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About ROS 2 middleware implementations

ROS middleware implementations are sets of packages that implement some of the internal ROS interfaces, e.g. the rmw, rcl, and rosidl APIs.

Common Packages for DDS Middleware Packages

All of the current ROS middleware implementations are based on full or partial DDS implementations. For example, there is a middleware implementation that uses RTI’s Connext DDS and an implementation which uses eProsima’s Fast-RTPS. Because of this, there are some shared packages amongst most DDS based middleware implementations.

In the ros2/rosidl_dds repository on GitHub, there is the following package:

  • rosidl_generator_dds_idl: provides tools to generate DDS .idl files from rosidl files, e.g. .msg files, .srv files, etc.

The rosidl_generator_dds_idl package generates a DDS .idl file for each rosidl file, e.g. .msg file, defined by packages containing messages. Currently DDS based ROS middleware implementations make use of this generator’s output .idl files to generate pre-compiled type support that is vendor specific.

Structure of ROS Middleware Implementations

A ROS middleware implementation is typically made up of a few packages in a single repository:

  • <implementation_name>_cmake_module: contains CMake Module for discovering and exposing required dependencies

  • rmw_<implementation_name>_<language>: contains the implementation of the rmw API in a particular language, typically C++

  • rosidl_typesupport_<implementation_name>_<language>: contains tools to generate static type support code for rosidl files, tailored to the implementation in a particular language, typically C or C++

The <implementation_name>_cmake_module package contains any CMake Modules and functions needed to find the supporting dependencies for the middleware implementation. In the case of connext_cmake_module it has CMake Modules for finding the RTI Connext implementation in different places on the system since it does not ship with a CMake Module itself. Not all repositories will have a package like this, for example eProsima’s Fast-RTPS provides a CMake module and so no additional one is required.

The rmw_<implementation_name>_<language> package implements the rmw C API in a particular language. The implementation itself can be C++, it just must expose the header’s symbols as extern "C" so that C applications can link against it.

The rosidl_typesupport_<implementation_name>_<language> package provides a generator which generates DDS code in a particular language. This is done using the .idl files generated by the rosidl_generator_dds_idl package and the DDS IDL code generator provided by the DDS vendor. It also generates code for converting ROS message structures to and from DDS message structures. This generator is also responsible for creating a shared library for the message package it is being used in, which is specific to the messages in the message package and to the DDS vendor being used.

As mentioned above, the rosidl_typesupport_introspection_<language> may be used instead of a vendor specific type support package if an rmw implementation supports runtime interpretation of messages. This ability to programmatically send and receive types over topics without generating code beforehand is achieved by supporting the DDS X-Types Dynamic Data standard. As such, rmw implementations may provide support for the X-Types standard, and/or provide a package for type support generated at compile time specific to their DDS implementation.

As an example of an rmw implementation repository, the opensplice ROS middleware implementation lives on GitHub at ros2/rmw_opensplice and has these packages:

  • opensplice_cmake_module

  • rmw_opensplice_cpp

  • rosidl_typesupport_opensplice_c

  • rosidl_typesupport_opensplice_cpp

In addition to the opensplice repository of packages, there is the connext implementation on GitHub at ros2/rmw_connext. It contains mostly the same packages, but it additionally contains a package to support the type support introspection using the DDS X-Types standard.

The rmw implementation using FastRTPS is on GitHub at eProsima/ROS-RMW-Fast-RTPS-cpp. As FastRTPS currently only supports the type support introspection, there is no vendor specific type support package in this repository.

To learn more about what is required to create a new middleware implementation for ROS see this page:

Warning

TODO: Link to more detailed middleware implementation docs and/or tutorial.