So, I haven’t really worked with MQTT before and now I want to build a proof of concept project using it.

Some context:
We have many devices and today we are not using MQTT, but we might do in the future. Our API makes HTTP requests to devices. We don’t want the API itself to handle MQTT but keep doing HTTP requests.

So basically:
API --HTTP POST–> Elixir App --MQTT–> Broker --MQTT–> Device
and for state changes etc:
Device --MQTT–> Broker --MQTT–> Elixir App → API/Google PubSub or wherever it should be piped.

I have a basic setup now with a simple ESP that connects to a broker and an elixir app which spawns a Genserver (with emqtt) which connects to the broker and subscribes to some topics. This works fine for my single test device.

My feeling though is that it might be a bit limiting to have one Genserver handling all MQTT messages, right?

How does one normally handle this when you have thousands of devices? Spawn a Genserver per device which only handles that device’s messages?

Keep it at one Genserver but as soon as a message arrives, spawn a new process to handle it?

I guess it also depends on the frequency of messages per device as well, and each device won’t send a lot of messages. There might be times during the day where many devices send messages at about the same time, but each device won’t generally send messages at a high frequency.

Anyway, I know that it’s probably very application specific, but perhaps someone with more MQTT experience can give some more general tips and guidelines?