What is MQTT?
MQTT is a lightweight publish/subscribe messaging protocol designed for M2M (machine to machine) telemetry in low bandwidth environments.
It was designed by Andy Stanford-Clark (IBM) and Arlen Nipper in 1999 for connecting Oil Pipeline telemetry systems over satellite.
Although it started as a proprietary protocol it was released Royalty free in 2010 and became an OASIS standard in 2014.
MQTT stands for MQ Telemetry Transport but previously was known as Message Queuing Telemetry Transport.
MQTT is fast becoming one of the main protocols for IOT (internet of things) deployments.
There are two versions of MQTT.
The original MQTT which was designed in 1999 and has been in use for many years and designed for TCP/IP networks.
Currently there is little support for version 5 but that should change in 2018.If you are wondering what happened to 4 then see here.
An overview of the key ideas in version 5 is here.
MQTT-SN which was specified in around 2013, and designed to work over UDP, ZigBee and other transports.
MQTT-SN doesn’t currently appear to be very popular. and the specification hasn’t changed for several years, but I expect that to change as IOT deployments start. See MQTT-SN working Notes.
Understanding MQTT-How MQTT Works
MQTT uses a publish /subscribe model which requires the use of a central Broker as shown in the diagram below:
Clients do not have addresses like in email systems, and messages are not sent to clients.
Instead messages are published to a broker on a topic.
The job of an MQTT broker is to filter messages based on topic, and then distribute them to subscribers..
A client can receive these messages by subscribing to that topic on the same broker
In this model there is no direct connection between a publisher and subscriber.
However unlike the broadcast TV/radio model in MQTT all clients can publish (broadcast) and subscribe (receive).
What are Topics?
These are like channels in the TV radio model.
The publisher broadcasts on a particular channel (topic) and a listener (subscriber) tunes into that channel (subscribes) to receive the broadcasts.
What Happens to Published Messages?
1.What happens to the published message after the subscriber receives it?
2. What happens to the published message if their are no subscribers?
To answer these questions just think of a TV or radio broadcast.
If you are tuned into the broadcast you simply miss it!
So for question1 and question 2 the answer is- the message is deleted from the broker.
When a client publishes a message on a topic then the broker will distribute that message to any connected clients that have subscribed to that topic.
Once the message has been sent to those clients it is removed from the broker (see note below).
If no clients have subscribed to the topic or they aren’t currently connected then the message is removed from the broker. (see note)
In general the broker doesn’t store messages.
Note: Retained messages, persistent connections and QOS levels can result in messages being stored on the broker/server.
MQTT Client-Broker Connections
MQTT uses TCP/IP to connect to the broker. TCP is a connection orientated protocol with error correction and guarantees that packets are received in order.
You can consider a TCP/IP connection to be similar to a telephone connection.
Once a telephone connection is established you can talk over it until one party hangs up.
Most MQTT clients will connect to the broker and remain connected even if they aren’t sending data.
MQTT clients publish a keepalive message at regular intervals (usually 60 seconds) which tells the broker that the client is still connected.
The Client name
All clients are required to have a client name.
The client name is used by the MQTT broker to track subscriptions etc.
Client names must also be unique.
If you attempt to connect to an MQTT broker with the same name as an existing client then the existing client connection is dropped.
Because most MQTT clients will attempt to reconnect following a disconnect this can result in a loop of disconnect and connect.
The screen shot below show what happens when I try and connect a client to the broker using the same id as an existing client.
MQTT clients will usually by default establish a clean session with a broker.
A clean session is one in which the broker isn’t expected to remember anything about the client when it disconnects.
With a non clean session the broker will remember client subscriptions and may hold undelivered messages for the client.
However this depends on the Quality of service used when subscribing to topics and the quality of service used when publishing topics.
Quality of Service (QOS)
MQTT support 3 QOS levels 0,1,2. QOS is set on published messages and on topic subscriptions.
- QOS -0 – Default and doesn’t guarantee message delivery.
- QOS -1 – Guarantees message delivery but could get duplicates.
- QOS -2 -Guarantees message delivery with no duplicates
Working out the overall QOS of a message is covered in other more in depth tutorials.
QOS level 2 involves more message overhead that QOS 1 which itself requires more messages than QOS 0.
MQTT Topic Structure
In MQTT there is no formal structure and a publisher is free to choose it’s own topic names and structure.
There are however naming rules:
Taken from the Specification MQTT V3.1
Topic names look very similar to URLs.
As long as a topic name follows the naming standards it is accepted by the Broker.
A Topic name is a UTF-8 string, and must comprise of at least one character.
Topics can be created in a hierarchical manner (levels) using a forward slash as a delimiter.
Simple topic examples are:
topic = bulb1, topic = bulb2, topic = bulb3 or
topic =bulbs/ bulb1, topic =bulbs/ bulb2, topic =bulbs/ bulb3
Topic names are not permanent as they only exist on a broker when someone has subscribed to that topic and is connected, or subscribed with the clean session flag set to False.
The $SYS topic hierarchy is used by most brokers for publishing broker statistics. However it isn’t mandatory.
Publishing Messages and The Retain Flag
When a client publishes a message to a broker it needs to send:
- The message topic
- The message QOS
- Whether the message should be retained.- Retain Flag
The retain Flag is normally set to False which means that the broker doesn’t keep the message.
If you set the retain flag to True then the last message received by the broker with the retained flag set will be kept.
The QOS of the published message has no effect on the retained message.
The main use of this is for sensors that don’t change very much and publish their status infrequently.
If you have a door sensor, for example, then it doesn’t make much sense publishing it’s status every second when it is almost always the same.
However if it only publishes it’s status when it changes what happens when a subscriber subscribes to the sensor.
In this case if the last status was published without the retain flag set then the subscriber wouldn’t know the status of the sensor until it published it again.
Last Will Messages
The idea of the last will message is to notify a subscriber that the publisher is unavailable due to network outage.
The last will message is set by the publishing client on a topic.
The message is stored on the broker and sent to any subscribing client if the connection to the publisher fails.
If the publisher disconnects normally the last Will Message is not sent.
Because MQTT clients don’t have addresses like email addresses, phone numbers etc. you don’t need to assign addresses to clients like you do with most messaging systems.
There is client software available in almost all programming languages and for the main operating systems Linux, Windows, Mac from the Eclipse Paho project.
On this site I will be using the Python client.
MQTT Brokers or Servers
Note: The original term was broker but it has now been standardized as Server. You will see Both terms used.
There are many MQTT brokers available that you can use for testing and for real applications.
If you don’t want to install and manage your own broker you can use a cloud based broker from Cloud service providers like IBM, Microsoft (Azure) etc
Eclipse has a free public MQTT broker and COAP server that you can also use for testing. The address is iot.eclipse.org and the port is 1883 or 8883(SSL).
See the MQTT Brokers and Servers article for a list of cloud hosting options
MQTT supports various authentications and data security mechanisms.
It is important to note that these security mechanisms are configured on the MQTT broker, and it is up to the client to comply with the mechanisms in place.
MQTT Overview Video
MQTT Over WebSockets
Websockets allows you to receive MQTT data directly into a web browser.
This is important as the web browser may become the de-facto interface for displaying MQTT data.
If you are familiar with the web and email then you will probably find, as I did, that MQTT is very different. These are some of the questions I had, and saw on other sites and forums that may clear things up a little.
Q- What happens to messages that get published to topics that no one subscribes to?
A- They are discarded by the broker.
Q-How can I find out what topics have been published?
A- You can’t do this easily as the broker doesn’t seem to keep a list of published topics as they aren’t permanent.
Q- Can I subscribe to a topic that no one is publishing to?
Q- Are messages stored on the broker?
A- Yes but only temporarily. Once they have been sent to all subscribers they are then discarded. But see next question.
Q- What are retained messages?
A- When you publish a message you can have the broker store the last published message. This message will be the first message that new subscribers see when they subscribe to that topic. MQTT only retains 1 message.
Real World MQTT Example Deployments
It’s often useful and interesting to see how a particular technology is actually being used. Here are some examples I’ve come across:
- Freight Farms-MQTT
- IP pinger swarm using MQTT
- How do you use MQTT- Mosquitto.org
- Owntracks -Location tracking
Fortunately when I started to learn about MQTT I came across the very good MQTT essentials series that explained the main features of MQTT.
However there is one thing reading about something and another seeing it really work so I started using Python to create simple scripts to demonstrate the main aspects of MQTT.
I use the Paho Python MQTT client for the client scripts.
However A knowledge of python isn’t really required to understand the examples.
I also used my own local Mosquitto MQTT broker but there are many free online brokers that you can use instead.
MQTT Tutorials By Subject
- Understanding MQTT Topics
- Understanding Topic Naming and Design Notes
- Subscribing to Topics Messages Using The Paho Python Client
- Clean Sessions and QOS (quality of service)
- Understanding MQTT QOS Levels- Part 1
- Understanding MQTT QOS Levels- Part 2
- MQTT Retained messages Explained
- Last will and testament-Example
- Publishing Messages Using The Paho Python Client
Test your MQTT knowledge with the MQTT basics quiz
- Using The Paho Python MQTT Client
- Really good introductory series- Recommended reading MQTT essentials.
- Very good MQTT pdf with detailed examples.
- Installing and setting up The Mosquitto broker.