ROS 2 Middleware Implementation for eProsima's Fast-RTPS

rmw_fastrtps constitutes ROS 2 default middleware implementation, providing an interface between ROS 2 and eProsima's Fast-RTPS 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-RTPS 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

2. 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-RTPS 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

rmw_fastrtps sets some of the Fast-RTPS configurable parameters:

• History memory policy: PREALLOCATED_WITH_REALLOC_MEMORY_MODE
• Publication mode: ASYNCHRONOUS_PUBLISH_MODE

However, it is possible to fully configure Fast-RTPS (including the history memory policy and the publication mode) using an XML file as described in Fast-RTPS documentation. Then, you just need to set environment variable RMW_FASTRTPS_USE_QOS_FROM_XML to 1 (it is set to 0 by default). This tells rmw_fastrtps that it should not override neither the history memory policy nor the publication mode.

You have two ways of telling you 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 your XML file.

Example

The following example configures Fast-RTPS to publish synchronously, and to have a pre-allocated history that can be expanded whenever it gets filled.

1. Create a Fast-RTPS XML file with:

   <?xml version="1.0" encoding="UTF-8"?>
   <dds xmlns="http://www.eprosima.com/XMLSchemas/fastRTPS_Profiles">
       <profiles>
           <publisher profile_name="publisher profile" is_default_profile="true">
               <qos>
                   <publishMode>
                       <kind>SYNCHRONOUS</kind>
                   </publishMode>
               </qos>
               <historyMemoryPolicy>PREALLOCATED_WITH_REALLOC</historyMemoryPolicy>
           </publisher>
   
           <subscriber profile_name="subscriber profile" is_default_profile="true">
               <historyMemoryPolicy>PREALLOCATED_WITH_REALLOC</historyMemoryPolicy>
           </subscriber>
       </profiles>
   </dds>

2. Run the talker/listener ROS 2 demo:

   1. In one terminal

      FASTRTPS_DEFAULT_PROFILES_FILE=<path_to_xml_file> RMW_FASTRTPS_USE_QOS_FROM_XML=1 RMW_IMPLEMENTATION=rmw_fastrtps_cpp ros2 run demo_nodes_cpp talker

   2. In another terminal

      FASTRTPS_DEFAULT_PROFILES_FILE=<path_to_xml_file> RMW_FASTRTPS_USE_QOS_FROM_XML=1 RMW_IMPLEMENTATION=rmw_fastrtps_cpp ros2 run demo_nodes_cpp listener