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The knowledge points of this article include: the concept of MQTT protocol, the application scenario of MQTT protocol and the server introduction of MQTT protocol. I believe you have a certain understanding of MQTT protocol after reading the complete article.

1. What is MQTT?

MQTT full name message queuing Telemetry Transport (Message Queuing Telemetry Transport). It mainly provides two message modes of subscription / publish, which is more simple, lightweight and easy to use. It is especially suitable for message distribution in limited environment (low bandwidth, high network delay, unstable network communication). It belongs to a standard transport protocol of the Internet of things (Internet of Thing).

2. History of MQTT

Andy Stanford-Clark of IBM and Alan Nip of Arcom wrote the first version of the agreement in 1999.

IBM submitted the MQTT version 3.1 specification to the Organization for the Promotion of structured Information Standards in 2013, with a charter to ensure that only minor changes can be made to the specification.

In April 2019, MQTT released v5.0, which is now the official OASIS standard.

3. What are the uses of MQTT

Since 1999, MQTT has been widely used in various industries. Because MQTT has become the standard Internet of things transport protocol, many Internet of things devices use these protocols to transmit data, and various industries basically need a variety of Internet of things devices.

In addition to the MQTT protocol, the following protocols are common industrial protocols:

OPC UAModbus TCP / RTUPROFINETRS232/RS4854. Quality of Service of MQTT

MQTT supports three kinds of message publishing quality of service (QoS):

At most once "(QoS==0)

Message distribution is completely dependent on the underlying TCP/IP network. Messages can be lost or duplicated. This level can be used in the case of environmental sensor data, it doesn't matter if you lose a read record, because there will be a second transmission soon. At least once "(QoS==1)

Ensure that the message arrives, but message repetition may occur. Only once "(QoS==2)

Make sure the message arrives once. This level can be used in situations where duplicate or lost messages can lead to incorrect results in a billing system. Small transmission, low overhead (fixed-length header is 2 bytes), and protocol switching is minimized to reduce network traffic. 5. Common MQTT server Apache ApolloMosquittoApache ActiveMQRabbitMQEMQTT

Among the above common MQTT servers, emqtt has the highest concurrency, but cpu occupies a high level and has high stability, but messages are sent slowly. Moqtuitto, fast to send messages, high stability, low cpu usage and high concurrency. Other stability is not very high. 6. MQTT implementation

The realization of MQTT protocol requires communication between client and server. In the process of communication, there are three identities in MQTT protocol: publisher (Publish), agent (Broker) (server) and subscriber (Subscribe). Among them, the publisher and subscriber of the message are both clients, the message broker is the server, and the message publisher can be the subscriber at the same time.

The message transmitted by MQTT is divided into two parts: topic (Topic) and load (payload):

(1) Topic, which can be understood as the type of message. After subscribing to (Subscribe), subscribers will receive the message content (payload) of the topic.

(2) payload, which can be understood as the content of the message, refers to the specific content that the subscriber wants to use.

After reading the above, do you have any further understanding of the MQTT agreement? If you want to learn more skills or want to know more about it, you are welcome to follow the industry information channel, thank you for reading!

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