Beginning to look at MQTT…

April 6, 2015 | Embedded, Internet of Things, My Projects | By: Mark VandeWettering

My weekend experiments lead me eventually toward flashing nodemcu, a Lua based firmware that runs on the ESP8266. Having a simple programming language (albeit one I’m not super fluent in) is very cool, and enables a whole bunch of nifty experiments.

While reading up, I encountered the acronym MQTT again. From Wikipedia:

MQTT (formerly Message Queue Telemetry Transport) is a publish-subscribe based “light weight” messaging protocol for use on top of the TCP/IP protocol. It is designed for connections with remote locations where a “small code footprint” is required and/or network bandwidth is limited. The Publish-Subscribe messaging pattern requires a message broker. The broker is responsible for distributing messages to interested clients based on the topic of a message. Andy Stanford-Clark and Arlen Nipper of Cirrus Link Solutions authored the first version of the protocol in 1999.

So, I thought I’d experiment a bit. I installed an MQTT broker called Mosquitto on a spare Raspberry Pi along with some simple command line clients and the python library with this command:

sudo apt-get install mosquitto mosquitto-clients python-mosquitto

The mosquitto broker is started automatically. In its simplest form, you can run a client that “subscribes” to a given “topic” in a shell window.

mosquitto_sub -d -t hello/world 

The -d specifies a slightly noisy “debug” mode. You’ll keep keepalive ping messages from the broker. The -t specifies the topic as something that looks like a path expression. You can specify more than one pattern, and there are expressions to wild card match subexpressions within the pattern. By default, these will connect to a broker on your localhost, but if you specify a -h flag, you can contact brokers running remotely (indeed, this will be the common case for IoT applications.)

From another window, you can “publish” to all clients which are subscribed to this topic:

mosquitto_pub -d -t hello/world -m "Hello world."

This sends the message to anybody subscribed to hello/world. You can send pretty much anything: it’s up to the application to know what to expect and what to do with it. You can even send the contents of complete files.

mosquitto_pub -d -t hello/world -f sendme.txt

So far, it doesn’t look all that exciting, but looks can be deceiving. First of all, the client code is relatively small and simple. This means that it’s easy to put into the small, low memory, low power nodes that you will typically use as sensors. There are already libraries for the Arduino and as I mentioned at the start, it’s already builtin to the nodemcu firmware for the ESP8266. Because it’s lightweight, even very modest brokers can handle huge numbers of messages. To give you some idea of scaleability, Facebook Messenger uses MQTT to route messages between users with very low latency, and without chewing the battery power on your mobile device. And it enables two way communications to sensor nodes, clients can both publish and subscribe at the same time.

As an experiment, I’m thinking of creating a notification service for my hummingbird camera that will update me when new captures occur. It seems pointless (and is) but it’s all a learning experiment. Eventually, I’m sure I’ll get back to the ESP8266.

Oh, and incidently, I’ve ordered some MCP23017 I2C I/O extenders, and hope to experiment with those on the ESP8266. They are nifty little chips which provide 16 additional digital I/O lines all under I2C control. Combined with the ESP8266, you’ll be able to sense lots of buttons and light lots of LEDs. Perhaps with the addition of an I2C A/D converter (or any other I2C sensor), you can do some serious tinkering.