How MQTT Works -Beginners Guide
How Does MQTT Work ?-MQTT is a messaging protocol i.e it was designed for
 transferring messages, and uses a publish and subscribe model.

This model makes it possible to send messages to 0,1 or multiple clients.

A useful analogy is TV or radio.

A TV broadcaster broadcasts a TV program using a specific channel and a
 viewer tunes into this channel to view the broadcast.

There is no direct connection between the broadcaster and the viewer.

In MQTT a publisher publishes messages on a topic and a subscriber must
 subscribe to that topic to view the message.

MQTT requires the use of a central Broker as shown in the diagram below:
	
	
	MQTT- Publish-Subscribe-Model



Important Points to Note

Clients do not have addresses like in email systems, and messages are not
 sent to clients.
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
There is no direct connection between a publisher and subscriber.
All clients can publish (broadcast) and subscribe (receive).
MQTT brokers do not normally store messages.



Introductory MQTT Video
This video takes you through the basics of MQTT.
MQTT is an OASIS standard messaging protocol for the Internet of Things
 (IoT). It is designed as an extremely lightweight publish/subscribe
 messaging transport that is ideal for connecting remote devices with a
 small code footprint and minimal network bandwidth. MQTT today is used in
 a wide variety of industries, such as automotive, manufacturing,
 telecommunications, oil and gas, etc.





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.

Connections are acknowledged by the broker using a Connection
 acknowledgement message.

    You cannot publish or subscribe unless you are connected.

mqtt-message-flow

MQTT clients publish a keepalive message at regular intervals (usually 60
 seconds) which tells the broker that the client is still connected.

See MQTT Keep Alive Interval Explained
The Client Name or Client ID

All clients are required to have a client name or ID.

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 client id as an existing connected client.

mqtt-broker-duplicate-ids
Clean Sessions

MQTT clients 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 to those topics.

Understanding Clean sessions Video
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, and is set on a per
 topic basis which means that each topic can have its own last will
 message.

This means that each topic can have its own last will message associated
 with it.

The message is stored on the broker and sent to any subscribing client (to
 that topic) if the connection to the publisher fails.

    If the publisher disconnects normally the last Will Message is not
 sent.

The actual will messages is including with the connect request message.

See Last Will and Testament example using Python

Python Client Example

This tutorial takes you though creating a client connection using the Paho
 MQTT client.

It looks at successful and failed connections and how to deal with them.
 – Working with Client Connections

Next ==> MQTT Topics and Topic Structure

Resources:

    Understanding the MQTT Protocol Packet Structure
    MQTT and Websockets