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MQTT Intro Why MQTT Publisher/Subscriber model. Topics Message Quality Connecting Test brokers Mosquitto Using it
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MQTT

Intro

MQTT - a lightweight messaging protocol. Ideal for use with IOT devices.

Created by Andy Stanford-Clark and Arlen Nipper 1999. Also used for Facebook messenger.

MQTT Logo

Why MQTT

Always use existing standards when possible. Custom protocols cause fragmentation.

Why MQTT?

  • Low bandwidth
    • Good when communication is high cost, i.e. power.
    • Good when communication is unreliable, i.e. long distance.
  • High latency
    • Good when responses can take a long time.
  • Free
    • Important for low cost devices.
  • Widespread
    • Large ecosystem.
    • Lots of support.

Publisher/Subscriber model.

MQTT Diagram

Topics

MQTT clients subscribe/publish to topics.

Topics can be arranged into levels. Allows grouping of related actions/information.

  • myhouse/upstairs/bedroom/temperature
  • myhouse/upstairs/bedroom/light
  • myhouse/upstairs/bathroom/light
  • myhouse/heating

Wildcards let you subscribe to multiple topics. Single level wildcard (+), e.g. myhouse/upstairs/+/light

  • %myhouse/upstairs/+/light
  • %myhouse/upstairs/+/light

Multi-level wildcard (#), e.g. myhouse/upstairs/#

  • myhouse/upstairs/bedroom/temperature
  • myhouse/upstairs/bedroom/light
  • myhouse/upstairs/bathroom/light

Message Quality

Different QOS levels depending on message type.

  1. Send once, it might get through.
    • E.g. simple sensor readings.
  2. Send at least once, will get through, might be duplicated.
    • Keep sending until confirmed.
    • E.g. data requests.
  3. Send exactly once, will get through.
    • E.g. device control.

Don't use QOS level 2 for everything.

  • Takes longer.
  • Needs more resources.

Connecting

Test brokers

NOT to be used for a real system. Have NO security, NO privacy.

But good enough to test with.

  • iot.eclipse.org
  • mqtt.pubnub.com
  • test.mosquitto.org
  • broker.mqttdashboard.com

Real systems need username/password or more sophisticated authentication. For example the Coventry University MQTT broker is available on mqtt.coventry.ac.uk and will require you to use a username and password and a CA certificate to connect. The CA certificate is available in this repository in the mqtt.crt file.

Mosquitto

Mosquitto is an open source MQTT broker implementation. Given its popularity it's a good choice if you need to run your own broker.

It also has a number of command line tools that can be used to publish and subscribe to MQTT topics. These are incredibly useful when checking that the MQTT portion your application is working correctly.

The most used commands are mosquitto_sub for subscribing to MQTT topics and mosquitto_pub for publishing to MQTT topics. If you are on a linux based system and need to install these tools, they are part of the mosquitto_clients package.

Using it

Use the --help flag to see the full range of commands but general usage will typically be some variation of:

mosquitto_sub -h mqtt.coventry.ac.uk -t "mytopic/wibble" -v and mosquitto_pub -h mqtt.coventry.ac.uk -t "mytopic/wibble" -m "42".

  • -h is the host to connect to, in this case mqtt.coventry.ac.uk
  • -t is the topic to publish/subscribe to, i.e. mytopic/wibble
  • -v enables verbose mode in mosquitto_sub, causes additional useful information to be displayed when messages are recieved
  • -m is the message that is sent to the topic, i.e. 42

If you are using a secure MQTT broker (i.e. mqtt.coventry.ac.uk) then some or all of the following flags may also be required.

  • -u the username to use
  • -P the password to use
  • --cafile the certificate file to use
  • -p the port to connect on, normally 8883 for secure brokers

For example mosquitto_sub -h mqtt.coventry.ac.uk -t "mytopic/wibble" -v -u USERNAME -P PASSWORD --cafile mqtt.crt -p 8883

For security, users need to be prevented from seeing each others topics. The easiest way to achieve this and to prevent clashes when different users try to create topics with the same names, users may be limited to subscribing/publishing topics that start with their own username. This is definitely the case on the Coventry broker, your topics will have to start with your own username i.e. USERNAME/mytopic/wibble