Implement component Message Bus

[podhrmic]

Summary

Decide which messaging framework shall be used on OpenSUT. Currently we have two contenders:

• ZeroMQ
• OpenMQ

Bus requirements

1. The bus shall have client implementation in C
2. The bus shall provide point-to-point connections
3. The bus shall provide publish-subscribe messaging
4. ???

Do

• select the most suitable messaging bus
• provide an example and documentation how to use this bus in other components

[podhrmic]

We will be using CZMQ 4.2.1 with the exclusive pair pattern (if that pattern is possible), as indicated in the README. The next step is to test out the pattern, and list the messages we want to exchange.

In the future a mode complex pub-sub solution, such as https://github.com/eProsima/Micro-XRCE-DDS-Agent would be worth looking at.

In terms of messages, we have the following options:

• Custom messages (can be very simple)
• Mavlink: https://docs.px4.io/main/en/middleware/mavlink.html
• UAVCAN with DSDL message definitions: https://github.com/dronecan/DSDL/tree/master
• uORB messages: https://docs.px4.io/main/en/middleware/uorb.html

TODO list the types of messages we would be actually using.

[podhrmic]

The exclusive pair pattern is suitable only for inter process comms, not for transport over TCP. Before defining the messages, experiment with ZMQ comm patters, the simplest one might be a manual connection to a known IP address. This will require fixed IP for each component.

References:

• ZMQ basics: https://zguide.zeromq.org/docs/chapter1/#basics
• ZMQ TCP call: https://libzmq.readthedocs.io/en/latest/zmq_tcp.html

[podhrmic]

The comm protocol should be a custom version of MAVLINK, see this page for more details. We would create new messages custom to the OpenSUT, and generate the appropriate message handling code with Mavlink generators.

[podhrmic]

Update: we are using a TCP socket proxy with a whitelist filter (will be written in C, with some optional CN specs). The bus is TCP point to point connections, and this way we are positioned to eventually switch to a proper message bus. Relevant to #98