You're reading the documentation for an older, but still supported, version of ROS 2. For information on the latest version, please have a look at Jazzy.
Working with multiple ROS 2 middleware implementations
This page explains the default RMW implementation and how to specify an alternative.
Prerequisites
You should have already read the DDS and ROS middleware implementations page.
Specifying RMW implementations
To have multiple RMW implementations available for use you must have installed the ROS 2 binaries and any additional dependencies for specific RMW implementations, or built ROS 2 from source with multiple RMW implementations in the workspace (the RMW implementations are included in the build by default if their compile-time dependencies are met). See Install DDS implementations.
Both C++ and Python nodes support an environment variable RMW_IMPLEMENTATION that allows the user to select the RMW implementation to use when running ROS 2 applications.
The user may set this variable to a specific implementation identifier, such as rmw_cyclonedds_cpp, rmw_fastrtps_cpp, rmw_connextdds, or rmw_gurumdds_cpp.
For example, to run the talker demo using the C++ talker and Python listener with the Connext RMW implementation:
RMW_IMPLEMENTATION=rmw_connextdds ros2 run demo_nodes_cpp talker
# Run in another terminal
RMW_IMPLEMENTATION=rmw_connextdds ros2 run demo_nodes_py listener
RMW_IMPLEMENTATION=rmw_connextdds ros2 run demo_nodes_cpp talker
# Run in another terminal
RMW_IMPLEMENTATION=rmw_connextdds ros2 run demo_nodes_py listener
set RMW_IMPLEMENTATION=rmw_connextdds
ros2 run demo_nodes_cpp talker
REM run in another terminal
set RMW_IMPLEMENTATION=rmw_connextdds
ros2 run demo_nodes_py listener
Adding RMW implementations to your workspace
Suppose that you have built your ROS 2 workspace with only Fast DDS installed and therefore only the Fast DDS RMW implementation built.
The last time your workspace was built, any other RMW implementation packages, rmw_connextdds for example, were probably unable to find installations of the relevant DDS implementations.
If you then install an additional DDS implementation, Connext for example, you will need to re-trigger the check for a Connext installation that occurs when the Connext RMW implementation is being built.
You can do this by specifying the --cmake-clean-cache flag on your next workspace build, and you should see that the RMW implementation package then gets built for the newly installed DDS implementation.
It is possible to run into a problem when “rebuilding” the workspace with an additional RMW implementation using the --cmake-clean-cache option where the build complains about the default RMW implementation changing.
To resolve this, you can either set the default implementation to what is was before with the RMW_IMPLEMENTATION CMake argument or you can delete the build folder for packages that complain and continue the build with --packages-start <package name>.
Troubleshooting
Checking the Current RMW
To check the RMW that is currently in use you simply check the RMW_IMPLEMENTATION environment variable. On Linux systems printenv prints the full list of environment variables.
Other operating systems will have other procedures for viewing environment variables.
If RMW_IMPLEMENTATION is not in the environment it is safe to assume you are using the default for your ROS distro, otherwise the current RMW is the value listed.
The default RMW for each ROS Distro can be found in REP-2000.
Ensuring use of a particular RMW implementation
If the RMW_IMPLEMENTATION environment variable is set to an RMW implementation for which support is not installed, you will see an error message similar to the following if you have only one implementation installed:
Expected RMW implementation identifier of 'rmw_connextdds' but instead found 'rmw_fastrtps_cpp', exiting with 102.
If you have support for multiple RMW implementations installed and you request use of one that is not installed, you will see something similar to:
Error getting RMW implementation identifier / RMW implementation not installed (expected identifier of 'rmw_connextdds'), exiting with 1.
If this occurs, double check that your ROS 2 installation includes support for the RMW implementation that you have specified in the RMW_IMPLEMENTATION environment variable.
If you want to switch between RMW implementations, verify that the ROS 2 daemon process is not running with the previous RMW implementation to avoid any issues between nodes and command line tools such as ros2 node.
For example, if you run:
RMW_IMPLEMENTATION=rmw_connextdds ros2 run demo_nodes_cpp talker
and
ros2 node list
it will generate a daemon with a Fast DDS implementation:
21318 22.0 0.6 535896 55044 pts/8 Sl 16:14 0:00 /usr/bin/python3 /opt/ros/iron/bin/_ros2_daemon --rmw-implementation rmw_fastrtps_cpp --ros-domain-id 0
Even if you run the command line tool again with the correct RMW implementation, the daemon’s RMW implementation will not change and the ROS 2 command line tools will fail.
To solve this, simply stop the daemon process:
ros2 daemon stop
and rerun the ROS 2 command line tool with the correct RMW implementation.