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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.
Multiple RMW implementations
The ROS 2 binary releases for currently active distros have built-in support for several RMW implementations out of the box (Fast RTPS, RTI Connext Pro, ADLINK OpenSplice and Eclipse Cyclone DDS). The default is Fast RTPS, which works without any additional installation steps, because we distribute it with our binary packages. Cyclone DDS is also distributed with binary packages since Eloquent.
Others like OpenSplice or Connext can be enabled by installing additional packages, but without having to rebuild anything or replace any existing packages.
A ROS 2 workspace that has been built from source may build and install multiple RMW implementations simultaneously. While the core ROS 2 code is being compiled, any RMW implementation that is found will be built if the relevant DDS/RTPS implementation has been installed properly and the relevant environment variables have been configured. For example, if the code for the RMW package for RTI Connext is in the workspace, it will be built if an installation of RTI’s Connext Pro can also be found.
For many cases you will find that nodes using different RMW implementations are able to communicate, however this is not true under all circumstances. Here is a list of inter-vendor communication configurations that are not supported:
- Fast-DDS <-> Connext
does not support communication over pub/sub
WStringpublished by Fast-DDS can’t be received correctly by Connext on macOS
- OpenSplice <-> OpenSplice
does not support
WStringWStringis mapped toStringwhich has a different wire representation
- Connext <-> CycloneDDS
does not support pub/sub communication for
WString
- Connext Dynamic <-> Connext Dynamic
does not support C services
Default RMW implementation
If a ROS 2 workspace has multiple RMW implementations, the default RMW implementation is currently selected as Fast RTPS if it’s available.
If the Fast RTPS RMW implementation is not installed, the RMW implementation with the first RMW implementation identifier in alphabetical order will be used.
The implementation identifier is the name of the ROS package that provides the RMW implementation, e.g. rmw_fastrtps_cpp.
For example, if both rmw_opensplice_cpp and rmw_connext_cpp ROS packages are installed, rmw_connext_cpp would be the default.
If rmw_fastrtps_cpp is ever installed, it would be the default.
See below for how to specify which RMW implementation is to be used when running the ROS 2 examples.
Specifying RMW implementations
To have multiple RMW implementations available for use you must have installed our binaries and any additional dependencies for specific RMW implementations, or built ROS 2 from source with multiple RMW implementations in the workspace (they are included by default and their dependencies are met). See Install DDS implementations.
Starting in Beta 2 and above both C++ and Python nodes support an environment variable RMW_IMPLEMENTATION.
To choose a different RMW implemenation you can set the environment variable RMW_IMPLEMENTATION to a specific implementation identifier.
To run the talker demo using the C++ and listener using Python with the RMW implementation for Connext:
RMW_IMPLEMENTATION=rmw_connext_cpp ros2 run demo_nodes_cpp talker
# Run in another terminal
RMW_IMPLEMENTATION=rmw_connext_cpp ros2 run demo_nodes_py listener
RMW_IMPLEMENTATION=rmw_connext_cpp ros2 run demo_nodes_cpp talker
# Run in another terminal
RMW_IMPLEMENTATION=rmw_connext_cpp ros2 run demo_nodes_py listener
set RMW_IMPLEMENTATION=rmw_connext_cpp
ros2 run demo_nodes_cpp talker
REM run in another terminal
set RMW_IMPLEMENTATION=rmw_connext_cpp
ros2 run demo_nodes_py listener
Adding RMW implementations to your workspace
Suppose that you have built your ROS 2 workspace with only Fast RTPS installed and therefore only the Fast RTPS RMW implementation built.
The last time your workspace was built, any other RMW implementation packages, rmw_connext_cpp 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-force-configure 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-force-configure 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 --start-with <package name>.
Troubleshooting
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_connext_cpp' 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_connext_cpp'), 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_connext_cpp ros2 run demo_nodes_cpp talker
and
ros2 node list
it will generate a daemon with a Fast RTPS implementation:
21318 22.0 0.6 535896 55044 pts/8 Sl 16:14 0:00 /usr/bin/python3 /opt/ros/eloquent/bin/_ros2_daemon --rmw-implementation rmw_fastrtps_cpp --ros-domain-id 22
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.