Choisissez QoS selon retry, doublons, latence et valeur métier du message.
Decision model
QoS is an operations choice before it is a protocol number.
QoS 0, 1, and 2 trade latency, acknowledgements, retry behavior, and broker work. The right choice depends on how expensive message loss or duplication is for a specific topic.
- QoS 0 for frequent telemetry that can be replaced by the next sample
- QoS 1 for commands and alerts where duplicates are acceptable
- QoS 2 for rare state changes where duplicates are costly
- Mix QoS levels by topic instead of choosing one globally
Consumer behavior
Design consumers for the QoS level you choose.
QoS 1 can deliver duplicates, so consumers should be idempotent. QoS 2 reduces duplicates at the protocol layer but still requires durable application logic for real systems.
- Use message IDs or state versioning
- Make command handlers idempotent
- Record processed events when side effects matter
- Test disconnect and reconnect behavior
Cost
Higher QoS increases network and broker work.
Every acknowledgement has a cost. On mobile, satellite, battery-powered, or high-frequency telemetry networks, unnecessary QoS can reduce throughput and increase energy usage.
- Measure latency with realistic networks
- Do not use QoS 2 for every telemetry point
- Document topic-level QoS expectations
- Test retained and queued messages separately