One of the goals of this project is to get my hands dirty with integrating data and events coming from hardware (sensors attached to the raspberry pi or functions of the pi itself) and content in the browser. The raspberry pi ecosystem leans heavily on python, so while I was already using node.js for the web server, I didn’t want to get tied into a system which limited my implementation choices for publishing or consuming data. I wanted lightweight, language-agnostic messaging.
My first thought was to simply use the filesystem: writing to some files and watching for changes. That seemed fraught with potential problems and reinventing too much of the wheel. So I went shopping for messaging solutions.
After looking into ZeroQ and ZeroRPC, I settled on Mosquitto and MQTT. The Mosquitto project is a message broker (think central hub for dispatching messages to subscribers) that implements the MQTT protocol. As well as the broker (available as a raspbian-ready package) it also has command-line clients for publishing and subscribing which proved useful. There are MQTT implementations for both python and node.js (as well as a bunch of other languages.) The pub-sub paradigm turns out to be a great fit here. My “hello world” was to use the RPi.GPIO python module to watch for rising-edge button events on the GPIO pin I had jumpered a momentary button to, and publish a message over MQTT when the button is pressed. Logged into the raspberry pi, I run:
$ mosquitto_sub -t sensors/buttonup
.. to see those messages from the broker. In node.js land, the mqtt module provides the same functionality. We connect to the host/port the broker is on, subscribe to some topics and register callbacks for when messages arrive.
Finally, to bring the browser into the equation, I ended up making an adapter to subscribe to MQTT topics, and use socket.io to relay to the dashboard page. There is some (new) websocket support in mosquitto, and probably other more direct ways to accomplish this, but this works for now. As a proof of concept, I handle ‘gpio/button’ events from socket.io, and dispatch a ‘hardbuttonup’ event on the window object. A listener for this event toggles a class to flash the screen blue. This opens up a lot of opportunities as I have flexibility at every stage: the button is a stand-in for any sensor or input device that can toggle a GPIO pin high/low. The MQTT message produced can be caught by any program running on this device, or even anywhere on the network potentially. Turning this message into a DOM event enables a seamless tie-in so you can use existing frameworks to respond to the event.
Its a bit subtle, but in the video I’m clicking the button on the breadboard, and the display is flashing blue.