MQTT Tutorial | MQTT architecture, MQTT protocol use cases

This MQTT tutorial covers MQTT architecture and MQTT protocol working operation with different MQTT use cases. This tutorial on MQTT protocol architecture describes MQTT message flow between broker and clients.

The figure-1 depicts MQTT architecture. As mentioned MQTT is broker based protocol. In these end devices (i.e. Clients) communicate via a broker. The broker is a server which can be installed on any machine in the cloud. There are different types of brokers such as HiveMQ, Mosquitto etc. The single client and broker can also communicate with each other.

As MQTT runs above TCP/IP layer, it is also connection oriented protocol. The client establishes connection with the broker (i.e. Server) before the communication. MQTT is a publish-subscribe protocol. Here both client and server publish about any information (i.e. A Parameter such as temperature, humidity, event (ON/OFF) etc.) to each other using "PUBLISH" message. Any number of clients or end devices can subscribe for an event with the broker. Due to this subscription, when there is a change in any event or parameter, the broker will intimate to the subscribed clients about the change in event or parameter (i.e. Temperature, humidity, etc.).

MQTT Message format

Following table mentions MQTT protocol messages exchanged between client and Broker.

MQTT message	Description with direction of flow
CONNECT	Client to server (i.e. Broker), the client requests to connect to server.
CONNACK	Server to client, connect acknowledgment
PUBLISH	Client to server OR server to client Publish message
PUBACK	Client to server OR server to client Publish acknowledgment
PUBREC	Client to server OR server to client Publish received
PUBREL	Client to server OR server to client Publish release
PUBCOMP	Client to server OR server to client Publish complete
SUBSCRIBE	Client to server Client subscribe request
SUBACK	Server to client Subscribe acknowledgment
UNSUBSCRIBE	Client to server Unsubscribe request
UNSUBACK	Server to client Unsubscribe acknowledgment
PINGREQ	Client to server Ping request
PINGRESP	Server to client PING RESPONSE
DISCONNECT	Client to server , Client is disconnecting

MQTT Protocol Message Format

The figure-2 depicts MQTT message format. It consists of a fixed message header, variable header and payload. The first two bytes are used by most of the message types. Few of the message types use variable header and payload.

MQTT Protocol Architecture Working Operation

Let us understand the working operation of MQTT protocol architecture. The figure-3 depicts MQTT message flow between client and broker. We will take two MQTT use cases to understand the working operation of MQTT architecture.

MQTT Use Case#1: Broker wants to switch ON or OFF the light connected with remote client#1
➨Initially connection is established by client#1 with a broker using CONNECT and CONNACK messages.
➨Next Broker communicates with Client#1 to switch ON or OFF the light interfaced with it. The messages such as PUBLISH and PUBREC are used for it.
This use case is used to switch ON/OFF the street lights in Zigbee or LoRaWAN network. The lights are usually connected with end nodes or end devices in these wireless networks. The single Zigbee or LoRaWAN gateway controls multiple end nodes. Multiple such gateways are needed to cover the entire city.

MQTT Use Case#2: Client#2 or client#3 wants to update temperature/humidity status to the broker based on sensors
➨Client#2 and Client#3 will intimate temperature or humidity update to the broker using PUBLISH message. This information is stored in the database and will be sent to all the subscribers who have subscribed to these topics (i.e. Temperature, humidity). This information is "pushed" to all the subscribed clients of the topics.
➨If client#1 has already subscribed for subscription to topics (i.e. Temperature, humidity), it will get the information from broker using PUSH operation.
This use case is used for obtaining different types of sensing information automatically whenever there are any updates. For this purpose, different types of sensors (such as humidity sensor, temperature sensor, etc.) are interfaced with end nodes. These end nodes publish information (of any event updates) to the broker. The broker intimates changes to all the subscribed clients. The communication between gateway

There are two dominant data exchange protocol architectures viz. broker based and bus based. In this MQTT tutorial we have studied broker based MQTT protocol architecture. Wireless IoT technologies such as zigbee, LoRaWAN uses MQTT for communication between clients and router. Protocols such as AMPQ, CoAP and JMS also use broker based architecture. Protocols such as DDS, REST and XMPP use bus based architecture. Refer Bus based vs Broker based architecture >> for more information.

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