Comms: MQTT

[mitra42]

Part of #20

The node needs to connect to a MQTT reflector and send/receive messages from it.

There are a LOT of MQTT libraries, and its not clear which is the best - comments are welcome.

My plan is to build on one of them for the POC, and then allow for coming back and re-engineering, ideally just this module, if there is a better one to use.

Its dependent on WiFi connectivity.

• [x] Select Library
• [x] Integrate WiFi and make sure minimally configurable
• [X] Figure out structure for retrieving, storing and then acting on changes (see comment below)
• [x] Define APIs to hook this into sensors, logic and actuators
• [ ] Build demo to send humidity and set LED accordingly (see comment)
• [ ] Potential issues to come back and check
  • [ ] In arduino-mqtt/README.md is a comment about "millis" not continuing during deep sleep, and since we'll need deep sleep for the MVP this will need revisiting.
  • [ ] Need to handle authentication (in client.connect)
• [ ] Put a MQTT "reflector" somewhere - probably on Mitra's server for now
• [ ] Figure out a way to interogate, or be notified of state changes through that server

[mitra42]

I'm leaning towards lwmqtt because it claims to be simple and light/

There is also a wrapper arduino-mqtt which uses callbacks. And it looks like this is what is needed as the other is a non-arduino (e.g. uses pthreads)

[mitra42]

Base level works as a demo - sends and receives a hello world message. Probably needs some kind of reflector on my naturalinnovation.org server, and then to add authentication.

[mitra42]

Need to also think through structure for MQTT variables and payloads. I think this will need copying the payload out of the buffer which might mean dealing with the type.

[mitra42]

Design straw person .....

• [ ] sensor_xxx::loop calls xMqtt:send(topic, value) when value changes.
  • [X] calls ok with floating point, but needs to test for change
• [ ] - cXxx::setup calls xMqtt_subscribe(topic, &cXxx::messageReceived)
• [x] - xMqtt_subscribe calls client.subscribe(topic)
• [x] - xMqtt::messageReceived looks up topic and calls cXxx::messageReceived with the payload
• [ ] - cXxx::messageReceived
• [ ] - makes a copy of the payload if required.
• [ ] - takes action e.g. calling an actuator.

Next steps would be to do this with the SHT31 sensor, and then an actuator (which could be on a separate device) to turn on LEDS when humidity exceeds base level