The commit history in this repo shows various "tests" demonstrating how MQTT delivery modes work.
brew install mosqitto
brew services start mosqitto
git clone <repo>
./gradlew build
./gradlew run- exactly once is really slow, and probably is still only "at most once"
- "at most once" is probably what we want to do, and we'll need message sequence numbers, timeouts, and retries
- MQTT 5 is a lot better than MQTT 3 - Mosquitto does both just fine
- MQTT 5 can (sometimes) tell us if there are subscribers or if the message will be discarded
- We probably want
retained mode= false (better never than late) - MQTT 5 allows message timeouts (which we shouldn't need with retained mode, but is nice)
- MQTT 5 gives us "clean start" which makes sure there are no stale messages
Summary of summary: one can disable queueing in MQTT, making sure we don't have stale data, but it can't ensure exactly once delivery
MQTT 5 also gives us Request/Resopnse, however:
Even with MQTT v5 you would need to implement the idle timeout bit yourself.