Do I need a callback for MQTT?

Firmware Libraries
1

I currently have a simple temperature monitor (https://github.com/m00dawg/SolarStats/tree/master/particle) which sends data back to my local server via a crafted HTTP request. It works but I don’t find it particularly elegant - making deep sleep work took the better part of the day - and was already looking at using something like rabbitMQ to distribute data from my temperature sensor as well as from my home energy monitor to various sources (like a local DB, Weather Underground, etc.).

So MQTT seems like a natural fit as an all around better option. I found the MQTT library on the web IDE, but I’m curious about the callback:

MQTT client(“server_name”, 1883, callback);

I wasn’t planning on sending data to the Core. Instead I was going to have to just have it wake up, check temp, send temp data to the queue, and then go back into a deep sleep. As a result, do I need a callback here? I tried omitting it entirely and using a null value. It works if I provide an empty function, but that seems a little ugly?

Thoughts? Also I’m open to suggestions if folks have suggestions other than MQTT. I noticed there is a Redis library available as well so was pondering that.

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Hey, did you figure this out? Looking to do something similar with GPS coordinates.

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I think you need a callback to satisfy the MQTT constructor signature, so using the empty function is probably the way to go if you don’t plan on actually receiving messages. A little ugly, but not too bad.

And, from my limited knowledge, MQTT is definitely the right choice for your proposed workflow: wake, send MQTT message, sleep.

(I know this reply is 2 years late, but just in case others are wondering…)

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Even if you’re not subscribing to a topic, you’ll still need to define a method with the required signature in your code. The method won’t be called. That’s all.

The signature looks like this:

void callback(char* topic, byte* payload, unsigned int length) {
}

the length argument is the length of the payload.

MQTT is a great fit for devices. I use it in all of my applications. Most of them are publish and subscribe, but a few only publish (like in your case). I use Mosquito. I’ve installed it on one of my servers for devices that are home bound and I also have it installed in the cloud. During development as well as demos, I also use the Eclipse instance that is free to use (iot.eclipse.org).

Since MQTT can be used from multiple devices (Netduino, Arduino Zero (and other Arduino WiFi enabled devices), ESP8266, Adafruit Feather M0, Feather Huzzah, various Embed devices, TI MSP432 etc. its a really great choice for inter-communication between devices as well as regular desktop applications, Web browser applications (using WebSockets) and web server applications. Oh, and let’s not forget, the Electron!

I even have applications built using the UNO and these application are connected to a PC via USB because I communicate with an application running on the PC. These application publish messages to MQTT on behalf of the application on the UNO.

It’s an extremely light-weight protocol and if you keep your data small as well, it’s super fast, even over the internet.

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