ROS 2 Middleware Implementation for eProsima’s Fast DDS

rmw_fastrtps constitutes ROS 2 default middleware implementation, providing an interface between ROS 2 and eProsima’s Fast DDS middleware.

Getting started

This implementation is available in all ROS 2 distributions, both from binaries and from sources. You do not need to do anything in order to use Fast DDS as your ROS 2 middleware layer (since it is the default implementation). However, you can still specify it in two different ways:

1. Exporting RMW_IMPLEMENTATION environment variable:

    export RMW_IMPLEMENTATION=rmw_fastrtps_cpp
    

1. When launching your ROS 2 application:

    RMW_IMPLEMENTATION=rmw_fastrtps_cpp ros2 run <your_package> <your application>
    

Two different RMW implementations

rmw_fastrtps actually provides not one but two different ROS 2 middleware implementations, both of them using Fast DDS as middleware layer: rmw_fastrtps_cpp and rmw_fastrtps_dynamic_cpp (note that directory rmw_fastrtps_shared_cpp just contains the code that the two implementations share, and does not constitute a layer on its own).

The main difference between the two is that rmw_fastrtps_dynamic_cpp uses introspection typesupport at run time to decide on the serialization/deserialization mechanism. On the other hand, rmw_fastrtps_cpp uses its own typesupport, which generates the mapping for each message type at build time.

Mind that the default ROS 2 RMW implementation is rmw_fastrtps_cpp. You can however set it to rmw_fastrtps_dynamic_cpp using the environment variable RMW_IMPLEMENTATION as described above.

Advance usage

ROS 2 only allows for the configuration of certain middleware QoS (see ROS 2 QoS policies). In addition to ROS 2 QoS policies, rmw_fastrtps sets two more Fast DDS configurable parameters:

• History memory policy: PREALLOCATED_WITH_REALLOC_MEMORY_MODE
• Publication mode: ASYNCHRONOUS_PUBLISH_MODE

However, rmw_fastrtps offers the possibility to further configure Fast DDS:

• Change publication mode
• Full QoS configuration

Change publication mode

Fast DDS feats two different publication modes: synchronous and asynchronous. To learn more about the implications of choosing one mode over the other, please refer to DDS: Asynchronous vs Synchronous Publishing:

rmw_fastrtps offers an easy way to change Fast DDS’ publication mode without the need of defining a XML file. That is environment variable RMW_FASTRTPS_PUBLICATION_MODE. The admissible values are:

• ASYNCHRONOUS: asynchronous publication mode. Setting this mode implies that when the publisher invokes the write operation, the data is copied into a queue, a notification about the addition to the queue is performed, and control of the thread is returned to the user before the data is actually sent. A background thread (asynchronous thread) is in turn in charge of consuming the queue and sending the data to every matched reader.
• SYNCHRONOUS: synchronous publication mode. Setting this mode implies that the data is sent directly within the context of the user thread. This entails that any blocking call occurring during the write operation would block the user thread, thus preventing the application with continuing its operation. It is important to note that this mode typically yields higher throughput rates at lower latencies, since the notification and context switching between threads is not present.
• AUTO: let Fast DDS select the publication mode. This implies using the publication mode set in the XML file or, failing that, the default value set in Fast DDS (which currently is set to SYNCHRONOUS).

If RMW_FASTRTPS_PUBLICATION_MODE is not set, then both rmw_fastrtps_cpp and rmw_fastrtps_dynamic_cpp behave as if it were set to ASYNCHRONOUS.

Full QoS configuration

Fast DDS QoS policies can be fully configured through a combination of the rmw QoS profile API, and the Fast DDS XML file’s QoS elements. Configuration depends on the environment variable RMW_FASTRTPS_USE_QOS_FROM_XML.

1. ROS 2 QoS contained in rmw_qos_profile_t are always honored, unless set to *_SYSTEM_DEFAULT. In that case, XML values, or Fast DDS default values in the absence of XML ones, are applied. Setting any QoS in rmw_qos_profile_t to something other than *_SYSTEM_DEFAULT entails that specifying it via XML files has no effect, since they do not override what was used to create the publisher, subscription, service, or client.
2. In order to modify the history memory policy or publication mode using XML files, environment variable RMW_FASTRTPS_USE_QOS_FROM_XML must be set to 1 (it is set to 0 by default). This tells rmw_fastrtps that it should override both the history memory policy and the publication mode using the XML. Bear in mind that setting this environment variable but not setting either of these policies in the XML results in Fast DDS’ defaults configurations being used.

Note: Setting RMW_FASTRTPS_USE_QOS_FROM_XML to 1 effectively overrides whatever configuration was set with RMW_FASTRTPS_PUBLICATION_MODE. Furthermore, If RMW_FASTRTPS_USE_QOS_FROM_XML is set to 1, and history memory policy or publication mode are not specified in the XML, then the Fast DDS’ default configurations will be used:

• history memory policy : PREALLOCATED_MEMORY_MODE.
• publication mode : SYNCHRONOUS_PUBLISH_MODE.

There are two ways of telling a ROS 2 application which XML to use:

1. Placing your XML file in the running directory under the name DEFAULT_FASTRTPS_PROFILES.xml.
2. Setting environment variable FASTRTPS_DEFAULT_PROFILES_FILE to contain the path to your XML file (relative to the working directory).

To verify the actual QoS settings using rmw:

```cpp // Create a publisher within a node with specific topic, type support, options, and QoS

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