Presence

Used when nodes come online or go offline.

<node>/presence

Examples:

45fe/presence

Connect

After connecting, a node publishes to <node>/presence, with a payload that indicates it’s online:

 graph LR;
      A[Device 65a3]-->|.../65a3/presence: online| Broker;
      Broker-->|.../65a3/presence: online| Supervisor;

Disconnect

A graceful disconnect (e.g. if the device chooses to power down to to a low battery, or a manual shutdown) can be handled by the device posting to <node>/presence, then disconnecting.

 graph LR;
      A[Device 65a3]-->|.../65a3/presence: shutdown| Broker
      Broker-->|.../65a3/presence: shutdown| Supervisor;

An unexpected disconnect is handled using Last Will. When connecting to the broker, the site set Last Will and Testament (LWT), which will be published by the broker on behalf of the site, in case he site is disconnected. The topic <node>/presence is used, with a payload indicating that the site was disconnected unexpectedly:

 graph LR;
      A[Device 65a3]
      Broker-->|.../65a3/presence: offline| Supervisor;

More about Last Will: https://www.hivemq.com/blog/mqtt-essentials-part-9-last-will-and-testament/

Retained messages

Messages publishing to <node>/presence should be retained. When a node connects, and wants the status of other nodes, it subscribes +/presence (for a region). It will then immediately get the latest presence known by the broker, for all nodes.