.rti.-
com/products/dds/benchmarks.html. Updated results for new releases are typically published
within two months after general availability of that release.
39
5.1 Automatic Application Discovery
40
5.1 Automatic Application Discovery
As you’ve been running the code example described in this guide, you may have noticed that you have not
had to start any server processes or configure any network addresses. Its built-in automatic discovery cap-
ability is one important way in which Connext DDS differs from other networking middleware imple-
mentations. It is designed to be low-overhead and require minimal configuration, so in many cases there is
nothing you need to do; it will just work. Nevertheless, it’s helpful to understand the basics so that you can
decide if and when a custom configuration is necessary.
Before applications can communicate, they need to “discover” each other. By default, Connext DDS
applications discover each other using shared memory or UDP loopback if they are on the same host or
using multicast
a
if they are on different hosts. Therefore, to run an application on two or more computers
using multicast, or on a single computer with a network connection, no changes are needed. They will dis-
cover each other automatically! The section on Discovery in the RTIConnext DDS Core Libraries User's
Manual describes the process in more detail.
If you want to use computers that do not support multicast (or you need to use unicast for some other
reason), or if you want to run on a single computer that does not have a network connection (in which case
your operating system may have disabled your network stack), there is a simple way to control the dis-
covery process—you won’t even have to recompile. Application discovery can be configured through the
NDDS_DISCOVERY_PEERS environment variable or in your QoS configuration file.
5.1.1 When to Set the Discovery Peers
There are only a few situations in which you must set the discovery peers:
(In the following, replace N with the number of Connext DDS applications you want to run.)
1. If you cannot use multicast
b
:
Set your discovery peers to a list of all of the hosts that need to discover each other. The list can con-
tain host names and/or IP addresses; each entry should be of the form N@built-
in.udpv4://<hostname|IP>.
2. If you do not have a network connection:
Some operating systems—for example, Microsoft Windows—disable some functionality of their net-
work stack when they detect that no network interfaces are available. This can cause problems when
applications try to open network connections.
a
With the exception of LynxOS. On LynxOS systems, multicast is not used for discovery by default
unless NDDS_DISCOVERY_PEERS is set.
b
To see if your platform supports multicast, see the RTIConnext DDS Core Libraries Platform Notes.
5.1.2 How to Set Your Discovery Peers
If your system supports shared memory
a
, set your discovery peers to N@builtin.shmem://. This will
enable the shared memory transport only.
If your system does not support shared memory (or it is disabled), set your discovery peers to the loop-
back address, N@builtin.udpv4://127.0.0.1.
5.1.2 How to Set Your Discovery Peers
As stated above, in most cases you do not need to set your discovery peers explicitly.
If setting them is required, there are two easy ways to do so:
l Set the NDDS_DISCOVERY_PEERS environment variable to a comma-separated list of the
names or addresses of all the hosts that need to discover each other.
l
Example on Windows systems:
set NDDS_DISCOVERY_PEERS=3@builtin.udpv4://mypeerhost1,
4@builtin.udpv4://mypeerhost2
l
Example on UNIX-based systems when using csh or tcsh:
setenv NDDS_DISCOVERY_PEERS 3@builtin.udpv4://mypeerhost1,
4@builtin.udpv4://mypeerhost2
l Set the discovery peer list in your XML QoS configuration file.
a
To see if your platform supports RTI’s shared memory transport, see the RTIConnext DDS Core Librar-
ies Platform Notes.
41
5.2 Customizing Behavior: QoS Configuration
42
For example, to turn on shared memory only:
<participant_qos>
<discovery>
<!--
The initial_peers list are those "addresses" to which the
middleware will send discovery announcements.
-->
<initial_peers>
<element>4@builtin.shmem://</element>
</initial_peers>
<!--
The multicast_receive_addresses list identifies where the
DomainParticipant listens for multicast announcements
from others. Set this list to an empty value to disable
listening over multicast.
-->
<multicast_receive_addresses>
<!-- empty -->
</multicast_receive_addresses>
</discovery>
<transport_builtin>
<!--
The transport_builtin mask identifies which builtin
transports the DomainParticipant uses. The default value
is UDPv4 | SHMEM, so set this mask to SHMEM to prevent
other nodes from initiating communication with this node
via UDPv4.
-->
<mask>SHMEM</mask>
</transport_builtin>
...
</participant_qos>
For more information, please see the Connext DDS Platform Notes, User’s Manual, and API Reference
HTML documentation (from the main page, select Modules, Infrastructure Module, QoS Policies,
DISCOVERY).
5.2 Customizing Behavior: QoS Configuration
Almost every object in the Connext DDS API is associated with QoS policies that govern its behavior.
These policies govern everything from the amount of memory the object may use to store incoming or out-
going data, to the degree of reliability required, to the amount of meta-traffic that Connext DDS will send
on the network, and many others. The following is a short summary of just a few of the available policies:
l Reliability and Availability for Consistent Behavior with Critical Data:
l Reliability: Specifies whether or not the middleware will deliver data reliably. The reliability
of a connection between a DataWriter and DataReader is entirely user configurable. It can be
done on a per DataWriter-DataReader connection.
5.2 Customizing Behavior: QoS Configuration
For some use cases, such as the periodic update of sensor values to a GUI displaying the
value to a person, best-effort delivery is often good enough. It is the fastest, most efficient, and
least resource-intensive (CPU and network bandwidth) method of getting the newest/latest
value for a topic from DataWriters to DataReaders. But there is no guarantee that the data
sent will be received. It may be lost due to a variety of factors, including data loss by the phys-
ical transport such as wireless RF or Ethernet.
However, there are data streams (topics) in which you want an absolute guarantee that all data
sent by a DataWriter is received reliably by DataReaders. This means that the middleware
must check whether or not data was received, and repair any data that was lost by resending a
copy of the data as many times as it takes for the DataReader to receive the data.
l History: Specifies how much data must be stored by the middleware for the DataWriter or
DataReader. This QoS policy affects the Reliability and Durability QoS policies.
When a DataWriter sends data or a DataReader receives data, the data sent or received is
stored in a cache whose contents are controlled by the History QosPolicy. The History
QosPolicy can tell the middleware to store all of the data that was sent or received, or only
store the last n values sent or received. If the History QosPolicy is set to keep the last n values
only, then after n values have been sent or received, any new data will overwrite the oldest
data in the queue. The queue thus acts like a circular buffer of length n.
This QoS policy interacts with the Reliability QosPolicy by controlling whether or not the
middleware guarantees that (a) all of the data sent is received (using the KEEP_ALL setting
of the History QosPolicy) or (b) that only the last n data values sent are received (a reduced
level of reliability, using the KEEP_LAST setting of the History QosPolicy). See the Reli-
ability QosPolicy for more information.
Also, the amount of data sent to new DataReaders whose Durability QosPolicy (see below)
is set to receive previously published data is controlled by the History QosPolicy.
l Lifespan: Specifies how long the middleware should consider data sent by a user application
to be valid.
The middleware attaches timestamps to all data sent and received. When you specify a finite
Lifespan for your data, the middleware will compare the current time with those timestamps
and drop data when your specified Lifespan expires. You can use the Lifespan QosPolicy to
ensure that applications do not receive or act on data, commands or messages that are too old
and have "expired."
l Durability: Specifies whether or not the middleware will store and deliver previously pub-
lished data to new DataReaders. This policy helps ensure that DataReaders get all data that
was sent by DataWriters, even if it was sent while the DataReader was disconnected from
the network. It can increase a system's tolerance to failure conditions.
43
5.2 Customizing Behavior: QoS Configuration
44
l Fault Tolerance for increased robustness and reduced risk:
l Liveliness: Specifies and configures the mechanism that allows DataReaders to detect when
DataWriters become disconnected or "dead." It can be used during system integration to
ensure that systems meet their intended responsiveness specifications. It can also be used dur-
ing run time to detect possible losses of connectivity.
l Ownership and Ownership Strength: Along with Ownership Strength, Ownership spe-
cifies if a DataReader can receive data of a given instance from multiple DataWriters at the
same time. By default, DataReaders for a given topic can receive data of all instances from
any DataWriter for the same topic. But you can also configure a DataReader to receive data
of a given instance from only one DataWriter at a time. The DataWriter with the highest
Ownership Strength value will be the owner of the instance and the one whose data is
delivered to DataReaders. Data of that instance sent by all other DataWriters with lower
Ownership Strength will be dropped by the middleware.
When the DataWriter with the highest Ownership strength loses its liveliness (as controlled
by the Liveliness QosPolicy) or misses a deadline (as controlled by the Deadline QosPolicy)
or whose application quits, dies, or otherwise disconnects, the middleware will change own-
ership of the topic to the DataWriter with the highest Ownership Strength from the remaining
DataWriters. This QoS policy can help you build systems that have redundant elements to
safeguard against component or application failures. When systems have active and hot
standby components, the Ownership QosPolicy can be used to ensure that data from standby
applications are only delivered in the case of the failure of the primary.
l Built-in Support for Periodic Data:
l Deadline: For a DataReader, this QoS specifies the maximum expected elapsed time
between arriving DDS data samples. For a DataWriter, it specifies a commitment to publish
DDS samples with no greater than this elapsed time between them.
This policy can be used during system integration to ensure that applications have been coded
to meet design specifications. It can be used during run time to detect when systems are per-
forming outside of design specifications. Receiving applications can take appropriate actions
to prevent total system failure when data is not received in time. For topics on which data is
not expected to be periodic, the deadline period should be set to an infinite value.
You can specify an object's QoS two ways: (a) programmatically, in your application's source code or (b)
in an XML configuration file. The same parameters are available, regardless of which way you choose.
For complete information about all of the policies available, see Chapter 4 in the RTIConnext DDS Core
Libraries User's Manual or see the API Reference HTML documentation.
The examples covered in this document are intended to be configured with XML files.
By default, examples are copied into your home directory the first time you run RTI Launcher or any script
in <NDDSHOME>/bin. This document refers to the location of the copied examples as <path to
examples>.
5.3 Compact, Type-Safe Data Programming with DDSData Types
Wherever you see <path to examples>, replace it with the appropriate path.
Default path to the examples:
l macOS systems: /Users/<your user name>/rti_workspace/6.0.1/examples
l Linux systems: /home/<your user name>/rti_workspace/6.0.1/examples
l Windows systems: <your Windows documents folder>\rti_workspace\6.0.1\examples
Where 'your Windows documents folder' depends on your version of Windows. For example, on
Windows 10, the folder is C:\Users\<your user name>\Documents.
Note: You can specify a different location for rti_workspace. You can also specify that you do not want
the examples copied to the workspace. For details, see Controlling Location for RTIWorkspace and Copy-
ing of Examples in the RTIConnext DDS Installation Guide.
You can find several example configurations, called profiles, in the directory examples/connext_dds/qos.
The easiest way to use one of these profile files is to either set the environment variable NDDS_QOS_
PROFILES to the path of the file you want, or copy that file into your current working directory with the
file name USER_QOS_PROFILES.xml before running your application.
5.3 Compact, Type-Safe Data Programming with DDSData Types
How data is stored or laid out in memory can vary from language to language, compiler to compiler, oper-
ating system to operating system, and processor to processor. This combination of lan-
guage/compiler/operating system/processor is called a platform. Any modern middleware must be able to
take data from one specific platform (say C/gcc 3.2.2/Solaris/Sparc) and transparently deliver it to another
(for example, Java/JDK 1.6/Windows/Pentium). This process is commonly called seri-
alization/deserialization or marshalling/demarshalling. Messaging products have typically taken one of two
approaches to this problem:
l Do nothing. With this approach, the middleware does not provide any help and user code must take
into account memory-layout differences when sending messages from one platform to another. The
middleware treats messages as an opaque buffer of bytes. The JMS BytesMessage is an example of
this approach.
l Send everything, every time. Self-describing messages are at the opposite extreme, and embed full
reflective information, including data types and field names, with each message. The JMS MapMes-
sage and the messages in TIBCO Rendezvous are examples of this approach.
The “do nothing” approach is lightweight on its surface but forces you, the user of the middleware API, to
consider all data encoding, alignment, and padding issues. The “send everything” alternative results in
large amounts of redundant information being sent with every packet, impacting performance.
45
5.3 Compact, Type-Safe Data Programming with DDSData Types
46
Figure 5.1: Self-Describing Messages vs. Type Definitions
Connext DDS exchanges data type definitions, such as field names and types, once at application start-up time. This
increases performance and reduces bandwidth consumption compared to the conventional approach, in which each mes-
sage is self-describing and thus includes a substantial amount of meta-data along with the actual data.
Connext DDS takes an intermediate approach. Just as objects in your application program belong to some
data type, DDS data samples sent on the same Topic share a DDS data type. This DDS type defines the
fields that exist in the DDS data samples and what their constituent types are; users in the aerospace and
defense industries will recognize such type definitions as a form of Interface Definition Document (IDD).
Connext DDS stores and propagates this meta-information separately from the individual DDS data
samples, allowing it to propagate DDS samples efficiently while handling byte ordering and alignment
issues for you.
5.3 Compact, Type-Safe Data Programming with DDSData Types
Figure 5.2: Example IDD
This example IDD shows one legacy approach to type definition. RTI supports multiple standard type definition formats
that are machine readable as well as human readable.
With RTI, you have a number of choices when it comes to defining and using DDS data types. You can
choose one of these options, or you can mix and match them—these approaches interoperate with each
other and across programming languages and platforms. So, your options are:
l Use the built-in DDS types. If a message is simply a string or a buffer of bytes, you can use RTI's
built-in DDS types, described in 5.3.1 Using Built-in DDS Types on the next page.
l Define a DDS type at compile-time using a language-independent description language and RTI
Code Generator, rtiddsgen, as described in 5.3.2 Using DDS Types Defined at Compile Time on
the next page. This approach offers the strongest compile-time type safety.
Whereas in-house middleware implementation teams often define data formats in word processing
or spreadsheet documents and translate those formats into application code by hand, RTI relies on
standard type definition formats that are both human- and machine-readable and generates code in
compliance with open international standards. The code generator accepts data-type definitions in a
number of formats to make it easy to integrate Connext DDS with your development processes and
IT infrastructure:
l OMG IDL. This format is a standard component of both the DDS and CORBA spe-
cifications. It describes data types with a C++-like syntax. This format is described in the
RTIConnext DDS Core Libraries User's Manual.
l XML schema (XSD), whether independent or embedded in a WSDL file. XSD may be the
format of choice for those using Connext DDS alongside or connected to a web services infra-
structure. This format is described in RTIConnext DDS Core Libraries User's Manual.
l XML in a DDS-specific format. This XML format is terser, and therefore easier to read and
write by hand, than an XSD file. It offers the general benefits of XML—extensibility and ease
of integration—while fully supporting DDS-specific data types and concepts. This format is
described in the RTIConnext DDS Core Libraries User's Manual.
47
5.3.1 Using Built-in DDS Types
48
l Define a dynamic type programmatically at run time.
a
This method may be appropriate for
applications with dynamic data description needs: applications for which types change frequently or
cannot be known ahead of time. It allows you to use an API similar to those of Tibco Rendezvous
or JMS MapMessage to manipulate messages without sacrificing efficiency. It is described in 5.3.4
Running with Dynamic DDS Types on page56.
The following sections of this document describe each of these models.
5.3.1 Using Built-in DDS Types
Connext DDS provides a set of standard types that are built into the middleware. These DDS types can be
used immediately. The supported built-in DDS types are String, KeyedString, Octets, and KeyedOctets.
(On Java and .NET platforms, the latter two types are called Bytes and KeyedBytes, respectively; the
names are different but the data is compatible across languages.) String and KeyedStrings can be used to
send variable-length strings of single-byte characters. Octets and KeyedOctets can be used to send vari-
able-length arrays of bytes.
These built-in DDS types may be sufficient if your data-type needs are simple. If your data is more com-
plex and highly structured, or you want Connext DDS to examine fields within the data for filtering or
other purposes, this option may not be appropriate, and you will need to take additional steps to use com-
pile-time types (see 5.3.2 Using DDS Types Defined at Compile Time below) or dynamic types (see
5.3.4 Running with Dynamic DDS Types on page56).
5.3.2 Using DDS Types Defined at Compile Time
In this section, we define a DDS type at compile time using a language-independent description and RTI
Code Generator (rtiddsgen).
RTI Code Generator accepts data-type definitions in a number of formats, such as OMG IDL, XML
Schema (XSD), and a DDS-specific format of XML. This makes it easy to integrate Connext DDS with
your development processes and IT infrastructure. In this section, we will define DDS types using IDL.
(In case you would like to experiment with a different format, rtiddsgen can convert from any supported
format to any other: simply pass the arguments -convertToIdl, -convertToXml, -convertToXsd, or -con-
vertToWsdl.)
As described in the Release Notes, some platforms are supported as both a host and a target, while others
are only supported as a target. The rtiddsgen tool must be run on a computer that is supported as a host.
For target-only platforms, you will need to run rtiddsgen and build the application on a separate host com-
puter.
The following sections will take your through the process of generating example code from your own data
type.
a
Dynamic types are not supported when using the separate add-on product, RTI Ada Language Support.
5.3.3 Generating Code with RTI Code Generator
5.3.3 Generating Code with RTI Code Generator
Don't worry about how DDS types are defined for now. For this example, just copy and paste the fol-
lowing into a new file, HelloWorld.idl.
const long HELLO_MAX_STRING_SIZE = 256;
struct HelloWorld {
string<HELLO_MAX_STRING_SIZE> message;
};
Next, we will invoke RTI Code Generator (rtiddsgen), which can be found in the $NDDSHOME/bin dir-
ectory that should already be on your path, to create definitions of your data type in a target programming
language, including logic to serialize and deserialize instances of that type for transmission over the net-
work. Then we will build and run the generated code.
For a complete list of the arguments rtiddsgen understands, and a brief description of each of them, run it
with the -help argument. More information about rtiddsgen, including its command-line parameters and
the list of files it creates, can be found in the RTIConnext DDS Core Libraries User's Manual.
Note:Running rtiddsgen on a Red Hat Enterprise Linux 4.0 target platform is not supported
because it uses an older JRE. You can, however, run rtiddsgen on a newer Linux platform to
generate code that can be used on the Red Hat Enterprise Linux 4.0 target.
Instructions for Traditional C++
Generate C++ code from your IDL file with the following command (replace the architecture
namei86Linux3gcc4.8.2 with the name of your own architecture):
> rtiddsgen -ppDisable \
-language C++ \
-example i86Linux3gcc4.8.2 \
-replace \
HelloWorld.idl
The generated code publishes identical DDS data samples and subscribes to them, printing the received
data to the terminal. Edit the code to modify each DDS data sample before it's published: Open Hel-
loWorld_publisher.cxx. In the code for publisher_main(), locate the "for" loop and add the bold line
seen below, which puts "Hello World!" and a consecutive number in each DDS sample that is sent.
a
a
If you are using Visual Studio, consider using sprintf_s instead of sprintf: sprintf_s(instance->msg, 128, "Hello World!
(%d)", count);
49
5.3.3 Generating Code with RTI Code Generator
50
for (count=0; (sample_count == 0) || (count < sample_count); ++count) {
printf("Writing HelloWorld, count %d\n", count);
/* Modify the data to be sent here */
sprintf(instance->message, "Hello World! (%d)", count);
retcode = HelloWorld_writer->write(*instance, instance_handle);
if (retcode != DDS_RETCODE_OK) {
printf("write error %d\n", retcode);
}
NDDSUtility::sleep(send_period);
}
Instructions for Modern C++
Generate C++03 or C++11 code from your IDL file with the following command (replace the architecture
name i86Linux3gcc4.8.2 with the name of your own architecture):
> rtiddsgen -ppDisable \
-language C++03 \
-example i86Linux3gcc4.8.2 \
-replace \
HelloWorld.idl
Or:
> rtiddsgen -ppDisable \
-language C++11 \
-example i86Linux3gcc4.8.2 \
-replace \
HelloWorld.idl
The only differences between using C++03 or C++11 is that the latter creates a different subscriber
example and adds a flag to enable C++11 in supported compilers that require explicit activation. This also
activates C++11 features in the DDSAPI headers. See the API Reference HTMLdocumentation (Mod-
ules >Conventions) for more information.
The generated code publishes identical DDS data samples and subscribes to them, printing the received
data to the terminal. Edit the code to modify each DDS data sample before it's published: Open Hel-
loWorld_publisher.cxx. In the code for publisher_main(), locate the "for" loop and add the bold line
seen below, which puts "Hello World!" and a consecutive number in each DDS sample that is sent.
a
a
If you are using Visual Studio, consider using sprintf_s instead of sprintf: sprintf_s(instance->msg, 128,
"Hello World! (%d)", count);
5.3.3 Generating Code with RTI Code Generator
for (int count = 0; count < sample_count || sample_count == 0; count++) {
// Modify the data to be written here
sample.message("Hello World! ("+ std::to_string(count) +")");
std::cout << "Writing MyOtherType, count " << count << "\n";
writer.write(sample);
rti::util::sleep(dds::core::Duration(4));
}
Instructions for Java
Generate Java code from your IDL file with the following command
a
(replace the architecture name
i86Linux3gcc4.8.2 with the name of your own architecture):
> rtiddsgen -ppDisable \
-language Java \
-example i86Linux3gcc4.8.2 \
-replace \
HelloWorld.idl
The generated code publishes identical DDS data samples and subscribes to them, printing the received
data to the terminal. Edit the code to modify each DDS data sample before it's published: Open Hel-
loWorldPublisher.java. In the code for publisherMain(), locate the "for" loop and add the bold line seen
below, which puts "Hello World!" and a consecutive number in each DDS sample that is sent.
for
(int count = 0; sampleCount == 0) || (count < sampleCount); ++count) {
System.out.println("Writing HelloWorld, count " + count);
/* Modify the instance to be written here */
instance.msg = "Hello World! (" + count + ")";
/* Write data */
writer.write(instance, InstanceHandle_t.HANDLE_NIL);
try {
Thread.sleep(sendPeriodMillis);
} catch (InterruptedException ix) {
System.err.println("INTERRUPTED");
break;
}
}
Instructions for Ada
Generate Ada code from your IDL file with the following command (replace the architecture name
x64Linux2.6gcc4.4.5 with the name of your own architecture):
a
The argument -ppDisable tells the code generator not to attempt to invoke the C preprocessor (usually cpp
on UNIX systems and cl on Windows systems) on the IDL file prior to generating code. In this case, the
IDL file contains no preprocessor directives, so no preprocessing is necessary. However, if the pre-
processor executable is already on your system path (on Windows systems, running the Visual Studio
script vcvars32.bat will do this for you) you can omit this argument.
51
5.3.3.1 Building the Generated Code
52
rtiddsgen -ppDisable -language Ada \
-example x64Linux2.6gcc4.4.5 -replace HelloWorld.idl
Notes:
l Ada support requires a separate add-on product, Ada Language Support.
l The generated publisher and subscriber source files are under the samples directory. There are two
generated project files: one at the top level and one in the samples directory. The project file in the
samples directory, samples/helloworld-samples.gpr, should be the one that you will use to compile
the example.
l The generated Ada project files need two directories to compile a project: .obj and bin. If your Ada
IDE does not automatically create these directories, you will need to create them outside the Ada
IDE, in both the top-level directory and in samples directory.
for the “Count in 0 .. Sample_Count “loop
Put_Line ("Writing HelloWorld, count " & Count'Img);
declare
Msg : DDS.String := DDS.To_DDS_String
("Hello World! (" & Count'Img & ")");
begin
if Instance.message /= DDS.NULL_STRING then
Finalize (Instance.message);
end if;
Standard.DDS.Copy(Instance.message, Msg);
end;
HelloWorld_Writer.Write (
Instance_Data => Instance,
Handle => Instance_Handle'Unchecked_Access);
delay Send_Period;
end loop;
5.3.3.1 Building the Generated Code
You have now defined your data type, generated code for it, and customized that code. It's time to compile
the example applications.
Building a Generated C, C++, or .NET Example on Windows Systems
With the NDDSHOME environment variable set, start Visual Studio and open the rtiddsgen-generated
solution file (.sln). Select the Release configuration in the Build toolbar in Visual Studio
a
. From the Build
menu, select Build Solution. This will build two projects: <IDL name>_publisher and <IDL name>_
subscriber.
Building a Generated Ada Example on a Linux System
Ada support requires a separate add-on product, Ada Language Support.
a
The Connext DDS .NET language binding is currently supported for C# and C++/CLI.
5.3.3.1 Building the Generated Code
Use the generated Ada project file to compile an example on any system.
Note: The generated project file assumes the correct version of the compiler is already on your path,
NDDSHOME is set, and $NDDSHOME/lib/gnat is in your ADA_PROJECT_PATH.
gmake -f makefile_HelloWorld_<architecture>
After compiling the Ada example, you will find the application executables in the directory, samples/bin.
The build command in the generated makefile uses the static release versions of the Connext DDS. To
select dynamic or debug versions of the libraries, change the Ada compiler variables RTIDDS_
LIBRARY_TYPE and RTIDDS_BUILD in the build command in the makefile to build with the desired
version of the libraries. For example, if the application must be compiled with the relocatable debug ver-
sion of the libraries, compile with the following command:
gprbuild -p -P samples/helloworld-samples.gpr -XOS=Linux \
-XRTIDDS_LIBRARY_TYPE=relocatable -XRTIDDS_BUILD=debug -XARCH=${ARCH}
Building a Generated Example on Other Platforms
Use the generated makefile to compile a C or C++ example on a UNIX-based system or a Java example
on any system. Note: The generated makefile assumes the correct version of the compiler is already on
your path and that NDDSHOME is set. If you do not have gmake on your system, you can copy the lines
from the generated makefile to compile your example.
gmake -f makefile_HelloWorld_<architecture>
After compiling the C or C++ example, you will find the application executables in a directory objs/<ar-
chitecture>.
The generated makefile includes the static release versions of the Connext DDS. To select dynamic or
debug versions of the libraries, edit the makefile to change the library suffixes. Generally, Connext DDS
uses the following convention for library suffixes: "z" for static release, "zd" for static debug, none for
dynamic release, and "d" for dynamic debug. For a complete list of the required libraries for each con-
figuration, see the Connext DDS Platform Notes.
For example, to change a C++ makefile from using static release to dynamic release libraries, change this
directive:
LIBS = -L$(NDDSHOME)/lib/<architecture> \
-lnddscppz -lnddscz -lnddscorez $(syslibs_<architecture>)
to:
53
5.3.3.2 Running the Example Applications
54
LIBS = -L$(NDDSHOME)/lib/<architecture> \
-lnddscpp -lnddsc -lnddscore $(syslibs_<architecture>)
5.3.3.2 Running the Example Applications
Run the example publishing and subscribing applications and see them communicate:
Running the Generated C++ Example
First, start the subscriber application, HelloWorld_subscriber:
./objs/<architecture>/HelloWorld_subscriber
In this command window, you should see that the subscriber wakes up every four seconds to print a mes-
sage:
HelloWorld subscriber sleeping for 4 sec...
HelloWorld subscriber sleeping for 4 sec...
HelloWorld subscriber sleeping for 4 sec...
Next, open another command prompt window and start the publisher application, HelloWorld_publisher.
For example:
./objs/<architecture>/HelloWorld_publisher
In this second (publishing) command window, you should see:
Writing HelloWorld, count 0 Writing HelloWorld, count 1 Writing HelloWorld, count 2
Look back in the first (subscribing) command window. You should see that the subscriber is now receiv-
ing messages from the publisher:
HelloWorld subscriber sleeping for 4 sec...
msg: “Hello World! {0}“
HelloWorld subscriber sleeping for 4 sec...
msg: “Hello World! {1}“
HelloWorld subscriber sleeping for 4 sec...
msg: “Hello World! {2}“
Running the Generated Java Example
You can run the generated applications using the generated makefile. On most platforms, the generated
makefile assumes the correct version of java is already on your path and that the NDDSHOME envir-
onment variable is set
a
.
First, run the subscriber:
a
One exception on LynxOS systems; see Getting Started on Embedded UNIX-like Systems in the Getting Started Guide,
Addendum for Embedded Platforms.
5.3.3.2 Running the Example Applications
gmake -f makefile_HelloWorld_<architecture> HelloWorldSubscriber
In this command window, you should see that the subscriber wakes up every four seconds to print a mes-
sage:
HelloWorld subscriber sleeping for 4 sec...
HelloWorld subscriber sleeping for 4 sec...
HelloWorld subscriber sleeping for 4 sec...
Next, run the publisher:
gmake -f makefile_HelloWorld_<architecture> HelloWorldPublisher
In this second (publishing) command window, you should see:
Writing HelloWorld, count 0
Writing HelloWorld, count 1
Writing HelloWorld, count 2
Look back in the first (subscribing) command window. You should see that the subscriber is now receiv-
ing messages from the publisher:
HelloWorld subscriber sleeping for 4 sec...
msg: “Hello World! {0}“
HelloWorld subscriber sleeping for 4 sec...
msg: “Hello World! {1}“
HelloWorld subscriber sleeping for 4 sec...
msg: “Hello World! {2}“
Running the Generated Ada Example
Ada support requires a separate add-on product, Ada Language Support.
First, start the subscriber application, helloworld_idl_file-helloworld_subscriber:
./samples/bin/helloworld_idl_file-helloworld_subscriber
In this command window, you should see that the subscriber wakes up every four seconds to print a mes-
sage:
HelloWorld subscriber sleeping for 4.000000000 sec.
HelloWorld subscriber sleeping for 4.000000000 sec.
HelloWorld subscriber sleeping for 4.000000000 sec.
Next, open another command prompt window and start the publisher application, helloworld_idl_file-hel-
loworld_publisher. For example:
./samples/bin/helloworld_idl_file-helloworld_publisher
55
5.3.4 Running with Dynamic DDS Types
56
In this second (publishing) command window, you should see:
Writing HelloWorld, count 0
Writing HelloWorld, count 1
Writing HelloWorld, count 2
Look back in the first (subscribing) command window. You should see that the subscriber is now receiv-
ing messages from the publisher:
HelloWorld subscriber sleeping for 4 sec...
msg: “Hello World! {0}“
HelloWorld subscriber sleeping for 4 sec...
msg: “Hello World! {1}“
HelloWorld subscriber sleeping for 4 sec...
msg: “Hello World! {2}“
5.3.4 Running with Dynamic DDS Types
Dynamic DDS types are not supported when using Ada Language Support
This method may be appropriate for applications for which the structure (type) of messages changes fre-
quently or for deployed systems in which newer versions of applications need to interoperate with existing
applications that cannot be recompiled to incorporate message-type changes.
As your system evolves, you may find that your data types need to change. And unless your system is rel-
atively small, you may not be able to bring it all down at once in order to modify them. Instead, you may
need to upgrade your types one component at a time-or even on the fly, without bringing any part of the
system down.
While covering dynamic DDS types is outside the scope of this section, you can learn more about the sub-
ject in Chapter 3 of the RTIConnext DDS Core Libraries User's Manual. You can also view and run the
Hello World example code located in examples/connext_dds/<language>/Hello_dynamic/src.
For the examples in Modern C++, see the Modern C++API reference, Modules >Programming How-
To's > DynamicType and DynamicData Use Cases.
Chapter 6 Design Patterns for Rapid
Development
In this section, you will learn how to implement some common functional design patterns. As you
have learned, one of the advantages to using Connext DDS is that you can achieve significantly dif-
ferent functionality without changing your application code simply by updating the XML-based
Quality of Service (QoS) parameters.
In this section, we will look at a simple newspaper example to illustrate these design patterns.
Newspaper distribution has long been a canonical example of publish-subscribe communication,
because it provides a simple metaphor for real-world problems in a variety of industries.
A radar tracking system and a market data distribution system share many features with the news sub-
scriptions we are all familiar with: many-to-many publish-subscribe communication with certain quality-of-ser-
vice requirements.
In a newspaper scenario (provided in an example called news example for all languages, a news
publishing application distributes articles from a variety of news outlets—CNN, Bloomberg, etc.—
57
6.1 Building and Running the News Examples
58
on a periodic basis. However, the period differs from outlet to outlet. One or more news subscribers poll
for available articles, also on a periodic basis, and print out their contents. Once published, articles remain
available for a period of time, during which subscribing applications can repeatedly access them if they
wish. After that time has elapsed, the middleware will automatically expire them from its internal cache.
This section describes 6.1 Building and Running the News Examples below and includes the following
design patterns:
l 6.2 Subscribing Only to Relevant Data on page60
l 6.3 Accessing Historical Data when Joining the Network on page69
l 6.4 Caching Data within the Middleware on page71
l 6.5 Receiving Notifications When Data Delivery Is Late on page75
You can find more examples at http://www.rti.com/examples. This page contains example code snippets
on how to use individual features, examples illustrating specific use cases, as well as performance test
examples.
6.1 Building and Running the News Examples
By default, examples are copied into your home directory the first time you run RTI Launcher or any script
in <NDDSHOME>/bin. This document refers to the location of the copied examples as <path to
examples>.
Wherever you see <path to examples>, replace it with the appropriate path.
Default path to the examples:
l macOS systems: /Users/<your user name>/rti_workspace/6.0.1/examples
l Linux systems: /home/<your user name>/rti_workspace/6.0.1/examples
l Windows systems: <your Windows documents folder>\rti_workspace\6.0.1\examples
Where 'your Windows documents folder' depends on your version of Windows. For example, on
Windows 10, the folder is C:\Users\<your user name>\Documents.
Note: You can specify a different location for rti_workspace. You can also specify that you do not want
the examples copied to the workspace. For details, see Controlling Location for RTIWorkspace and Copy-
ing of Examples in the RTIConnext DDS Installation Guide.
Source code for the news example is in <path to examples>/connext_dds/<language>/news.
The example performs these steps:
1. Parses the command-line arguments.
6.1 Building and Running the News Examples
2. Loads the quality-of-service (QoS) file, USER_QOS_PROFILES.xml, from the current working
directory. The middleware does this automatically; you will not see code in the example to do this.
For more information on how to use QoS profiles, see the section on "Configuring QoS with
XML" in the RTIConnext DDS Core Libraries User's Manual.
3. On the publishing side, sends news articles periodically.
4. On the subscribing side, receives these articles and print them out periodically.
The steps for compiling and running the program are similar to those described in 4.1 Building and Run-
ning “Hello, World” on page14. As with the Hello World example, Java users should use the build and
run command scripts in <path to examples>/connext_dds/java/news. Non-Java Windows developers
will find the necessary Microsoft Visual Studio solution files in <path to examples>/connext_dds/<lan-
guage>/news/win32. Appropriate makefiles for building the C, C++, or Ada
a
examples on UNIX plat-
forms are in <path to examples>/connext_dds/<language>/news/make.
The news example combines the publisher and subscriber in a single program, which is run from the
<path to examples>/connext_dds/<language>/news directory with the argument pub or sub to select
the desired behavior. When running with default arguments, you should see output similar to that shown in
4.1 Building and Running “Hello, World” on page14. To see additional command-line options, run the
program without any arguments.
Both the publishing application and the subscribing application operate in a periodic fashion:
The publishing application writes articles from different news outlets at different rates. Each of these art-
icles is numbered consecutively with respect to its news outlet. Every two seconds, the publisher prints a
summary of what it wrote in the previous period.
The subscribing application polls the cache of its DataReader every two seconds and prints the data it
finds there. (Many real-world applications will choose to process data as soon as it arrives rather than
polling for it. For more information about the different ways to read data, select Modules, Programming
How-To’s, DataReader Use Cases in the API Reference HTML documentation. In this case, periodic
polling makes the behavior easy to illustrate.) Along with each article it prints, it includes the time at which
that article was published and whether that article has been read before or whether it was cached from a
previous read.
By default, both the publishing and subscribing applications run for 20 seconds and then quit. (To run
them for a different number of seconds, use the -r command-line argument.) Figure 6.1: Example Output
for Both Applications on the next page shows example output using the default run time and a domain ID
of 13 (set with the command-line option, -d 13).
a
Ada support requires a separate add-on product, Ada Language Support.
59
6.2 Subscribing Only to Relevant Data
60
Figure 6.1: Example Output for Both Applications
6.2 Subscribing Only to Relevant Data
From reading AQuick Overview (Chapter 4 on page14), you already understand how to subscribe only
to the topics in which you’re interested. However, depending on your application, you may be interested
in only a fraction of the data available on those topics. Extraneous, uninteresting, or obsolete data puts a
drain on your network and CPU resources and complicates your application logic.
Fortunately, Connext DDS can perform much of your filtering and data reduction for you. Data reduction
is a general term for discarding unnecessary or irrelevant data so that you can spend your time processing
the data you care about. You can define the set of data that is relevant to you based on:
l Its content. Content-based filters can examine any field in your data based on a variety of criteria,
such as whether numeric values meet various equality and inequality relationship or whether string
values match certain regular expression patterns. For example, you may choose to distribute a stream
of stock prices using a single topic, but indicate that you’re interested only in the price of IBM, and
only when that price goes above $20.
l How old it is. You can indicate that data is relevant for only a certain period of time (its lifespan)
and/or that you wish to retain only a certain number of DDS data samples (a history depth). For
6.2.1 Content-Based Filtering
example, if you are interested in the last ten articles from each news outlet, you can set the history
depth to 10.
l How fast it’s updated. Sometimes data streams represent the state of a moving or changing
object—for example, an application may publish the position of a moving vehicle or the changing
price of a certain financial instrument. Subscribing applications may only be able to—or interested
in—processing this data at a certain maximum rate. For example, if the changing state is to be plot-
ted in a user interface, a human viewer is unlikely to be able to process more than a couple of
changes per second. If the application attempts to process updates any faster, it will only confuse the
viewer and consume resources unnecessarily.
6.2.1 Content-Based Filtering
A DataReader can filter incoming data by subscribing not to a given Topic itself, but to a Con-
tentFilteredTopic that associates the Topic with an SQL-like expression that indicates which DDS data
samples are of interest to the subscribing application. Each subscribing application can specify its own con-
tent-based filter, as it desires; publishing applications’ code does not need to change to allow subscribers to
filter on data contents.
6.2.1.1 Implementation
In Traditional C++:
DDSContentFilteredTopic cft = participant->create_contentfilteredtopic(
cftName.c_str(),
topic,
contentFilterExpression.c_str(),
noFilterParams);
if (cft == NULL) {
throw std::runtime_error("Unable to create ContentFilteredTopic");
}
The Topic and ContentFilteredTopic classes share a base class: TopicDescription.
The variable contentFilterExpression in the above example code is a SQL expression; see below.
In Modern C++:
using namespace dds::topic;
Topic<Stock> topic(participant, "My Stock");=
ContentFilteredTopic<Stock> cft(
topic, "My Stock (filtered)", Filter("key='CNN' OR key='Reuters'");
The Topic and ContentFilteredTopic classes share a base class: TopicDescription.
61
6.2.1.2 Running & Verifying
62
In Java:
ContentFilteredTopic cft = participant.create_contentfilteredtopic(
topic.get_name() + " (filtered)", topic,
contentFilterExpression, null);
if (cft == null) {
throw new IllegalStateException(
"Unable to create ContentFilteredTopic");
}
In Ada:
declare
cft : DDS.ContentFilteredTopic.Ref_Access;
cftName : DDS.String := DDS.To_DDS_String (DDS.To_Standard_String(
topic.Get_Name) & " (filtered)");
begin
cft := participant.Create_Contentfilteredtopic(
cftName, topic, contentFilterExpression, null);
DDS.Finalize (cftName);
if cft = null then
Put_Line (Standard_Error,
"Unable to create ContentFilteredTopic");
return;
end if;
end;
In Ada, ContentFilteredTopic class inherits from TopicDescription while Topic has a function ‘As_Top-
icDescription’ that is useful to create a DataReader where a TopicDescription is needed.
6.2.1.2 Running & Verifying
The following shows a simple filter with which the subscribing application indicates it is interested only in
news articles from CNN or Reuters; you can specify such a filter with the –f or --filterExpression com-
mand-line argument, like this in the Java example:
> ./run.sh sub -f "key='CNN' OR key='Reuters'"
This example uses the built-in KeyedString data type. This type has two fields: key, a string field that is
the type’s only key field, and value, a second string. This example uses the key field to store the news out-
let name and the value field to store the article text. The word “key” in the content filter expression “key-
='CNN'” refers to the field’s name, not the fact that it is a key field; you can also filter on the value field if
you like. In your own data types, you will use the name(s) of the fields you define. For example output,
see Figure 6.2: Using a Content-Based Filter on the next page.
You can find more detailed information in the API Reference HTML documentation under Modules, Con-
next DDSAPI Reference, Queries and Filters Syntax. For Ada, open <NDDSHOME>/-
doc/api/connext_dds/api_ada/index.html and look under Connext DDSAPI Reference,
DDSQueryAndFilterSyntaxModule.
6.2.2 Lifespan and History Depth
Figure 6.2: Using a Content-Based Filter
6.2.2 Lifespan and History Depth
One of the most common ways to reduce data is to indicate for how long a DDS data sample remains valid
once it has been sent. You can indicate such a contract in one (or both) of two ways: in terms of a number
of DDS samples to retain (the history depth) and the elapsed time period during which a DDS sample
should be retained (the lifespan). For example, you may be interested in only the most recent data value
(the so-called “last values”) or in all data that has been sent during the last second.
The history depth and lifespan, which can be specified declaratively with QoS policies (see below), apply
both to durable historical data sent to late-joining subscribing applications as well as to current subscribing
applications. For example, suppose a subscribing application has set history depth = 2, indicating that it is
only interested in the last two DDS samples. A publishing application sends four DDS data samples in
close succession, and the middleware on the subscribing side receives them before the application chooses
to read them from the middleware. When the subscribing application does eventually call DataRead-
er::read(), it will see only DDS samples 3 and 4; DDS samples 1 and 2 will already have been over-
written. If an application specifies both a finite history depth and a finite lifespan, whichever limit is
reached first will take effect.
6.2.2.1 Implementation
History depth is part of the History QoS policy. The lifespan is specified with the Lifespan QoS policy.
The History QoS policy applies independently to both the DataReader and the DataWriter; the values spe-
cified on both sides of the communication need not agree. The Lifespan QoS policy is specified only on
the DataWriter, but it is enforced on both sides: the DataReader will enforce the lifespan indicated by
each DataWriter it discovers for each DDS sample it receives from that DataWriter.
63
6.2.2.1 Implementation
64
For more information about these QoS policies, consult the API Reference HTML documentation. Open
ReadMe.html and select a programming language, then select Modules, RTI Connext DDSAPI Refer-
ence, Infrastructure, QoS Policies. For Ada: open <NDDSHOME>/doc/api/connext_dds/api_ada/in-
dex.html and select Infrastructure Module, DDSQosTypesModule.
You can specify these QoS policies either in your application code or in one or more XML files. Both
mechanisms are functionally equivalent; the News example provided for C, C++, Java, and Ada uses
XML files. For more information about this mechanism, see the section on “Configuring QoS with XML”
in the RTIConnext DDS Core Libraries User's Manual.
History Depth:
The DDS specification, which Connext DDS implements, recognizes two “kinds” of history: KEEP_
ALL and KEEP_LAST. KEEP_ALL history indicates that the application wants to see every DDS data
sample, regardless of how old it is (subject, of course, to Lifespan and other QoS policies). KEEP_LAST
history indicates that only a certain number of back DDS samples are relevant; this number is indicated by
a second parameter, the history depth. (The depth value, if any, is ignored if the kind is set to KEEP_
ALL.)
To specify a default History kind of KEEP_LAST and a depth of 10 in a QoS profile:
<history>
<kind>KEEP_LAST_HISTORY_QOS</kind>
<depth>10</depth>
</history>
You can see this in the News example (provided for C, C++, Java, and Ada) in the file USER_QOS_
PROFILES.xml.
Lifespan Duration:
The Lifespan QoS policy contains a field duration that indicates for how long each DDS sample remains
valid. The duration is measured relative to the DDS sample’s reception timestamp, which means that it
doesn’t include the latency of the underlying transport.
To specify a Lifespan duration of six seconds:
<lifespan>
<duration>
<sec>6</sec>
<nanosec>0</nanosec>
</duration>
</lifespan>
You can see this in the News example (provided for C, C++, Java, and Ada) in the file USER_QOS_
PROFILES.xml.
6.2.2.2 Running & Verifying
6.2.2.2 Running & Verifying
The News example never takes DDS samples from the middleware, it only reads DDS samples, so DDS
data samples that have already been viewed by the subscribing application remain in the middleware’s
internal cache until they are expired by their history depth or lifespan duration contract. These previously
viewed DDS samples are displayed by the subscribing application as “cached” to make them easy to spot.
See how “CNN” articles are expired:
65
6.2.2.2 Running & Verifying
66
Figure 6.3: Using History and Lifespan QoS
Remember that there is a strong analogy between DDS and relational databases: the key of a Topic is like
the primary key of a database table, and the instance corresponding to that key is like the table row cor-
responding to that primary key.
6.2.3 Time-Based Filtering
It's important to understand that the data type used by this example is keyed on the name of the news outlet
and that the history depth is enforced on a per-instance basis. You can see the effect in Figure 6.3: Using
History and Lifespan QoS on the previous page: even though Reuters publishes articles faster than CNN,
the Reuters articles do not "starve out" the CNN articles; each outlet gets its own depth DDS samples. (Fig-
ure 6.3: Using History and Lifespan QoS on the previous page only shows articles from CNN and Reu-
ters, because that makes it easier to fit more data on the page. If you run the example without a content
filter, you will see the same effect across all news outlets.).
Next, we will change the history depth and lifespan duration in the file USER_QOS_PROFILES.xml
to see how the example's output changes.
Set the history depth to 1.
Run the example again with the content-based filter, shown here in the Java example:
> ./run.sh sub -f "key='CNN' OR key='Reuters'"
You will only see articles from CNN and Reuters, and only the last value published for each (as of the end
of the two-second polling period).
Now set the history depth to 10 and decrease the lifespan to three seconds.
With these settings and at the rates used by this example, the lifespan will always take effect before the his-
tory depth. Run the example again, this time without a content filter. Notice how the subscribing applic-
ation sees all of the data that was published during the last two-second period as well as the data that was
published in the latter half of the previous period.
Reduce the lifespan again, this time to 1 second.
If you run the example now, you will see no cached articles at all. Do you understand why? The sub-
scribing application’s polling period is two seconds, so by the time the next poll comes, everything seen
the previous time has expired.
Try changing the history depth and lifespan duration in the file USER_QOS_PROFILES.xml to see
how the example’s output changes.
6.2.3 Time-Based Filtering
A time-based filter allows you to specify a minimum separation between the DDS data DDS samples your
subscribing application receives. If data is published faster than this rate, the middleware will discard the
intervening DDS samples.
Such a filter is most often used to down-sample high-rate periodic data (see also 6.5 Receiving Noti-
fications When Data Delivery Is Late on page75) but it can also be used to limit data rates for aperiodic-
but-bursty data streams. Time-based filters have several applications:
67
6.2.3.1 Implementation
68
l You can limit data update rates for applications in which rapid updates would be unnecessary or
inappropriate. For example, a graphical user interface should typically not update itself more than a
few times each second; more frequent updates can cause flickering or make it difficult for a human
operator to perceive the correct values.
l You can reduce the CPU requirements for less-capable subscribing machines to improve their per-
formance. If your data stream is reliable, helping slow readers keep up can actually improve the
effective throughput for all readers by preventing them from throttling the writer.
l In some cases, you can reduce your network bandwidth utilization, because the writer can trans-
parently discard unwanted data before it is even sent on the network.
6.2.3.1 Implementation
Time-based filters are specified using the TimeBasedFilter QoS policy. It only applies to DataReaders.
For more information about this QoS policy, consult the API Reference HTML documentation. Open
ReadMe.html and select a programming language, then select Modules, RTI Connext DDSAPI Refer-
ence, Infrastructure, QoS Policies. For Ada: open <NDDSHOME>/doc/api/connext_dds/api_ada/in-
dex.html and select Infrastructure Module, DDSQosTypesModule.
You can specify the QoS policies either in your application code or in one or more XML files. Both mech-
anisms are functionally equivalent; the News example uses the XML mechanism. For more information,
see the section on “Configuring QoS with XML” in the RTIConnext DDS Core Libraries User's Manual.
To specify a time-based filter in a QoS policy:
<!--
<time_based_filter>
<minimum_separation>
<sec>1</sec>
<nanosec>1</nanosec>
</minimum_separation>
</time_based_filter>
<deadline>
<period>
<sec>3</sec>
<nanosec>0</nanosec>
</period>
</deadline>
-->
You can see this in the file USER_QOS_PROFILES.xml provided with the C, C++, Java, and Ada
News example (uncomment it to specify a time-based filter).
At the same time we implement a time-based filter in the News example, we increase the deadline period.
See the accompanying comment in the XML file as well as the API Reference HTML documentation for
more information about using the Deadline and TimeBasedFilter QoS policies together.
6.2.3.2 Running & Verifying
6.2.3.2 Running & Verifying
Figure 6.4: Using a Time-Based Filter below shows some of the output after activating the filter:
Figure 6.4: Using a Time-Based Filter
Because you set the time-based filter to one second, you will not see more than two updates for any single
news outlet in any given period, because the period is two seconds long.
6.3 Accessing Historical Data when Joining the Network
In 6.2 Subscribing Only to Relevant Data on page60, you learned how to specify which data on the net-
work is of interest to your application. The same QoS parameters can also apply to late-joining subscribers,
applications that subscribe to a topic after some amount of data has already been published on that topic.
This concept—storing sent data within the middleware—is referred to as durability. You only need to
indicate the degree of durability in which you’re interested:
l Volatile. Data is relevant to current subscribers only. Once it has been acknowledged (if reliable
communication has been turned on), it can be removed from the service. This level of durability is
the default; if you specify nothing, you will get this behavior.
l Transient local. Data that has been sent may be relevant to late-joining subscribers (subject to any
history depth, lifespan, and content- and/or time-based filters defined). Historical data will be cached
with the DataWriter that originally produced it. Once that writer has been shut down for any reason,
intentionally or unintentionally, however, the data will no longer be available. This lightweight level
of durability is appropriate for non-critical data streams without stringent data availability require-
ments.
l Transient. Data that has been sent may be relevant to late-joining subscribers and will be stored
externally to the DataWriter that produced that data. This level of durability requires one or more
instances of RTI Persistence Service on your network. As long as one or more of these persistence
69
6.3.1 Implementation
70
service instances is functional, the durable data will continue to be available, even if the original
DataWriter shuts down or fails. However, if all instances of the persistence service shut down or
fail, the durable data they were maintaining will be lost. This level of durability provides a higher
level of fault tolerance and availability than does transient-local durability without the performance
or management overhead of a database.
l Persistent. Data that has been sent may be relevant to late-joining subscribers and will be stored
externally to the DataWriter that produced that data in a relational database. This level of durability
requires one or more instances of RTI Persistence Service on your network. It provides the greatest
degree of assurance for your most critical data streams, because even if all data writers and all per-
sistence server instances fail, your data can nevertheless be restored and made available to sub-
scribers when you restart the persistence service.
As you can see, the level of durability indicates not only whether historical data will be made available to
late-joining subscribers; it also indicates the level of fault tolerance with which that data will be made avail-
able. Learn more about durability, including the RTI Persistence Service, by reading the section on “Mech-
anisms for Achieving Information Durability and Persistence” in the RTIConnext DDS Core Libraries
User's Manual.
6.3.1 Implementation
To configure data durability, use the Durability QoS policy on your DataReader and/or DataWriter. The
degrees of durability described in 6.3 Accessing Historical Data when Joining the Network on the pre-
vious page are represented as an enumerated durability kind.
For more information about this QoS policy, consult the API Reference HTML documentation. Open
ReadMe.html and select the API documentation for your language then select Modules, RTI Connext
DDSAPI Reference, Infrastructure, QoS Policies. For Ada: open <NDDSHOME>/doc/api/connext_
dds/api_ada/index.html and select Infrastructure Module, DDSQosTypesModule.
You can specify the QoS policies either in your application code or in one or more XML files. Both mech-
anisms are functionally equivalent; the News example uses the XML mechanism. For more information,
see the chapter on “Configuring QoS with XML” in the RTIConnext DDS Core Libraries User's Manual.
Here is an example of how to configure the Durability QoS policy in a QoS profile:
<durability>
<kind>TRANSIENT_LOCAL_DURABILITY_QOS</kind>
</durability>
You can see this in the news example (provided for C, C++, Java, and Ada) in the file USER_QOS_
PROFILES.xml.
The above configuration indicates that the DataWriter should maintain data it has published on behalf of
later-joining DataReaders, and that DataReaders should expect to receive historical data when they join
6.3.2 Running & Verifying
the network. However, if a DataWriter starts up, publishes some data, and then shuts down, a DataReader
that subsequently starts up will not receive that data.
Durability, like some other QoS policies, has request-offer semantics: the DataWriter must offer a level of
service that is greater than or equal to the level of service requested by the DataReader. For example, a
DataReader may request only volatile durability, while the DataWriter may offer transient durability. In
such a case, the two will be able to communicate. However, if the situation were reversed, they would not
be able to communicate.
6.3.2 Running & Verifying
Run the NewsPublisher and wait several seconds. Then start the NewsSubscriber. Look at the time
stamps printed next to the received data: you will see that the subscribing application receives data that was
published before it started up.
Now do the same thing again, but first modify the configuration file by commenting the durability con-
figuration. You will see that the subscribing application does not receive any data that was sent prior to
when it joined the network.
6.4 Caching Data within the Middleware
When you receive data from the middleware in your subscribing application, you may be able to process
all of the data immediately and then discard it. Frequently, however, you will need to store it somewhere in
order to process it later. Since you’ve already expressed to the middleware how long your data remains rel-
evant (see 6.2 Subscribing Only to Relevant Data on page60), wouldn’t it be nice if you could take
advantage of the middleware’s own data cache rather than implementing your own? You can.
When a DataReader reads data from the network, it places the DDS samples, in order, into its internal
cache. When you’re ready to view that data (either because you received a notification that it was available
or because you decided to poll), you use one of two families of methods:
l take: Read the data from the cache and simultaneously remove it from that cache. Future access to
that DataReader’s cache will not see any data that was previously taken from the cache. This beha-
vior is similar to the behavior of “receive” methods provided by traditional messaging middleware
implementations. The generated Ada example uses this method.
l read: Read the data from the cache but leave it in the cache so that it can potentially be read again
(subject to any lifespan or history depth you may have specified). The news example for C, C++
and Java uses this method.
When you read or take data from a DataReader, you can indicate that you wish to access all of the data in
the cache, all up to a certain maximum number of DDS samples, all of the new DDS samples that you
have never read before, and/or various other qualifiers. If your topic is keyed, you can choose to access the
DDS samples of all instances at once, or you can read/take one instance at a time. For more information
71
6.4.1 Implementation
72
about keys and instances, see Section 2.2.2, “DDS Samples, Instances, and Keys” in the RTIConnext
DDS Core Libraries User's Manual.
6.4.1 Implementation
The call to read looks like this:
In C++:
DDS_ReturnCode_t result = _reader->read(
articles, // fill in data here
articleInfos, // fill in parallel meta-data here
DDS_LENGTH_UNLIMITED, // any # articles
DDS_ANY_SAMPLE_STATE, DDS_ANY_VIEW_STATE, DDS_ANY_INSTANCE_STATE);
if (result == DDS_RETCODE_NO_DATA) {
// nothing to read; go back to sleep
}
if (result != DDS_RETCODE_OK) {
// an error occurred: stop reading
throw std::runtime_error("A read error occurred: " + result);
}
// Process data...
_reader->return_loan(articles, articleInfos);
In Java:
try {
_reader.read(
articles, // fill in data here
articleInfos, // fill in parallel meta-data here
ResourceLimitsQosPolicy.LENGTH_UNLIMITED, // any # articles
SampleStateKind.ANY_SAMPLE_STATE,
ViewStateKind.ANY_VIEW_STATE,
InstanceStateKind.ANY_INSTANCE_STATE);
// Process data...
} catch (RETCODE_NO_DATA noData) {
// nothing to read; go back to sleep
} catch (RETCODE_ERROR ex) {
// an error occurred: stop reading
throw ex;
} finally {
_reader.return_loan(articles, articleInfos);
}
In Ada:
declare
received_data : aliased DDS.KeyedString_Seq.Sequence;
sample_info : aliased DDS.SampleInfo_Seq.Sequence;
begin
reader.Read(
received_data'Access, sample_info'Access,
6.4.1 Implementation
DDS.LENGTH_UNLIMITED,
DDS.ANY_SAMPLE_STATE,
DDS.ANY_VIEW_STATE,
DDS.ANY_INSTANCE_STATE);
for i in 1 .. DDS.KeyedString_Seq.Get_Length(received_data'Access)
loop
printArticle(
DDS.KeyedString_Seq.Get (received_data'Access, i),
DDS.SampleInfo_Seq.Get (sample_info'Access, i));
end loop;
reader.Return_Loan (received_data'Access, sample_info'Access);
exception
when DDS.NO_DATA =>
null; -- ignore this error
-- nothing to read; go back to sleep
when DDS.ERROR =>
-- an error occurred: stop reading
raise;
end;
end;
The read method takes several arguments:
l Data and SampleInfo sequences:
The first two arguments to read or take are lists: for the DDS samples themselves and, in parallel,
for meta-data about those DDS samples. In most cases, you will pass these collections into the mid-
dleware empty, and the middleware will fill them for you. The objects it places into these sequences
are loaned directly from the DataReader’s cache in order to avoid unnecessary object allocations or
copies. When you are done with them, call return_loan.
If you would rather read the DDS samples into your own memory instead of taking a loan from the
middleware, simple pass in sequences in which the contents have already been deeply allocated.
The DataReader will copy over the state of the objects you provide to it instead of loaning you its
own objects. In this case, you do not need to call return_loan.
l Number of DDS samples to read:
If you are only prepared to read a certain number of DDS samples at a time, you can indicate that to
the DataReader. In most cases, you will probably just use the constant LENGTH_UNLIMITED,
which indicates that you are prepared to handle as many DDS samples as the DataReader has avail-
able.
l Sample state:
The sample state indicates whether or not an individual DDS sample has been observed by a pre-
vious call to read. (The news example uses this state to decide whether or not to append “(cached)”
73
6.4.1 Implementation
74
to the data it prints out.) By passing a sample state mask to the DataReader, you indicate whether
you are interested in all DDS samples, only those DDS samples you’ve never seen before, or only
those DDS samples you have seen before. The most common value passed here is ANY_
SAMPLE_STATE.
l View state:
The view state indicates whether the instance to which a DDS sample belongs has been observed by
a previous call to read or take. By passing a view state mask to the DataReader, you can indicate
whether you’re interested in all instances, only those instances that you have never seen before
(NEW instances), or only those instances that you have seen before (NOT_NEW instances). The
most common value passed here is ANY_VIEW_STATE.
l Instance state:
The instance state indicates whether the instance to which a DDS sample belongs is still alive
(ALIVE), whether it has been disposed (NOT_ALIVE_DISPOSED), or whether the DataWriter
has simply gone away or stopped writing it without disposing it (NOT_ALIVE_NO_WRITERS).
The most common value passed here is ANY_INSTANCE_STATE. For more information about
the data lifecycle, consult the RTIConnext DDS Core Libraries User's Manual and the API Refer-
ence HTML documentation.
Unlike reading from a socket directly, or calling receive in JMS, a read or take is non-blocking: the
call returns immediately. If no data was available to be read, it will return (in C and C++) or throw
(in Java, .NET
a
, and Ada) a NO_DATA result.
Connext DDS also offers additional variations on the read() and take() methods to allow you to
view different “slices” of your data at a time:
l You can view one instance at a time with read_instance(), take_instance(), read_next_
instance(), and take_next_instance().
l You can view a single DDS sample at a time with read_next_sample() and take_next_
sample().
For more information about these and other variations, see the API Reference HTML documentation:
open ReadMe.html, select a language, and look under Modules, Subscription Module, DataReader
Support, FooDataReader. For Ada, open <NDDSHOME>/doc/api/connext_dds/api_ada/index.html
and look under Subscription Module, DDSReaderModule, DDS.Typed_DataReader_Generic.
a
Connext DDS .NET language binding is currently supported for C# and C++/CLI.
6.4.2 Running & Verifying
6.4.2 Running & Verifying
To see the difference between read() and take() semantics, replace “read” with “take” (the arguments are
the same), rebuild the example, and run again. You will see that “(cached)” is never printed, because every
DDS sample will be removed from the cache as soon as it is viewed for the first time.
6.5 Receiving Notifications When Data Delivery Is Late
Many applications expect data to be sent and received periodically (or quasi-periodically). They typically
expect at least one DDS data sample to arrive during each period; a failure of data to arrive may or may
not indicate a serious problem, but is probably something about which the application would like to
receive notifications. For example:
l A vehicle may report its current position to its home base every second.
l Each sensor in a sensor network reports a new reading every 0.5 seconds.
l A radar reports the position of each object that it’s tracking every 0.2 seconds.
If any vehicle, any sensor, or any radar track fails to yield an update within its promised period, another
part of the system, or perhaps its human operator, may need to take a corrective action.
(In addition to built-in deadline support, Connext DDS has other features useful to applications that publish
and/or receive data periodically. For example, it’s possible to down-sample high-rate periodic data; see 6.2
Subscribing Only to Relevant Data on page60.
6.5.1 Implementation
Deadline enforcement is comprised of two parts: (1) QoS policies that specify the deadline contracts and
(2) listener callbacks that are notified if those contracts are violated.
Deadlines are enforced independently for DataWriters and DataReaders. However, the Deadline QoS
policy, like some other policies, has request-offer semantics: the DataWriter must offer a level of service
that is the same or better than the level of service the DataReader requests. For example, if a DataWriter
promises to publish data at least once every second, it will be able to communicate with a DataReader that
expects to receive data at least once every two seconds. However, if the DataReader declares that it
expects data twice a second, and the DataWriter only promises to publish updates only once a second,
they will not be able to communicate.
6.5.1.1 Offered Deadlines
A DataWriter promises to publish data at a certain rate by providing a finite value for the Deadline QoS
policy, either in source code or in one or more XML configuration files. (Both mechanisms are func-
tionally equivalent; the News example in C, C++, Java, and Ada uses XML files. For more information
about this mechanism, see the section on “Configuring QoS with XML” in the RTIConnext DDS Core
Libraries User's Manual.)
75
6.5.1.2 Requested Deadlines
76
The file USER_QOS_PROFILES.xml in the News example for C, C++, Java, and Ada contains the fol-
lowing Deadline QoS policy configuration, which applies to both DataWriters and DataReaders:
<deadline>
<period>
<sec>2</sec>
<nanosec>0</nanosec>
</period>
</deadline>
The DataWriter thus promises to publish at least one DDS data sample—of each instance—every two
seconds.
If a period of two seconds elapses from the time the DataWriter last sent a DDS sample of a particular
instance, the writer’s listener—if one is installed—will receive a callback to its on_offered_deadline_
missed method. The News example does not actually install a DataWriterListener. See the section on
requested deadlines below; the DataWriterListener works in a way that’s parallel to the DataRead-
erListener.
6.5.1.2 Requested Deadlines
The DataReader declares that it expects to receive at least one DDS data sample of each instance within a
given period using the Deadline QoS policy. See the example XML configuration above.
If the declared deadline period elapses since the DataReader received the last DDS sample of some
instance, that reader’s listener—if any—will be invoked. The listener’s on_requested_deadline_missed()
will receive a call informing the application of the missed deadline.
To install a DataReaderListener:
In C++:
DDSDataReader* reader = participant->create_datareader(
topic,
DDS_DATAREADER_QOS_DEFAULT,
&_listener, // listener
DDS_STATUS_MASK_ALL); // all callbacks
if (reader == NULL) {
throw std::runtime_error("Unable to create DataReader");
}
In Java:
DataReader reader = participant.create_datareader(
topic,
Subscriber.DATAREADER_QOS_DEFAULT,
new ArticleDeliveryStatusListener(), // listener
6.5.1.2 Requested Deadlines
StatusKind.STATUS_MASK_ALL); // all callbacks
if (reader == null) {
throw new IllegalStateException("Unable to create DataReader");
}
In Ada:
declare
readerListener : ArticleDeliveryStatusListener;
begin
reader := participant.Create_DataReader
(topic.As_TopicDescription,
DDS.Subscriber.DATAREADER_QOS_DEFAULT,
readerListener'Unchecked_Access, -- listener
DDS.STATUS_MASK_ALL); -- all callbacks
if reader = null then
Put_Line (Standard_Error, "Unable to create DataReader");
return;
end if;
end;
There are two listener-related arguments to provide:
l
Listener: The listener object itself, which must implement some subset of the callbacks defined by
the DataReaderListener supertype.
l
Listener mask: Which of the callbacks you’d like to receive. In most cases, you will use one of the
constants STATUS_MASK_ALL (if you are providing a non-null listener object) of STATUS_
MASK_NONE (if you are not providing a listener). There are some cases in which you might want
to specify a different listener mask; see the RTIConnext DDS Core Libraries User's Manual and
API Reference HTML documentation for more information.
Let’s look at a very simple implementation of the on_requested_deadline_missed callback that prints the
value of the key (i.e., the news outlet name) for the instance whose deadline was missed. You can see this
in the News example provided with C, C++, and Java.
In C++:
void ArticleDeliveryStatusListener::on_requested_deadline_missed(
DDSDataReader* reader,
const DDS_RequestedDeadlineMissedStatus& status) {
DDS_KeyedString keyHolder;
DDSKeyedStringDataReader* typedReader =
DDSKeyedStringDataReader::narrow(reader);
typedReader->get_key_value(keyHolder,
status.last_instance_handle);
std::cout << "->Callback: requested deadline missed: "
<< keyHolder.key
77
6.5.2 Running & Verifying
78
<< std::endl;
}
In Java:
public void on_requested_deadline_missed(
DataReader reader,
RequestedDeadlineMissedStatus status) {
KeyedString keyHolder = new KeyedString();
reader.get_key_value_untyped(keyHolder,
status.last_instance_handle);
System.out.println("->Callback: requested deadline missed: " +
keyHolder.key);
}
In Ada:
procedure On_Requested_Deadline_Missed
(Self : not null access Ref;
The_Reader : in DDS.DataReaderListener.DataReader_Access;
Status : in DDS.RequestedDeadlineMissedStatus)
is
pragma Unreferenced (Self);
pragma Unreferenced (The_Reader);
pragma Unreferenced (Status);
begin
Put_Line ("->Callback: requested deadline missed.");
end On_Requested_Deadline_Missed;
6.5.2 Running & Verifying
Modify the file USER_QOS_PROFILES.xml to decrease the deadline to one second:
<deadline>
<period>
<sec>1</sec>
<nanosec>0</nanosec>
</period>
</deadline>
Note that, if you have a DataReader- or DataWriter-level deadline specified (inside the file's datareader_
qos or datawriter_qos elements, respectively)—possibly because you previously modified the con-
figuration in 6.2.3 Time-Based Filtering on page67—it is overriding the topic-level configuration. Be
careful that you don't modify a deadline specification that will only be overridden later and not take effect.
You will see output similar to Figure 6.5: Using a Shorter Deadline on the next page:
6.5.2 Running & Verifying
Figure 6.5: Using a Shorter Deadline
79
Chapter 7 Design Patterns for High
Performance
In this section, you will learn how to implement some common performance-oriented design pat-
terns. As you have learned, one of the advantages to using Connext DDS is that you can easily
tune your application without changing its code, simply by updating the XML-based Quality of
Service (QoS) parameters.
We will build on the examples used in 4.1 Building and Running “Hello, World” on page14 to
demonstrate the different use-cases. The example applications (Hello_builtin, Hello_idl, and
Hello_dynamic
a
), provide the same functionality but use different data types in order to help you
understand the different type-definition mechanisms offered by Connext DDS and their tradeoffs.
They implement a simple throughput test: the publisher sends a payload to the subscriber, which
periodically prints out some basic statistics. You can use this simple test to quickly see the effects
of your system design choices: programming language, target machines, QoS configurations, and
so on.
The QoS parameters do not depend on the language used for your application, and seldom on the
operating system (there are few key exceptions), so you should be able to use the XML files with
the example in the language of your choice.
This section includes:
l 7.1 Building and Running the Code Examples on the next page
l 7.2 Reliable Messaging on page84
l 7.3 High Throughput for Streaming Data on page88
l 7.4 Sending Large Data on page91
l 7.5 Streaming Data over Unreliable Network Connections on page95
a
Dynamic DDS types are not supported when using Ada Language Support.
80
7.1 Building and Running the Code Examples
81
7.1 Building and Running the Code Examples
By default, examples are copied into your home directory the first time you run RTI Launcher or any script
in <NDDSHOME>/bin. This document refers to the location of the copied examples as <path to
examples>.
Wherever you see <path to examples>, replace it with the appropriate path.
Default path to the examples:
l macOS systems: /Users/<your user name>/rti_workspace/6.0.1/examples
l Linux systems: /home/<your user name>/rti_workspace/6.0.1/examples
l Windows systems: <your Windows documents folder>\rti_workspace\6.0.1\examples
Where 'your Windows documents folder' depends on your version of Windows. For example, on
Windows 10, the folder is C:\Users\<your user name>\Documents.
Note: You can specify a different location for rti_workspace. You can also specify that you do not want
the examples copied to the workspace. For details, see Controlling Location for RTIWorkspace and Copy-
ing of Examples in the RTIConnext DDS Installation Guide.
You can find the source for the Hello_builtin example for Java in <path to examples>/connext_dds/-
java/hello_builtin; the equivalent source code in other supported languages is in the directories <path to
examples>/connext_dds/<language>. The hello_idl and hello_dynamic examples are in parallel dir-
ectories under <path to examples>/connext_dds/<language>.
The examples perform these steps:
1. Parse their command-line arguments.
2. Check if the Quality of Service (QoS) file can be located.
The XML file that sets the Quality of Service (QoS) parameters is either loaded from the file
USER_QOS_PROFILES.xml in the current directory of the program, or from the environment
variable NDDS_QOS_PROFILES. For more information on how to use QoS profiles, see the sec-
tion on "Configuring QoS with XML" in the RTIConnext DDS Core Libraries User's Manual.
3. On the publishing side, send text strings as fast as possible, prefixing each one with a serial number.
4. On the subscribing side, receive these strings, keeping track of the highest number seen, as well as
other statistics. Print these out periodically.
7.1.1 Understanding the Performance Results
Figure 7.1: Example Output from Subscribing Applications in C, C++, or Java
The steps for compiling and running the program are the same as mentioned in 4.1 Building and Running
“Hello, World” on page14.
Run the publisher and subscriber from the <path to examples>/connext_dds/<language>/hello_builtin
directory using one of the QoS profile files provided in examples/connext_dds/qos by copying it into
your current working directory with the file name USER_QOS_PROFILES.xml. You should see output
like the following from the subscribing application:
7.1.1 Understanding the Performance Results
You will see several columns in the subscriber-side output, similar to Figure 7.1: Example Output from
Subscribing Applications in C, C++, or Java above.
l
Seconds from start:
The number of seconds the subscribing application has been running. It will print out one line per
second.
l
Total DDS samples:
The number of DDS data samples that the subscribing application has received since it started run-
ning.
82
7.1.1 Understanding the Performance Results
83
l
Total lost DDS samples:
The number of DDS samples that were lost in transit and could not be re-paired, since the sub-
scribing application started running. If you are using a QoS profile configured for strict reliability,
you can expect this column to always display 0. If you are running in best-effort mode, or in a lim-
ited-reliability mode (e.g., you have configured your History QoS policy to only keep the most
recent DDS sample), you may see non-zero values here.
l
Current lost DDS samples:
The number of DDS samples that were lost in transit and could not be repaired, since the last status
line was printed. See the description of the previous column for information about what values to
expect here.
l
Average DDS samples per second:
The mean number of DDS data samples received per second since the subscribing application star-
ted running. By default, these example applications send DDS data samples that are 1 KB in size.
(You can change this by passing a different size, in bytes, to the publishing application with --size.)
l
Current DDS samples per second:
The number of DDS data samples received since the subscribing application printed the last status
line.
l
Throughput megabits per second:
The throughput from the publishing application to the subscribing application, in bits per second,
since the subscribing application printed the previous status line. The value in this column is equi-
valent to the current DDS samples per second multiplied by the number of bits in an individual DDS
sample.
With small sample sizes, the fixed "cost" of traversing your operating system's network stack is
greater than the cost of actually transmitting the data; as you increase the sample size, you will see
the throughput more closely approach the theoretical throughput of your network.
By batching multiple DDS data samples into a single network packet, as the high-throughput
example QoS profile does, you should be able to saturate a gigabit Ethernet network with sample
sizes as small as 100-200 bytes. Without batching DDS samples, you should be able to saturate the
network with DDS samples of a few kilobytes. The difference is due to the performance limitations
of the network transport; enterprise-class platforms with commodity Ethernet interfaces can typically
execute tens of thousands of send()’s per second. In contrast, saturating a high-throughput network
link with data sizes of less than a kilobyte requires hundreds of thousands of DDS samples per
second.
7.1.2 Is this the Best Possible Performance?
7.1.2 Is this the Best Possible Performance?
The performance of an application depends heavily on the operating system, the network, and how it con-
figures and uses the middleware. This example is just a starting point; tuning is important for a production
application. RTI can help you get the most out of your platform and the middleware.
To get a sense for how an application's behavior changes with different QoS contracts, try the other
provided example QoS profiles and see how the printed results change.
7.2 Reliable Messaging
Packets sent by a middleware may be lost by the physical network or dropped by routers, switches and
even the operating system of the subscribing applications when buffers become full. In reliable messaging,
the middleware keeps track of whether or not data sent has been received by subscribing applications, and
will resend data that was lost on transmission.
Like most reliable protocols (including TCP), the reliability protocol used by RTI uses additional packets
on the network, called metadata, to know when user data packets are lost and need to be resent. RTI offers
the user a comprehensive set of tunable parameters that control how many and how often metadata packets
are sent, how much memory is used for internal buffers that help overcome intermittent data losses, and
how to detect and respond to a reliable subscriber that either falls behind or otherwise disconnects.
When users want applications to exchange messages reliably, there is always a need to trade-off between
performance and memory requirements. When strictly reliable communication is enabled, every written
DDS sample will be kept by Connext DDS inside an internal buffer until all known reliable subscribers
acknowledge receiving the DDS sample
a
.
If the publisher writes DDS samples faster than subscribers can acknowledge receiving, this internal buffer
will eventually be completely filled, exhausting all the available space—in that case, further writes by the
publishing application will block. Similarly, on the subscriber side, when a DDS sample is received, it is
stored inside an internal receive buffer, waiting for the application to take the data for processing. If the sub-
scribing application doesn't take the received DDS samples fast enough, the internal receive buffer may fill
up—in that case, newly received data will be discarded and would need to be repaired by the reliable pro-
tocol.
Although the size of those buffers can be controlled from the QoS, you can also use QoS to control what
Connext DDS will do when the space available in one of those buffers is exhausted. There are two pos-
sible scenarios for both the publisher and subscriber:
a
Connext DDS also supports reliability based only on negative acknowledgements ("NACK-only reliability"). This feature
is described in detail in the User's Manual (Section6.5.2.3) but is beyond the scope of this document.
84
7.2.1 Implementation
85
l Publishing side: If write() is called and there is no more room in the DataWriter’s buffer, Connext
DDS can:
1. Temporarily block the write operation until there is room on this buffer (for example, when
one or more DDS samples is acknowledged to have been received from all the subscribers).
2. Drop the oldest DDS sample from the queue to make room for the new one.
l Subscribing side: If a DDS sample is received (from a publisher) and there is no more room on the
DataReader’s buffer:
1. Drop the DDS sample as if it was never received. The subscribing application will send a neg-
ative acknowledgement requesting that the DDS sample be resent.
2. Drop the oldest DDS sample from the queue to make room for the new one.
7.2.1 Implementation
There are many variables to consider, and finding the optimum values to the queue size and the right
policy for the buffers depends on the type of data being exchanged, the rate of which the data is written,
the nature of the communication between nodes and various other factors.
The RTIConnext DDS Core Libraries User's Manual dedicates an entire section to the reliability protocol,
providing details on choosing the correct values for the QoS based on the system configuration. For more
information, refer to Chapter 10 in the User’s Manual.
The following sections highlight the key QoS settings needed to achieve strict reliability. In the reli-
able.xml QoS profile file, you will find many other settings besides the ones described here. A detailed
description of these QoS is outside the scope of this document, and for further information, refers to the
comments in the QoS profile and in the User’s Manual.
7.2.1.1 Enable Reliable Communication
The QoS that control the kind of communication is the Reliability QoS of the DataWriter and
DataReader:
7.2.1.2 Set History To KEEP_ALL
<datawriter_qos>
...
<reliability>
<kind>RELIABLE_RELIABILITY_QOS</kind>
<max_blocking_time>
<sec>5</sec>
<nanosec>0</nanosec>
</max_blocking_time>
</reliability>
...
</datawriter_qos>
...
<datareader_qos>
<reliability>
<kind>RELIABLE_RELIABILITY_QOS</kind>
</reliability>
</datareader_qos>
This section of the QoS file enables reliability on the DataReader and DataWriter, and tells the mid-
dleware that a call to write() may block up to 5 seconds if the DataWriter’s cache is full of unac-
knowledged DDS samples. If no space opens up in 5 seconds, write() will return with a timeout indicating
that the write operation failed and that the data was not sent.
7.2.1.2 Set History To KEEP_ALL
The History QoS determines the behavior of a DataWriter or DataReader when its internal buffer fills up.
There are two kinds:
l KEEP_ALL:The middleware will attempt to keep all the DDS samples until they are acknow-
ledged (when the DataWriter’s History is KEEP_ALL), or taken by the application (when the
DataReader’s History is KEEP_ALL).
l KEEP_LAST:The middleware will discard the oldest DDS samples to make room for new DDS
samples. When the DataWriter’s History is KEEP_LAST, DDS samples are discarded when a
new call to write() is performed. When the DataReader’s History is KEEP_LAST, DDS samples
in the receive buffer are discarded when new DDS samples are received. This kind of history is asso-
ciated with a depth that indicates how many historical DDS samples to retain.
<datawriter_qos>
<history>
<kind>KEEP_ALL_HISTORY_QOS</kind>
</history>
...
</datawriter_qos>
...
<datareader_qos>
<history>
<kind>KEEP_ALL_HISTORY_QOS</kind>
</history>
...
</datareader_qos>
86
7.2.1.3 Controlling Middleware Resources
87
The above section of the QoS profile tells RTI to use the policy KEEP_ALL for both DataReader and
DataWriter.
7.2.1.3 Controlling Middleware Resources
With the ResourceLimits QosPolicy, you have full control over the amount of memory used by the mid-
dleware. In the example below, we specify that both the reader and writer will store up to 10 DDS samples
(if you use a History kind of KEEP_LAST, the values specified here must be consistent with the value
specified in the History’s depth).
<datawriter_qos>
<resource_limits>
<max_samples>10</max_samples>
</resource_limits>
...
</datawriter_qos>
...
<datareader_qos>
<resource_limits>
<max_samples>2</max_samples>
</resource_limits>
...
</datareader_qos>
The above section tells RTI to allocate a buffer of 10 DDS samples for the DataWriter and 2 for the
DataReader. If you do not specify any value for max_samples, the default behavior is for the middleware
to allocate as much space as it needs.
One important function of the Resource Limits QoS policy, when used in conjunction with the Reliability
and History QoS policies, is to govern how far "ahead" of its DataReaders a DataWriter may get before it
will block, waiting for them to catch up. In many systems, consuming applications cannot acknowledge
data as fast as its producing applications can put new data on the network. In such cases, the Resource Lim-
its QoS policy provides a throttling mechanism that governs how many sent-but-not-yet-acknowledged
DDS samples a DataWriter will maintain. If a DataWriter is configured for reliable KEEP_ALL oper-
ation, and it exceeds max_samples or max_samples_per_instance, calls to write() will block until the
writer receives acknowledgements that will allow it to reclaim that memory.
The write operation can also block if the send window, configurable using the max/min_send_window_
size fields in the DDS_RtpsReliableWriterProtocol_t structure in the DataWriter Protocol policy, fills up.
The send window provides an alternative throttling mechanism that works with both KEEP_ALL and
KEEP_LAST configurations.
If you see that your reliable publishing application is using an unacceptable amount of memory, you can
specify a finite value for max_samples. By doing this, you restrain the size of the DataWriter's cache,
causing it to use less memory; however, a smaller cache will fill more quickly, potentially causing the
writer to block for a time when sending, decreasing throughput. If decreased throughput proves to be an
issue, you can tune the reliability protocol to process acknowledgements and repairs more aggressively,
7.3 High Throughput for Streaming Data
allowing the writer to clear its cache more effectively. A full discussion of the relevant reliability protocol
parameters is beyond the scope of this example. However, you can find a useful example in high_
throughput.xml. Also see the documentation for the DataReaderProtocol and DataWriterProtocol QoS
policies in the on-line API documentation.
7.3 High Throughput for Streaming Data
This design pattern is useful for systems that produce a large number of small messages at a high rate.
In such cases, there is a small but measurable overhead in sending (and in the case of reliable com-
munication, acknowledging) each message separately on the network. It is more efficient for the system to
manage many DDS samples together as a group (referred to in the API as a batch) and then send the entire
group in a single network packet. This allows Connext DDS to minimize the overhead of building a data-
gram and traversing the network stack.
Batching increases throughput when writing small DDS samples at a high rate. As seen in Figure 7.3:
Benefits of Batching:Sample Rates on the next page, throughput can be increased several-fold, much
more closely approaching the physical limitations of the underlying network transport.
Figure 7.2: Benefits of Batching
Batching delivers tremendous benefits for messages of small size.
88
7.3 High Throughput for Streaming Data
89
Figure 7.3: Benefits of Batching:Sample Rates
A subset of the batched throughput data above, expressed in terms of DDS samples per second.
Collecting DDS samples into a batch implies that they are not sent on the network (flushed) immediately
when the application writes them; this can potentially increase latency. However, if the application sends
data faster than the network can support, an increased share of the network's available bandwidth will be
spent on acknowledgments and resending dropped data. In this case, reducing that meta-data overhead by
turning on batching could decrease latency even while increasing throughput. Only an evaluation of your
system's requirements and a measurement of its actual performance will indicate whether batching is appro-
priate. Fortunately, it is easy to enable and tune batching, as you will see below.
Batching is particularly useful when the system has to send a large number of small messages at a fast rate.
Without this feature enabled, you may observe that your maximum throughput is less than the maximum
bandwidth of your network. Simultaneously, you may observe high CPU loads. In this situation, the bot-
tleneck in your system is the ability of the CPU to send packets through the OS network stack.
For example, in some algorithmic trading applications, market data updates arrive at a rate of from tens of
thousands of messages per second to over a million; each update is some hundreds of bytes in size. It is
often better to send these updates in batches than to publish them individually. Batching is also useful
when sending a large number of small DDS samples over a connection where the bandwidth is severely
constrained.
7.3.1 Implementation
7.3.1 Implementation
RTI can automatically flush batches based on the maximum number of DDS samples, the total batch size,
or elapsed time since the first DDS sample was placed in the batch, whichever comes first. Your applic-
ation can also flush the current batch manually. Batching is completely transparent on the subscribing side;
no special configuration is necessary.
Figure 7.4: Batching Implementation
RTI collects DDS samples in a batch until the batch is flushed.
For more information on batching, see the RTIConnext DDS Core Libraries User's Manual (Section
6.5.2) or API Reference HTML documentation (the Batch QosPolicy is described in the Infrastructure
Module).
Using the batching feature is simple—just modify the QoS in the publishing application’s configuration
file.
For example, to enable batching with a batch size of 100 DDS samples, set the following QoS in your
XML configuration file:
<datawriter_qos>
...
<batch>
<enable>true</enable>
<max_samples>100</max_samples>
</batch>
...
</datawriter_qos>
To enable batching with a maximum batch size of 8K bytes:
90
7.4 Sending Large Data
91
<datawriter_qos>
...
<batch>
<enable>true</enable>
<max_data_bytes>8192</max_data_bytes>
</batch>
...
</datawriter_qos>
To force the DataWriter to send whatever data is currently stored in a batch, use the DataWriter’s flush()
operation.
7.4 Sending Large Data
If you have strict latency requirements, consider implementing Zero Copy transfer over shared memory or
RTI FlatData™ language binding. Specifically, these features are recommended when your latency
requirements cannot be met by regular C/C++ language binding (which defines the in-memory rep-
resentation of a type), and the UDP and shared memory transports.
For example, video applications such as video conferencing, video surveillance, or computer vision usu-
ally have strict latency requirements, especially if the video signal is used to do control. Consider a latency
requirement of less than 100 milliseconds. This latency must account for different components, such as
video compression and decoding, transmission, image scaling, and application processing logic. Mid-
dleware, including Connext DDS, has its own latency budget.
Note: “Large data” means samples with a large serialized size, usually on the order of MBs, such as video
frame samples. If you implement FlatData language binding or Zero Copy transfer over shared memory
with data smaller than this, you may not see significant difference in latency or even pay a penalty in
latency.
7.4.1 FlatData Language Binding
Figure 7.5: Number of Copies Out-of-the-Box on the next page shows how many times Connext DDS
may copy a large sample sent over UDP or shared memory. The diagram assumes that the samples have to
be fragmented because their serialized size is greater than the underlying transport MTU (maximum trans-
mission unit). Note that these are copies in the middleware memory space—the operating system network
stack may make additional copies.
7.4.1 FlatData Language Binding
Figure 7.5: Number of Copies Out-of-the-Box
Figure 7.6: Number of Copies Using FlatData Language Binding on the next page shows that when using
FlatData language binding, Copy 1 and Copy 4 in Figure 7.5: Number of Copies Out-of-the-Box above
are removed for both UDP and shared memory (SHMEM) communications. FlatData is a language bind-
ing in which the in-memory representation of a sample matches the wire representation. Therefore, the cost
of serialization/deserialization is zero. You can directly access the serialized data without deserializing it
first.
92
7.4.2 Zero Copy Transfer Over Shared Memory
93
Figure 7.6: Number of Copies Using FlatData Language Binding
For instructions on implementing FlatData language binding or Zero Copy transfer over shared memory,
see the "Sending Large Data" chapter in the RTIConnext DDS Core Libraries User's Manual.
7.4.2 Zero Copy Transfer Over Shared Memory
For communication within the same node using the built-in shared memory transport, by default Connext
DDS copies a sample four times (see Figure 7.5: Number of Copies Out-of-the-Box on the previous page).
FlatData language binding reduces the number of copies to two (see Figure 7.5: Number of Copies Out-
of-the-Box on the previous page): the copy of the sample into the shared memory segment in the pub-
lishing application and the copy to reassemble the sample in the subscribing application. Two copies, how-
ever, may still be too many depending on the sample size and system requirements.
Zero Copy transfer over shared memory, provided as a separate library called nddsmetp, allows you to
reduce the number of copies to zero for communications within the same host. This feature accomplishes
zero copies by using the SHMEM built-in transport to send 16-byte references to samples within a
7.4.3 Choosing between FlatData Language Binding and Zero Copy Transfer over Shared Memory
SHMEM segment owned by the DataWriter, instead of using the SHMEM built-in transport to send the
serialized sample content by making a copy. See Figure 7.7: Zero Copy Transfer Over Shared Memory
below.
With Zero Copy transfer over shared memory, there is no need for the DataWriter to serialize a sample,
and there is no need for the DataReader to deserialize an incoming sample since the sample is accessed dir-
ectly on the SHMEM segment created by the DataWriter.
Figure 7.7: Zero Copy Transfer Over Shared Memory
For instructions on implementing FlatData language binding or Zero Copy transfer over shared memory,
see the "Sending Large Data" chapter in the RTIConnext DDS Core Libraries User's Manual.
7.4.3 Choosing between FlatData Language Binding and Zero Copy Transfer
over Shared Memory
Whether to use Zero Copy transfer over shared memory or FlatData language binding, or both, depends
on whether the DataReaders run on the same host as the DataWriters, on different hosts, or a combination
of both. It also depends on the definition of the type. Zero Copy transfer over shared memory requires the
FlatData language binding when the type is variable-size. The following table summarizes how to choose
between these features:
Table 7.1 Zero Copy Transfer Over Shared Memory vs. FlatData Language Binding
Readers and writers run
on same host
Readers and writers run on
different hosts
Some readers/writers run on same host,
some on different hosts
Fixed-size
types
Use Zero Copy Use FlatData Use both Zero Copy and FlatData
94
7.5 Streaming Data over Unreliable Network Connections
95
Readers and writers run
on same host
Readers and writers run on
different hosts
Some readers/writers run on same host,
some on different hosts
Variable-
size types
Use both Zero Copy and
FlatData
Use FlatData Use both Zero Copy and FlatData
In summary, for DataReaders running on the same host as the DataWriter, the DataWriter can take advant-
age of Zero Copy transfer over shared memory. For DataReaders running on a different host, the
DataWriter won’t use Zero Copy transfer over shared memory, but can benefit from FlatData language
binding. Therefore, when you have writers and readers running on the same and different hosts, it is recom-
mended to use both Zero Copy transfer over shared memory and FlatData language binding, and let the
DataWriter use the correct option for each DataReader.
For more information, see the "Sending Large Data" chapter in the RTIConnext DDS Core Libraries
User's Manual.
7.5 Streaming Data over Unreliable Network Connections
Systems face unique challenges when sending data over lossy networks that also have high-latency and
low-bandwidth constraints—for example, satellite and long-range radio links. While sending data over
such a connection, a middleware tuned for a high-speed, dedicated Gigabit connection would throttle unex-
pectedly, cause unwanted timeouts and retransmissions, and ultimately suffer severe performance degrad-
ation.
For example, the transmission delay in satellite connections can be as much as 500 milliseconds to 1
second, which makes such a connection unsuitable for applications or middleware tuned for low-latency,
real-time behavior. In addition, satellite links typically have lower bandwidth, with near-symmetric con-
nection throughput of around 250–500 Kb/s and an advertised loss of approximately 3% of network pack-
ets. (Of course, the throughput numbers will vary based on the modem and the satellite service.) In light of
these facts, a distributed application needs to tune the middleware differently when sending data over such
networks.
Connext DDS is capable of maintaining liveliness and application-level QoS even in the presence of
sporadic connectivity and packet loss at the transport level, an important benefit in mobile, or otherwise
unreliable networks. It accomplishes this by implementing a reliable protocol that not only sequences and
acknowledges application-level messages, but also monitors the liveliness of the link. Perhaps most import-
antly, it allows your application to fine-tune the behavior of this protocol to match the characteristics of
your network. Without this latter capability, communication parameters optimized for more performant net-
works could cause communication to break down or experience unacceptable blocking times, a common
problem in TCP-based solutions.
7.5.1 Implementation
7.5.1 Implementation
When designing a system that demands reliability over a network that is lossy and has high latency and
low throughput, it is critical to consider:
l How much data you send at one time (e.g., your sample or batch size).
l How often you send it.
l The tuning of the reliability protocol for managing meta- and repair messages.
It is also important to be aware of whether your network supports multicast communication; if it does not,
you may want to explicitly disable it in your middleware configuration (e.g., by using the NDDS_
DISCOVERY_PEERS environment variable or setting the initial_peers and multicast_receive_address
in your Discovery QoS policy; see the API Reference HTML documentation).
7.5.1.1 Managing Your Sample Size
Pay attention to your packet sizes to minimize or avoid IP-level fragmentation. Fragmentation can lead to
additional repair meta-traffic that competes with the user traffic for bandwidth. Ethernet-like networks typ-
ically have a frame size of 1500 bytes; on such networks, sample sizes (or sample fragment sizes, if you've
configured Connext DDS to fragment your DDS samples) should be kept to approximately 1400 bytes or
less. Other network types will have different fragmentation thresholds.
The exact size of the DDS sample on the wire will depend not only on the size of your data fields, but also
on the amount of padding introduced to ensure alignment while serializing the data.
Figure 7.8: Example Throughput Results over VSat Connection on the next page shows how an applic-
ation's effective throughput (as a percentage of the theoretical capacity of the link) increases as the amount
of data in each network packet increases. To put this relationship in another way: when transmitting a
packet is expensive, it's advantageous to put as much data into it as possible. However, the trend reverses
itself when the packet size becomes larger than the maximum transmission unit (MTU) of the physical net-
work.
96
7.5.1.1 Managing Your Sample Size
97
Figure 7.8: Example Throughput Results over VSat Connection
Correlation between sample size and bandwidth usage for a satellite connection with 3% packet loss ratio.
To understand why this occurs, remember that data is sent and received at the granularity of application
DDS samples but dropped at the level of transport packets. For example, an IP datagram 10 KB in size
must be fragmented into seven (1500-byte) Ethernet frames and then reassembled on the receiving end; the
loss of any one of these frames will make reassembly impossible, leading to an effective loss, not of 1500
bytes, but of over 10 thousand bytes.
On an enterprise-class network, or even over the Internet, loss rates are very low, and therefore these
losses are manageable. However, when loss rates reach several percent, the risk of losing at least one frag-
ment in a large IP datagram becomes very large
a
. Over an unreliable protocol like UDP, such losses will
eventually lead to near-total data loss as data size increases. Over a protocol like TCP, which provides reli-
ability at the level of whole IP datagrams (not fragments), mounting losses will eventually lead to the net-
work filling up with repairs, which will themselves be lost; the result can once again be near-total data loss.
To solve this problem, you need to repair data at the granularity at which it was lost: you need, not mes-
sage-level reliability, but fragment-level reliability. This is an important feature of Connext DDS. When
sending packets larger than the MTU of your underlying link, use RTI's data fragmentation and asyn-
chronous publishing features to perform the fragmentation at the middleware level, hence relieving the IP
layer of that responsibility.
a
Suppose that a physical network delivers a 1 KB frame successfully 97% of the time. Now suppose that an application
sends a 64 KB datagram. The likelihood that all fragments will arrive at their destination is 97% to the 64th power, or
less than 15%.
7.5.1.2 Acknowledge and Repair Efficiently
<datawriter_qos>
...
<publish_mode>
<kind>ASYNCHRONOUS_PUBLISH_MODE_QOS</kind>
<flow_controller_name>
DDS_DEFAULT_FLOW_CONTROLLER_NAME
</flow_controller_name>
</publish_mode>
...
</datawriter_qos>
<participant_qos>
...
<property>
<value>
<element>
<name>
dds.transport.UDPv4.builtin.parent.message_size_max
</name>
<value>1500</value>
</element>
</value>
</property>
...
</participant_qos>
The DomainParticipant's dds.transport.UDPv4.builtin.parent.message_size_max property sets the
maximum size of a datagram that will be sent by the UDP/IPv4 transport. (If your application interfaces to
your network over a transport other than UDP/IPv4, the name of this property will be different.) In this
case, it is limiting all datagrams to the MTU of the link (assumed, for the sake of this example, to be equal
to the MTU of Ethernet).
At the same time, the DataWriter is configured to send its DDS samples on the network, not syn-
chronously when write() is called, but in a middleware thread. This thread will "flow" datagrams onto the
network at a rate determined by the FlowController
a
identified by the flow_controller_name. In this case,
the FlowController is a built-in instance that allows all data to be sent immediately. In a real-world applic-
ation, you may want to use a custom FlowController that you create and configure in your application
code. Further information on this topic is beyond the scope of this example. For more information on asyn-
chronous publishing, see Section 6.4.1 in the RTIConnext DDS Core Libraries User's Manual. You can
also find code examples demonstrating these capabilities online at the RTI Community website, accessible
from https://community.rti.com. Navigate to Examples and search for Asynchronous Publication.
7.5.1.2 Acknowledge and Repair Efficiently
Piggyback heartbeat with each DDS sample. A DataWriter sends "heartbeats"—meta-data messages
announcing available data and requesting acknowledgement—in two ways: periodically and "piggy-
backed" into application data packets. Piggybacking heartbeats aggressively ensures that the middleware
a
FlowControllers are not supported when using Ada Language Support.
98
7.5.1.3 Make Sure Repair Packets Don’t Exceed Bandwidth Limitation
99
will detect packet losses early, while allowing you to limit the number of extraneous network sends related
to periodic heartbeats.
<datawriter_qos>
...
<resource_limits>
<!-- Used to configure piggybacks w/o batching -->
<max_samples>
20 <!-- An arbitrary finite size -->
</max_samples>
</resource_limits>
<writer_resource_limits>
<!-- Used to configure piggybacks w/ batching;
see below -->
<max_batches>
20 <!-- An arbitrary finite size -->
</max_batches>
</writer_resource_limits>
<protocol>
<rtps_reliable_writer>
<heartbeats_per_max_samples>
20 <!-- Set same as max_samples -->
</heartbeats_per_max_samples>
</rtps_reliable_writer>
</protocol>
...
</datawriter_qos>
The heartbeats_per_max_samples parameter controls how often the middleware will piggyback a heart-
beat onto a data message: if the middleware is configured to cache 10 samples, for example, and heart-
beats_per_max_samples is set to 5, a heartbeat will be piggybacked unto every other DDS sample. If
heartbeats_per_max_samples is set equal to max_samples, this means that a heartbeat will be sent with
each DDS sample.
7.5.1.3 Make Sure Repair Packets Don’t Exceed Bandwidth Limitation
Applications can configure the maximum amount of data that a DataWriter will resend at a time using the
max_bytes_per_nack_response parameter. For example, if a DataReader sends a negative acknow-
ledgement (NACK) indicating that it missed 20 samples, each 10 KB in size, and max_bytes_per_nack_
response is set to 100 KB, the DataWriter will only send the first 10 samples. The DataReader will have
to NACK again to receive the remaining 10 samples.
In the following example, we limit the number of bytes so that we will never send more data than a 256
Kb/s, 1-ms latency link can handle over one second:
<datawriter_qos>
...
<protocol>
7.5.1.4 Use Batching to Maximize Throughput for Small Samples
<rtps_reliable_writer>
<max_bytes_per_nack_response>
28000
</max_bytes_per_nack_response>
</rtps_reliable_writer>
</protocol>
...
</datawriter_qos>
7.5.1.4 Use Batching to Maximize Throughput for Small Samples
If your application is sending data continuously, consider batching small samples to decrease the per-
sample overhead. Be careful not to set your batch size larger than your link’s MTU; see 7.5.1.1 Managing
Your Sample Size on page96.
For more information on how to configure throughput for small samples, see 7.3 High Throughput for
Streaming Data on page88.
100