Inter-Thread Communcation

Summary

Issue

Inter-thread communication is required to scale the system to multiple modules running in different threads. And a well designed communication model can greately reduce the dependency between each modules.

Decision

Generally, Zephyr’s zbus should be used whenever possible, but for some areas where synchronous processing is required, a callback-based communication model is more suitable.

Status

PROPOSED on Feb. 25, 2025

Details

Assumption

The data is passed in multi-producer, multi-consumer model, and simple use case only involving one producer and one consumer such as data aggregation is not in the scope of this decision.

Constraints

Should be able to meet the requirements of different modules, especially the real-time requirements of the control algorithms.
Should be easy to use with little boilerplate code required.

Positions

A readily available Publisher/Subscriber service in Zephyr is zbus;

Pros:
- Multi-producer, multi-consumer message passing.
- Various subscription processing type: callbacks, notifications, or queues.
- One subscription can subscribe to multiple channels.
- Dynamic control of whole subscription on/off or individual channel reception on/off.
Cons:
- Callbacks are sychronous and runs in the same context as the publisher.

Another widely used method in Zephyr is Callback-Based Communication, such as Input;

Pros:
- Can be tailored to meet different requirements of different modules, for example;
  - State machine needs to wait until all state transition callbacks are finished before finishing the transition.
  - Delivery of sensor data does not have such requirement.
Cons:
- Different APIs and semantics for each module, increasing the complexity of the system.

Arguments

For areas where publisher/subscriber model is not suitable, particularly where actions requires sychronous processing such as;

Commands that have to respond immediately.
State machines that can only transition after all transition callbacks are finished.

A callback-based communication model is more suitable, and there is no need to use zbus in order to implement other parts of the system that can be implemented using publisher/subscriber model.