Auto Subscribe

Auto Subscribe is an extended MQTT feature supported by EMQX. With Auto Subscription enabled, users can set multiple EMQX rules. After a client is successfully connected to EMQX, EMQX will complete the subscription process for the client automatically, and the clients no longer need to send SUBSCRIBE requests.

Before EMQX 5.0, this feature is called Proxy Subscription.

Configure Auto Subscribe via Dashboard

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

2. On the Delayed Publish page, click the Add button at the upper right corner.

3. In the pop-up dialog box, type the test topic a/1 in the Topic text box. Leave other settings as default.

   • Topic: Type the topic that is automatically subscribed to for the client.

   • QoS: Specify the quality of service of the topic. Options: 0, 1, and 2.

   • No local: Options: False or True.

   • Retain as Published: Specify if the message sent with the specified topic will be retained. Options: False or True.

   • Retained Handling: Options: 0, 1, and 2.

     

   Click the Add button on the dialogue box. The auto subscribe topic a/1 is created successfully.

   

Now the auto subscription function is enabled. New subscribers will subscribe to the topic a/1 automatically once they are connected to the broker.

Try Auto Subscription Using MQTTX Client

The topic a/1 is configured as the auto subscribe topic in Configure Auto Subscribe via Dashboard. The following procedure demonstrates how a client subscribes to the topic a/1 automatically once it is connected to the broker.

Prerequisite

• Basic publishing and subscribing operations using MQTTX Client

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

   • 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 named Publisher.

3. Type a/1 as the topic. Send a message on this topic.

   • The client Subscriber receives the message automatically without creating a new subscription.

   • The client Publisher also receives the message as it is also a new connection.

     TIP

     In the publish/subscribe pattern, a client can be both sender and subscriber.

4. Go to EMQX Dashboard. Click Monitoring -> Subscriptions in the left navigation menu. It shows two subscriptions automatically subscribe to the topic “a/1”.

   

Try Auto Subscription using MQTTX CLI

Prerequisite

• Basic publishing and subscribing operations using MQTTX CLI

1. Create a new connection with client ID as emqx_c.

   mqttx conn -c emqx_c

2. Go to EMQX Dashboard. Click Monitoring ->Subscriptions in the left navigation menu. It shows the client emqx_c subscribes to the topic a/1.