This course is for complete beginners wanting to understand the fundamentals of the MQTT protocol and how to use it in IOT Projects.
The course consists of a series of tutorials, videos and examples that take you through the basics of the MQTT protocol, and then the important features.
At the end of the course you should have a very good understanding of MQTT and the most important features.
Although many of the tutorials include examples written in Python no programming will be required. However demo scripts are available to download for many of the examples.
Note: This course/book is now available as a pdf download. It is a beta version and I would be grateful for feedback it is available for download on the feedback page.
MQTT Basics
How MQTT Works – In this tutorial we look at the publish and subscribe model used by MQTT and its advantages over the client server model used by http
Understanding MQTT Topics and Topic Structure – MQTT clients don;t have address like you are familiar with in services like email but use topics to share messages.
MQTT Publishing,Subscribing and Message Exchange – Covers how messages are exchanged between MQTT clients.
Authentication mechanisms
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.
See An Introduction to MQTT Security Mechanisms
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.
MQTT websocket support for web browsers is provided by the Javascript MQTT Client.
The Using MQTT Over WebSockets tutorial cover this is detail and includes example scripts.
Quiz
MQTT Basics
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Question 1 of 10
1. Question
In a Publish/Subscribe model how does the publisher know who is subscribing.
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Question 2 of 10
2. Question
MQTT uses
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Question 3 of 10
3. Question
MQTT-SN uses
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Question 4 of 10
4. Question
Each client that connects to a broker must have a unique client id?
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Question 5 of 10
5. Question
When connecting to an MQTT broker you always need a username and password?
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Question 6 of 10
6. Question
An MQTT broker can use any port, but by default it uses port 1883?
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Question 7 of 10
7. Question
MQTT transmits data to and from a broker in which format.
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Question 8 of 10
8. Question
Messages sent to an MQTT broker are encrypted by default?
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Question 9 of 10
9. Question
You can subscribe to multiple topics at once using wild cards?
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Question 10 of 10
10. Question
You must always start your topic hierachy with a / e.g. /topic name?
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MQTT v3.1 Features
There are several important features that you need to be aware of and understand how to use, These tutorial go into these features in detail and include example usage:
Clean Sessions and Persistent Connections
When a client connects to a broker it can connect using either
- a non persistent connection (clean session) or
- a persistent connection..
With a non persistent connection the broker doesn’t store any subscription information or undelivered messages for the client.
This mode is ideal when the client only publishes messages.
It can also connect as a durable client using a persistent connection.
In this mode the broker/server will, depending on the QOS of the published messages and subscribing client, store messages for the client if it is disconnected.
The Understanding Persistent Connections- Python Examples tutorial demonstrates the various options using python and includes sample code.
Retained Messages
We mention retained messages earlier when we talked about publishing messages.
Normally if a publisher publishes a message to a topic, and no one is subscribed to that topic the message is simply discarded by the broker.
However the publisher can tell the broker to keep the last message on that topic by setting the retained message flag.
This can be very useful, as for example, if you have sensor publishing its status with long time intervals
The MQTT Retained Messages Explained tutorial covers this in detail with examples using Python.
Last Will and Testament
The last will and testament message is used to notify subscribers of an unexpected shut down of the publisher.
Each topic can have a last will and testament message stored on the broker.
tutorial takes you through setting the last will and testament and triggering it.
Keep Alives
MQTT uses a TCP/IP connection which is normally left open by the client so that is can send and receive data at any time.
If no data flows over an open connection for a certain time period then the client will generate a PINGREQ and expect to receive a PINGRESP from the broker.
If this fails then the server considers the connection broken and closes it. The MQTT keep alive tutorial covers this process with examples.
Quiz
MQTT Basics -Retained Messages,Clean Sessions and Last Will
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Question 1 of 10
1. Question
The Last Will Message is used to:
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Question 2 of 10
2. Question
To Receive a last will message.
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Question 3 of 10
3. Question
The Last will message is sent from?
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Question 4 of 10
4. Question
When you publish a message to a topic with the retained flag set what happens to the message?
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Question 5 of 10
5. Question
To receive a retained message?
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Question 6 of 10
6. Question
To delete a retained message from a broker you must?
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Question 7 of 10
7. Question
When you connect to a broker with clean session flag set to true what Happens?
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Question 8 of 10
8. Question
You connect to a broker with the clean session flag set to false and subscribe to the topic house/sensor1.
You then disconnect and after about 10 minutes you reconnect with the same client. Do you need to Subscribe to the topic house/sensor1 again?Correct
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Question 9 of 10
9. Question
You connect to a broker with the clean session flag set to false, and subscribe to the topic house/sensor1 with QOS 1.You then disconnect, and after about 10 minutes you reconnect with the same client. If another client publishes messages on house/sensor1 with QOS 1 while you are disconnected. What happens when you reconnect?
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Question 10 of 10
10. Question
You connect to a broker with the clean session flag set to false, and subscribe to the topic house/sensor1 with QOS 0.You then disconnect, and after about 10 minutes you reconnect with the same client. If another client publishes messages on house/sensor1 with QOS 1 while you are disconnected. What happens when you reconnect?
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Hello, how are you? I’m an enthusiastic student of new technologies, and MQTT technology really interested me. I am wanting to implement an IOT service where my clients are ESP32 and I would like them to send information to my server, this my server has a GUI developed in React + MongoDB. How could you make this call? Could you help me with some tips?
When you say server do You mean MQTT broker.
In any case you need to understand that the broker/server is a message transfer agent it doesn’t store messages it only forwards them.
IN MQTT you send data between two clients via a server/broker.
In http you send data between a client and a server.
The React/mongodb needs to work as a client and subscribe to the topics that the ESPs publish on.
Does that help?
Rgds
Steve