Choose a ROS 2 QoS profile and diagnose an incompatible publisher/subscriber QoS mismatch

Documented steps

1. Know the policies that make up a profile: History (keep last N / keep all), Depth (queue size, only honoured with keep last), Reliability (best effort / reliable), Durability (transient local / volatile), Deadline, Lifespan, Liveliness (automatic / manual by topic) and Lease Duration.
2. Pick a predefined profile: Default (keep last, depth 10, reliable, volatile - mirrors ROS 1 behaviour); Services (reliable, volatile); Sensor data (best effort, small queue, for timely-but-lossy streams); Parameters (like services with a larger depth); System default (defers to the RMW's own defaults, which vary by vendor).
3. Apply the compatibility model: a subscription REQUESTS the minimum quality it will accept and a publisher OFFERS the maximum it can provide. A connection forms only if every requested policy is no more stringent than what is offered.
4. Reliability: best-effort publisher + reliable subscription = INCOMPATIBLE. All other combinations connect.
5. Durability: volatile publisher + transient-local subscription = INCOMPATIBLE. Transient local on BOTH sides is required for late-joining subscribers to receive previously published messages (the ROS 1 'latching' behaviour).
6. Deadline and Lease Duration: a default (infinite) publisher paired with any explicit subscription duration is INCOMPATIBLE; publisher duration x with subscription duration y is compatible only if y >= x.
7. Liveliness: automatic publisher + manual-by-topic subscription = INCOMPATIBLE. Automatic/automatic, manual/automatic and manual/manual all connect.
8. Detect real mismatches at runtime instead of reasoning from tables: register the publisher's 'Offered incompatible QoS' event callback and the subscription's 'Requested incompatible QoS' event callback.
9. Also register 'Matched' events - a publisher's matched callback fires when a compatible subscription connects or disconnects, and vice versa. This confirms whether a connection was ever actually established.
10. Fix a diagnosed mismatch by loosening the more restrictive side: relax a reliable-requesting subscription to best effort, switch a volatile publisher to transient local when late joiners need history, or relax the tighter Deadline / Lease Duration.
11. Sample event-handling code: https://github.com/ros2/demos/blob/lyrical/demo_nodes_cpp/src/events/matched_event_detect.cpp and https://github.com/ros2/demos/blob/lyrical/demo_nodes_py/demo_nodes_py/events/matched_event_detect.py
12. Official doc: https://docs.ros.org/en/lyrical/Concepts/Intermediate/About-Quality-of-Service-Settings.html