Persistent messaging

Overview

Within MQTT 5, clients can set a unique Message Expiry Interval in seconds for each PUBLISH message. This interval establishes the duration the broker preserves the PUBLISH message for subscribers that match the topic but are currently offline. If the interval isn’t defined, the broker must indefinitely hold the message for matching subscribers, yet disconnected subscribers. Furthermore, if the ‘retained=true’ option is selected during the PUBLISH message, the interval also dictates the length of time a message is retained on a particular topic, reference HiveMQ.

Example

This example uses the MQTT v5 capabilities to store published messages on the MQTT broker.

DSL

mqttPublisher:
  ...
  message expiry interval: 15
  retain: true

MQTT Configuration

To enable message persistence two key broker configuration attributes are rquired.

persistence true
persistence_location /var/lib/mosquitto/mosquitto.db

Message persistence attributes schema

Attribute	Description	Data Type	Required
message expiry interval	The maximum time in seconds the broker will hold the message for matching subscribers, who may be offline (see Session Management)	Long Default: 0 seconds
retain	If message is retained on the broker	Boolean Default: true