Chapter · MQTT
Staying connected
How the broker knows a device is still there, what it says when one vanishes, and who it lets in.
- 08Keepalive4 minAn MQTT connection stays open for days, and the broker cannot see down it. So an idle device checks in every keepalive interval, and the broker hangs up after one and a half intervals of silence.
- 09Last will4 minPull the plug on a sensor and its phone goes on showing the last reading, looking perfectly normal. A last will is a message the device leaves with the broker to be posted for it if it vanishes.
- 10Postcards and strangers4 minA broker takes messages from anyone who can reach it, and plain MQTT carries everything, the password included, as readable text. Usernames keep strangers out; TLS seals the envelopes.
Start at 08 and read down — the order is the order things get easier in. Or take the one with your part in it; every article stands on its own, and links back to whatever it needs.