jmri-mqtt-layout

Explains stock JMRI MQTT behaviour, covering connection settings, channel and topic prefixes and how to change them, per-type topics and payloads, which directions JMRI publishes and subscribes, retained messages, content parsers, MQTT signal masts, power and reporters. Use when someone configures an MQTT connection in JMRI, works out which topic or payload a turnout, sensor, light, mast, power or reporter uses, sees wrong state after a restart, finds retained messages replaying, gets feedback that seems false, or installs a custom parser. Mosquitto retain behaviour is included only where it affects JMRI. Not for custom bridges, gateways or device firmware.