ApsaraMQ for MQTT:FAQ about features

Does cloud-side message subscription support child topics?

Cloud-side message subscription does not support child topics. By default, all messages under the parent topic are received.

To distinguish between child topics, you can check the secondTopic field in MessageProperties after you receive a message.

Can the number of wildcard subscription relationships be increased?

Each parent topic allows a maximum of 100 wildcard subscription patterns. You cannot increase this limit by upgrading the instance. This limit is not related to the instance type.

Note: The number of wildcard subscriptions for an MQTT topic refers to the number of different wildcard patterns under the same topic. For example, topicA/+ and topicA/test/+ are counted as two wildcard patterns. If two clients subscribe to topicA/+ at the same time, it is counted as only one wildcard subscription for topicA.

Is there a limit on the number of exact topic subscriptions?

There is no limit on the number of exact topic subscriptions.

When are subscription relationships retained?

• You can set CleanSession to false to retain subscription relationships. This setting saves the subscription relationship for the client ID. After the client reconnects, it can continue to consume messages.

• If you unsubscribe using the Unsubscribe operation, the subscription relationship is not retained, even if CleanSession is set to false.

Why is there a forward slash (/) at the end of a topic?

For instances of version 3.2.0, a forward slash (/) is automatically added to the end of topics. This does not happen for instances of version 3.3.0. You cannot upgrade an instance from one version to the other.

What is the limit on the number of wildcard subscriptions?

Each parent topic allows a maximum of 100 wildcard subscription patterns.

Note that the number of wildcard subscriptions refers to the number of different wildcard patterns under a topic, not the number of subscription relationships. For example, topicA/+ and topicA/test/+ are counted as two wildcard patterns. If two clients subscribe to topicA/+ at the same time, it is counted as only one wildcard subscription for topicA.

The server-side saves only the 100 most recent wildcard subscription patterns. Any excess patterns are automatically deleted.

Why do shared subscriptions not support User Properties?

When you use the shared subscription feature of the MQTT 5.0 protocol, the User Properties in the messages received by the subscriber are empty. However, User Properties are received correctly in non-shared subscription mode.

This is because the underlying implementation uses the MQTT 3.1.1 protocol to ensure compatibility with shared subscriptions in MQTT 3.x protocols. Therefore, the User Properties feature is not supported. There are no plans to support this feature in the near future.

How do I clear the subscription relationships of a topic?

You can configure a cleanup period for the subscription relationships of a topic to automatically clear them. For more information, see Subscription relationship cleanup.

Note: If a topic is deleted, its subscription relationships cannot be automatically cleared. To clear the subscription relationships of a deleted topic, you must submit a ticket.

Do shared subscriptions support wildcards?

By default, shared subscriptions do not support wildcards. To enable wildcard support, you must submit a ticket.

Can an MQTT instance be restarted?

MQTT instances cannot be restarted.

What is the difference between the cloud-side SDK and the device-side SDK?

The cloud-side software development kit (SDK) uses the Advanced Message Queuing Protocol (AMQP) and the clustering consumption pattern. The device-side SDK uses the open source MQTT protocol and the broadcasting consumption pattern.

Why does the status of a connector task created from MQTT show as abnormal?

The task status may be abnormal because older MQTT instances do not support the connector feature. You can check the creation time of your MQTT instance. To use the connector feature, you must create a new MQTT instance.