Delayed Publish

MQTT messages are usually sent to subscribers as soon as possible by the MQTT broker, however, in some cases, clients may wish for some delay, or even in a more organized way: to schedule publishes.

The 'scheduling' is represented as the duration of delay from the PUBLISH packet is received by EMQX. When a client publishes a message to EMQX with the special topic prefix $delayed/{DelayInteval}, the delay-publish is triggered.

The specific format of the delay-publish topic is as below:

$delayed/{DelayInterval}/{TopicName}

• $delayed: Messages prefixed with $delay will be treated as messages that need to be delayed. The delay interval is determined by the content of the next topic level.
• {DelayInterval}: Specify the time interval for delaying the publish of this MQTT message with the unit of second. The maximum allowed interval is 4294967 seconds. If {DelayInterval} cannot be parsed as an integer number, EMQX Broker will discard the message and the client will not receive any information.
• {TopicName}: The topic name of the MQTT message.

E.g:

• $delayed/15/x/y: Publish MQTT message to the topic x/y after 15 seconds
• $delayed/60/a/b: Publish MQTT message to the topic a/b after 1 minute
• $delayed/3600/$SYS/topic: Publish MQTT message to the topic $SYS/topic after 1 hour

Config

delayed {
    enable = true
    max_delayed_messages = 12435
}

enable: enable or disable delayed publish.

max_delayed_messages: the max number of delayed messages.