Delayed Publish

Delayed publish is an extended MQTT feature supported by EMQX. When a client publishes a message to EMQX with the topic prefix $delayed/{DelayInteval}, it triggers the delayed publish feature. The messages will be published after a period of time predefined by the user.

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 publishing 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 will discard the message and the client will not receive any information.
• {TopicName}: The topic name of the MQTT message.

Example:

• $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

Configure Delayed Publish via Dashboard

1. Open EMQX Dashboard. In the left navigation menu, click Management-> Delayed Publish.

2. On the Delayed Publish page, you can configure as follows:

   • Enable: Enable or disable delayed publish. By default, it is enabled.
   • Max Delayed Messages: The max number of delayed messages.
     • If you select Unlimited from the drop-down list, you cannot specify the number.
     • If you select Custom from the drop-down list, you can specify the number.

   

Try Delayed Publish with MQTTX Client

Prerequisite

• Basic publishing and subscribing operations using MQTTX Client

1. Start the MQTTX Client. Click the New Connection to create an MQTT connection named Demo.

   • The localhost 127.0.0.1 is used as an example in this demonstration.

   TIP

   For detailed instructions on creating an MQTT connection, see MQTTX Client.

   

2. Create another MQTT connection. Configure it as a subscriber.

3. Select the connection named Demo in the Connections pane. Type the topic name $delayed/10/x/y in the topic text box and type the message as Delayed Message.

   • $delayed: Indicates it is a delay message.
   • 10: Indicates the delayed interval is 10 seconds.
   • x/y: Indicates the topic name of the message.

4. Select the connection named Subscriber. Click the New Subscription button to create a subscription. Type x/y in the Topic textbox to subscribe to this topic.

   

5. Select the connection named Demo in the Connections pane. Click the send button to send the Delayed Message with topic $delayed/10/x/y.

6. Wait for 10 seconds. You will see the connection named Subscriber receive the delayed message after 10 seconds.

   

Try Delayed Publish with MQTTX CLI

Prerequisite

• Basic publishing and subscribing operations using MQTTX CLI

1. Create a new connection as a subscriber and subscribe to the topic t/1.

   mqttx sub -t t/1 -v

2. Open a new window in the terminal for the client as publisher. Use the following command to send a delayed message. The subscriber will receive the message after 5 seconds.

   mqttx pub -t "\$delayed/5/t/1" -m "Hello Delayed msg"