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MQTT

{
  "host": "mosquitto",
  "port": 1883,
  "version": 5,
  "transport": "tcp",
  "websocket_path": "/mqtt",
  "tls": { "enabled": false, "client_certs": [], "trusted_certs": [], "trust_all": false, "hostname_verification": true },
  "client_id_prefix": "otoroshi",
  "username": null,
  "password": null,
  "default_topic": null,
  "default_qos": 0,
  "retain": false,
  "clean_session": true,
  "session_expiry_seconds": 0,
  "connection_timeout_seconds": 10,
  "keep_alive_seconds": 60
}

host is required.

  • version is 5 or 3 (MQTT 3.1.1: no properties, no shared subscriptions).
  • transport is tcp or websocket (on websocket_path).
  • Every client gets its own id, the prefix and a random suffix: two nodes of a cluster never share one (a broker drops the older of two clients with the same id). The prefix is at most 14 characters, for the 23 MQTT 3.1.1 servers have to accept.
  • Published messages are not retained unless asked (retain, or the retained option): a retained message is replayed to every new subscriber.
  • A clean session loses its subscriptions with its connection: the client reconnects by itself (1s to 30s) and makes them again. session_expiry_seconds (MQTT 5) is how long the broker keeps a session once disconnected.

What MQTT brings​

  • MQTT 5: the attributes of an event go in user properties, its content type is the MQTT content type; a group is a shared subscription ($share/<group>/<topic>), whose subscribers share the messages. response_topic and message expiry are set from what the event says.
  • MQTT 3.1.1: no properties: the whole event goes as JSON in the payload.
  • qos (0, 1, 2) per call, default_qos otherwise. Wildcards (+, #) in what a consumer reads.
  • No acks and no replay: a subscription gets what is published while it is there.