MQTT Bindings

This document defines how to describe MQTT-specific information on AsyncAPI.

It applies to all versions of MQTT, although specific binding fields may only apply to particular versions.

Version

Current version is 0.2.0.

Server Binding Object

This object contains information about the server representation in MQTT.

Fixed Fields

Field Name	Type	MQTT Versions	Description
clientId	string	3, 5	The client identifier.
cleanSession	boolean	3, 5	Whether to create a persistent connection or not. When false, the connection will be persistent. This is called clean start in MQTTv5.
lastWill	object	3, 5	Last Will and Testament configuration. topic, qos, message and retain are properties of this object as shown below.
lastWill.topic	string	3, 5	The topic where the Last Will and Testament message will be sent.
lastWill.qos	integer	3, 5	Defines how hard the broker/client will try to ensure that the Last Will and Testament message is received. Its value MUST be either 0, 1 or 2.
lastWill.message	string	3, 5	Last Will message.
lastWill.retain	boolean	3, 5	Whether the broker should retain the Last Will and Testament message or not.
keepAlive	integer	3, 5	Interval in seconds of the longest period of time the broker and the client can endure without sending a message.
sessionExpiryInterval	integer | Schema Object | Reference Object	5	Interval in seconds or a Schema Object containing the definition of the interval. The broker maintains a session for a disconnected client until this interval expires.
maximumPacketSize	integer | Schema Object | Reference Object	5	Number of bytes or a Schema Object representing the maximum packet size the client is willing to accept.
bindingVersion	string		The version of this binding. If omitted, "latest" MUST be assumed.

This object MUST contain only the properties defined above.

Examples

servers:
  production:
    bindings:
      mqtt:
        clientId: guest
        cleanSession: true
        lastWill:
          topic: /last-wills
          qos: 2
          message: Guest gone offline.
          retain: false
        keepAlive: 60
        sessionExpiryInterval: 600
        maximumPacketSize: 1200
        bindingVersion: 0.2.0

servers:
  production:
    bindings:
      mqtt:
        sessionExpiryInterval:
          type: integer
          minimum: 30
          maximum: 1200
        maximumPacketSize:
          type: integer
          minimum: 256
        bindingVersion: 0.2.0

Channel Binding Object

This object MUST NOT contain any properties. Its name is reserved for future use.

Operation Binding Object

This object contains information about the operation representation in MQTT.

Fixed Fields

Field Name	Type	Applies To	MQTT Versions	Description
qos	integer	Publish, Subscribe	3, 5	Defines the Quality of Service (QoS) levels for the message flow between client and server. Its value MUST be either 0 (At most once delivery), 1 (At least once delivery), or 2 (Exactly once delivery).
retain	boolean	Publish	3, 5	Whether the broker should retain the message or not.
messageExpiryInterval	integer | Schema Object | Reference Object	Publish	5	Interval in seconds or a Schema Object containing the definition of the lifetime of the message.
bindingVersion	string			The version of this binding. If omitted, "latest" MUST be assumed.

This object MUST contain only the properties defined above.

Examples

channels:
  user/signup:
    publish:
      bindings:
        mqtt:
          qos: 2
          retain: true
          messageExpiryInterval: 60
          bindingVersion: 0.2.0

channels:
  user/signup:
    publish:
      bindings:
        mqtt:
          messageExpiryInterval:
            type: integer
            minimum: 30
            maximum: 300
          bindingVersion: 0.2.0

channels:
  user/signup:
    subscribe:
      bindings:
        mqtt:
          qos: 2
          bindingVersion: 0.2.0

Message Binding Object

This object contains information about the message representation in MQTT.

Fixed Fields

Field Name	Type	MQTT Versions	Description
payloadFormatIndicator	integer	5	Either: 0 (zero): Indicates that the payload is unspecified bytes, or 1: Indicates that the payload is UTF-8 encoded character data.
correlationData	Schema Object | Reference Object	5	Correlation Data is used by the sender of the request message to identify which request the response message is for when it is received.
contentType	string	5	String describing the content type of the message payload. This should not conflict with the contentType field of the associated AsyncAPI Message object.
responseTopic	URI string | Schema Object | Reference Object	5	The topic (channel URI) for a response message.
bindingVersion	string		The version of this binding. If omitted, "latest" MUST be assumed.

This object MUST contain only the properties defined above.

Examples

channels:
  user/signup:
    subscribe:
      message:
        bindings:
          mqtt:
            contentType: "application/json"
            correlationData:
              type: string
              format: uuid
            bindingVersion: 0.2.0

channels:
  userSignup:
    address: user/signup
    messages:
      userSignup:
        bindings:
          mqtt:
            payloadFormatIndicator: 1
            contentType: "application/json"
            correlationData:
              type: string
              format: uuid
            responseTopic:
              type: string
              pattern: "response/client/([a-z1-9]+)"
            bindingVersion: 0.2.0