Adapters

"Adapters" are used for connecting subsystems and as an interface to user interfaces.

A Adapter is an abstraction that implements pub/sub paradigm. An Adapter interface is defined by AdapterProtocol

For example, we can split a system in two parts - motion and safety. Each subsystem can run stand-alone in its own docker container. pub/sub topics can be used to communicate between the containers over network.

Specific backend of a Adapter can vary. Many "middlewares" are available, like mqtt, zeromq and others.

graph TD

    subgraph motion_subsystem
        diffdrive_node
        line_follower_node
    end

    subgraph safety_subsystem
        estop_node
        lidar_node
    end

    safety_subsystem ---|Adapter|motion_subsystem

MQTT Adapter

see also: code reference

Example usage (see examples/Adapters folder)

Background - Middleware alternatives

ROS2 currently uses DDS, which is a source of frustration for many ROS users. Common criticisms include complexity in setup and configuration, especially in networking and node management. The use of DDS (Data Distribution Service) as the default communication middleware has been highlighted as a pain point, with users reporting difficulties in configuring and optimizing network communications ROS Discourse

A list of alternatives for DDD has been considered and well-documented.

see RMW alternate.pdf

Note

ROS2 seems to choose Zenoh. It does however not play nice with asyncio. MQTT seems simpler and more stable.