MQTT Operation Binding v0.2.0

The MQTT operation binding allows for defining behavior specific to an MQTT publish or subscribe operation. It includes configurations for Quality of Service (QoS) and the retain flag.

Overview

This binding provides crucial, operation-level control over how messages are handled by the MQTT broker. It allows an application designer to specify the delivery guarantees and message retention policies required for a particular business process.

Operation Properties

Property	Type	Required	Description
bindingVersion	string	No	Binding version (defaults to 0.2.0)
qos	integer	No	Defines the Quality of Service level for the message. Must be 0, 1, or 2. Defaults to 0.
retain	boolean	No	Whether the message should be retained by the broker. Defaults to false.
messageExpiryInterval	integer or Schema Object	No	Lifetime of the message in seconds.

Property Details

qos

Quality of Service (QoS) determines the level of guarantee for a message delivery.

• 0 (At most once): The message is sent, but with no acknowledgment. It's fast but unreliable. Best for non-critical, high-volume data where some loss is acceptable.
• 1 (At least once): Guarantees the message is delivered, but it might arrive more than once. The sender retries until it gets a PUBACK from the receiver. Best for commands or important notifications where duplicate handling is possible.
• 2 (Exactly once): The most reliable level, guaranteeing the message is delivered exactly once using a four-part handshake. It has the highest latency. Best for critical systems like billing or financial transactions.

retain

If true, the MQTT broker will store this message as the "last known good" value for its topic. When a new client subscribes to that topic, it will immediately receive the last retained message. This is extremely useful for state information (e.g., a device's on/off status) but should not be used for event streams.

messageExpiryInterval

A feature from MQTT 5, this defines the lifetime of the message in seconds. If the message is not delivered to a matching subscriber within this interval, the broker will discard it. This is useful for time-sensitive commands that become invalid after a certain period.

Examples

Publish Operation with QoS 1 and Retain

This operation sends a device status message that is guaranteed to arrive and will be retained by the broker to give immediate status to new subscribers.

operations:
  publishDeviceStatus:
    action: send
    channel:
      $ref: '#/channels/deviceStatus'
    bindings:
      mqtt:
        bindingVersion: '0.2.0'
        qos: 1
        retain: true

Subscribe Operation Requesting QoS 2

This operation defines a subscriber that requires the highest level of reliability for incoming messages. The effective QoS will be the minimum of the publisher's QoS and the subscriber's QoS.

operations:
  receiveCriticalCommand:
    action: receive
    channel:
      $ref: '#/channels/criticalCommands'
    bindings:
      mqtt:
        bindingVersion: '0.2.0'
        qos: 2

Time-Sensitive Publish Operation

This message will be discarded by the broker if it cannot be delivered within 5 minutes.

operations:
  sendTimedOffer:
    action: send
    channel:
      $ref: '#/channels/specialOffers'
    bindings:
      mqtt:
        bindingVersion: '0.2.0'
        qos: 1
        messageExpiryInterval: 300

Changelog

Version 0.2.0

• Initial release of the generic MQTT operation binding.
• Added qos, retain, and messageExpiryInterval properties.
• Schema validation for all properties.