The king of Internet of things protocol: mqtt

Hello , This is the network technology alliance station .

Today, I'd like to talk about the famous Internet of things protocols ：MQTT.

Let's go straight to ！

What is? MQTT？

• English full name ：Message Quueuing Telemetry Transport
• Chinese term ： Message queuing telemetry transmission protocol
• A simple messaging protocol for communication between IOT devices
• Specially designed for Limited bandwidth Designed for the equipment
• use Release / subscribe Communication mode
• be used for machine Yes machine ( M2M ) signal communication
• MQTT Based on the agreement OASIS Under standard TCP/IP
• It is widely used in wireless network communication , It has a wide coverage , Health care coverage 、 Automotive and other industries that use IOT applications .

MQTT The history of

MQTT By the first Andy Stanford-Clark Doctor and Arlen Nipper Doctor Yu 1999 Created in , For connecting oil pipeline telemetry system via satellite .

The two inventors specified several requirements for future protocols ：

• Simple implementation
• Service quality data delivery
• Lightweight and bandwidth efficiency
• The data is not known
• Persistent session

MQTT On 2013 Organization for the promotion of structured information standards (OASIS) Is standardized as open source ,OASIS Still manage MQTT standard .

MQTT The advantages of

MQTT There are the following advantages ：

Simple

Use MQTT It's fast and easy to start and run .

It only needs a few commands to build , The startup is also very fast , There is also a special management interface .

reliable

Many IOT devices rely on radio Connect to transmit and collect data , This means that the connection is not always reliable .

MQTT By providing built-in QoS Features to reduce connectivity issues , This function can queue messages , Keep them in MQTT The agent of , And let them wait until the target device is all set to accept it , This reduces the chance of message misplacement , So the message is destined to reach its destination .

Two way messaging

Any device 、 Things or applications can be published or subscribed by MQTT Any topic handled by the agent .

This is not one-way , For example, you can only publish or subscribe , It can publish and subscribe , Be similar to full duplex signal communication .

Mass messaging

because Broker The existence of , So whether it's 100 Wan Tai equipment , still 100 device , about MQTT It's all the same , No pressure , Because the final message is delivered to Broker in , No matter how many clients listen for messages , Also from the Broker Take in .

MQTT working principle

MQTT framework

MQTT Use PUSH/SUBSCRIBE（ Publish subscribe ） Model in TCP/IP Run on , stay MQTT Architecture , There are two types of systems ：

• client
• agent

A proxy is a server with which clients communicate , The agent receives communications from the client and sends them to other clients , There is no direct communication between clients , It's connected to agent , Each client can be a publisher 、 Subscribers or both .

MQTT It's a kind of Event driven The agreement , There is no regular or continuous data transmission , This keeps transmission to a minimum . The client only publishes when there is information to send , The agent only sends information to subscribers when new data arrives .

Release / subscribe

In publish and subscribe systems , The device can post messages on a topic , You can also subscribe to specific topics to receive messages .

Pictured , equipment 1 Post a message on a topic , equipment 2 Subscribed to and devices 1 The published topic is the same topic , Subsequent equipment 2 You can receive messages .

news

Messages are information exchanged between devices , For example, some instructions , Or some information about the equipment , Or other information that needs to be exchanged .

The theme

As shown in the figure , Let me explain in one sentence with a picture ： The topic is the type of official account you subscribe to through wechat .

Take the official account as an example. , Creators are publishers , The server of the official account is Broker, Readers are subscribers , Publishers in different fields are working on different topics （ field ） Publish articles , For example, Rego usually publishes articles in the field of science and technology , So all readers （ subscriber ） Anyone who has subscribed to the technology field can see my article .

This field can be roughly understood as the theme .

For theme Forward slash Separated strings represent , Each forward slash represents a Subject level .

As shown in the picture on the official account The topic of creating network type in the field of science and technology in the article type ：

Be careful ： Topics are case sensitive , The two themes are different ：

If readers want to read articles on the Internet ：

• The publisher is article/technology/network Theme release OSPF Related articles
• The subscriber subscribed to the topic article/technology/network
• The publisher is on the topic article/technology/network When there is new news on the , Subscribers can subscribe to .

Broker agent

An agent is actually agent .

MQTT The agent is responsible for receiving all messages , Filtering messages , Decide who is interested in them , Then publish the message to all subscribed clients .

MQTT subscribe

MQTT Clients are not directly connected to each other , Instead, subscribe to topics to receive messages .

To subscribe to a topic , The client must first send a subscription request to the proxy ,SUBSCRIBE The request can contain multiple topics , Proxy usage SUBACK（ confirm subscription ） Respond to SUBSCRIBE request , Subscriptions also have QoS Set up , Can be used to downgrade published messages QoS. under these circumstances , The message is always in a lower QoS Set Publishing .

Subscribe to wildcards

There are two types of subscription wildcards ：

Single stage

Single level wildcard replaces one topic level ：

This wildcard will cover the following topics ：

• article / entertainment / The Internet
• article / Technology / The Internet
• article / Agriculture, rural areas and farmers / The Internet
• article / Journalism / The Internet

multistage

Multi level wildcards replace multiple topic levels ：

This wildcard will cover the following topics ：

• article / entertainment / The Internet
• article / Technology /java
• article / Agriculture, rural areas and farmers / food
• article / Journalism / space flight

Service quality QoS

• QoS 0（ At most once ）： The client's message is sent only once , Whether or not it has reached the agent .
• QoS 1（ At least once ）： Client messages are sent over and over again , Until the agent responds with an acknowledgement of receipt , This may cause messages to arrive at the broker multiple times .
• QoS 2（ Exactly Once ）： The client sends a message , Also make sure that it has reached the agent ,QoS 2 Communication needs more than quality of service 0 or 1 More bandwidth .

MQTT Several important websites

• mqtt Official website ：https://mqtt.org/

• emqx Official website ：https://www.emqx.com/

summary

MQTT Is a lightweight open messaging protocol , It provides a simple method for resource constrained network clients to distribute telemetry information in a low bandwidth environment , Adopt release / Subscription communication mode , For machine to machine ( M2M ) signal communication .