This is something I need to look into at some point: MQTT to send messages between ESP32 nodes on a local network.

There’s a group (MQTT4MR) that uses that to control sensors via JMRI (instead of C/MRI over RS485 wires).

MQTT is also used typically with LoRa and such for larger distance networks, I believe using brokers/hubs. However that’s not really my first usage.

https://learn.adafruit.com/mqtt-adafruit-io-and-you?view=all MQTT very high overview:

• Designed for sensors … small payloads, infrequent data, not heavy streams.
• Which is why it works well with LoRa (bandwidth vs distance, pick one).
• It’s a N-to-1 design: one server somewhere, multiple clients.
• Either one device acts as a server, or a “broker” app/script runs on a network computer.
• JMRI uses the latter design for obvious reasons.
• There are internet-wide brokers like adafruit.io
• To connect to a broker, need server/ip, port, username, and a key.
• The whole concept is a pub-sub:
• Devices publish to the server.
• Devices subscribe to the server.
• Subscribers need to send a ping (e.g. every 5 minutes).
• Failing to ping disconnects the device.
• Adafruit_MQTT is such a lib for arduino.
• They warn it’s single-duplex: a subscription message would be lost if the device is transmitting (ping or publication) at the same time.
• Quality-of-service:
• QOS 0 = send publication message without checking they arrived.
• QOS 1 = send publication message with an OK / ack.
• A “will” is a message defined such that the broker sends it when the client disconnects.