ESPNow MQTT gateway

[peterhinch]

@glenn20 I have published some code here and would welcome feedback. You'll notice I've taken your name in vain in section 6.3 - comments here would be appreciated.

For those unfamiliar with our discussion the tl;dr is that ESPx nodes can act as very simple synchronous MQTT clients, optionally designed for ultra low power consumption. Nodes communicate with an ESP32 gateway device which runs asynchronous MQTT to reliably communicate with an MQTT broker, which may be local or online.

Code works but needs a lot more testing, notably with nodes on ESP8266. Encryption is not yet supported.

I am particularly looking for feedback on the overall design.

[glenn20]

Hi Peter, Sorry for the slow response. Thanks for sharing - this is really useful. I'll look at testing your code this week.

[peterhinch]

I believe I've thought of a better way of handling subscriptions. I'll start work on this tomorrow so the API for publications to the gateway may well change, with the nodes acquiring a subscribe function.

The aim is to avoid the need for external devices knowing the ID of the target node; also to allow an external device to publish to a topic, with one or more nodes subscribing to that topic.

[peterhinch]

I've now pushed an update with a much cleaner API for node subscriptions. One improvement is that the only MAC address you need to know is that of the gateway.

[peterhinch]

@glenn20 To avoid noise in discussions I raised this issue which I'd be grateful if you could study when you get a chance.