For Programmers

The mosquitto_* utilities are essential for tests or for embedding in shell scripts. MQTT APIs exist for many languages, from Java and JavaScript, to C, C#, Objective-C, Lua, Perl, and Python. The subscriber written in Python initially arranges a callback to be invoked when a message arrives (Listing 3).

01 #!/usr/bin/env python
02
03 import paho.mqtt.client as paho   # pip install paho-mqtt
04
05 def on_message(mosq, userdata, msg):
06     print "%s (qos=%s, r=%s) %s" % (msg.topic, str(msg.qos), msg.retain, str(msg.payload))
07
08 mqtt = paho.Client()
09 mqtt.on_message = on_message
10
11 mqtt.connect("localhost", 1883, 60)
12 mqtt.subscribe("linux/+", 0)
13
14 mqtt.loop_forever()

The client now connects to the broker and goes into a continuous loop in a controlled manner. If a message arrives from the broker with the right topic, the callback is executed, which outputs the message and stores it in a database.

Anyone wanting to monitor whether the client is still alive can insert a last will and testament into the script from Listing 3:

# Arrange LWT
mqtt.will_set('alerts/attention', payload="Died: %s" % sys.argv[0], qos=0, retain=False)

The user now receives information as soon as the client terminates (with Ctrl+C).

The project mqttwarn [4] provides a Python tool with many plugins for such monitoring (Figure 3). Mqttwarn links topics that it follows with services it invokes. The whole thing is controlled using a configuration file, as shown in Listing 4.

Figure 3: As per the last will and testament, Mqttwarn sends a warning via pushover.

01 [defaults]
02 launch   = log, pushover
03
04 [config:log]
05 targets = {
06     'info'   : [ 'info' ],
07   }
08
09 [config:pushover]
10 targets = {
11     'linux'  : ['ml1234567890..', 'buzQx...'],
12   }
13
14 [alerts/attention]
15 targets = pushover:linux, log:info
16 title: Linux Magazine
17 format = {payload}

WebSockets

Modern web browsers can use MQTT with the help of WebSockets. The Paho project [5] provides a JavaScript module for MQTT. All brokers have WebSockets support on freely configurable ports. Note that communication via WebSockets does not run via the standard MQTT port 1883.

The need to perform HTTP polling and refreshes is eliminated when you have WebSockets in the browser: The browser receives messages as soon as MQTT clients publish them via the broker. This process provides interactive applications: With just a bit of skill in HTML and JavaScript, you can develop interesting monitoring panels, such as a panel for representing sensor or position data for mobile devices in real time.

In Real Life

MQTT is used outside of the IoT in various products. An MQTT plugin was recently developed for the statistics daemon Collectd [6] and for the logging system Graylog [7]. The open source project OwnTracks [8] supplies iOS and Android clients that provide positioning data to the MQTT broker via MQTT; friends and family members can find each other (see Figure 4).

Figure 4: Owntracks provides position information on your acquaintances via MQTT.

Anyone who deals with home automation and openHAB can also benefit from MQTT [9]: You'll discover a variety of techniques for controlling openHAB using MQTT or obtaining openHAB status information via MQTT. MQTT is also in use with all the major companies: Facebook used MQTT for its messenger service due to the lower latency [10].