ApsaraMQ for MQTT:FAQ about features

Does cloud-side message subscription support child topics?

No. Cloud-side message subscription delivers all messages under the parent topic. There is no child topic-level filtering.

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

What is the wildcard subscription limit?

Each parent topic supports up to 100 wildcard subscription patterns. This limit is fixed. Upgrading the instance or changing the instance type does not increase it. The server keeps only the 100 most recent patterns and automatically deletes older ones.

This limit counts distinct wildcard *patterns*, not individual client subscriptions. For example:

• topicA/+ and topicA/test/+ count as two patterns.

• Two clients both subscribing to topicA/+ count as one pattern.

Is there a limit on exact topic subscriptions?

No. Exact (non-wildcard) topic subscriptions have no limit.

When are subscription relationships retained?

Set CleanSession to false to retain subscription relationships. This persists subscriptions for the client ID, so the client resumes message consumption after reconnecting.

Explicitly calling Unsubscribe removes the subscription relationship regardless of the CleanSession setting.

How do I clear subscription relationships for a topic?

Configure a cleanup period for the topic. For details, see Subscription relationship cleanup.

Deleting a topic does not automatically clear its subscription relationships. To clear subscriptions for a deleted topic, submit a ticket.

Do shared subscriptions support wildcards?

Not by default. To enable wildcard support for shared subscriptions, submit a ticket.

Why are User Properties empty in shared subscriptions?

In MQTT 5.0 shared subscriptions, User Properties in received messages are empty. Non-shared subscriptions deliver User Properties correctly.

The shared subscription implementation uses MQTT 3.1.1 internally to maintain compatibility with shared subscriptions in MQTT 3.x protocols. Because MQTT 3.1.1 does not define User Properties, this field is unavailable in shared subscriptions. There are no plans to support this feature in the near future.

Why is there a forward slash (/) appended to topics?

This depends on the instance version:

• Version 3.2.0: A forward slash (/) is automatically appended to topic names.

• Version 3.3.0: No forward slash is appended.

Instances cannot be upgraded from one version to the other.

Are unused topics automatically deleted?

No. ApsaraMQ for MQTT does not automatically delete topics or Group IDs. These resources are released only when the instance expires, at which point all topics and groups within it are removed.

What is the maximum retention period for offline messages?

Three days. Offline messages are retained only when QoS is set to 1 and cleanSession is set to false. Messages are automatically deleted after three days.

Platinum Edition instances support customization. To request customization, contact ApsaraMQ for MQTT technical support (DingTalk group: 116015007918).

When are offline messages delivered?

The server pushes offline messages to a client after the client comes online.

What is a good keepalive interval?

90 seconds, which is the default in ApsaraMQ for MQTT.

Can I connect by using an IP address instead of a domain name?

No. Always use a domain name to connect to ApsaraMQ for MQTT. The underlying IP address is dynamic and may change, which can cause connection failures.

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

The cloud-side SDK is designed for integrating cloud business applications. For details, see Overview of cloud-side development.

Does ApsaraMQ for MQTT support clustering consumption on the device side?

No. The device-side SDK supports only broadcasting consumption. The cloud-side SDK supports clustering consumption.

A common pattern: use the device-side SDK to publish messages and the cloud-side SDK to subscribe. This gives you clustering consumption on the receiving side. For details, see Overview of cloud-side development.

Can an MQTT instance be restarted?

No. MQTT instances cannot be restarted.

Can instances be migrated across Alibaba Cloud accounts?

No. Cross-account instance migration is not supported.

Why does a connector task show an abnormal status?

Older MQTT instances do not support the connector feature. Check the creation date of your instance. If it predates the connector feature, create a new instance to use connectors.

Does ApsaraMQ for MQTT support will messages and retained messages?

No. Will messages and retained messages are not currently supported.

Can monitoring alert recipients be changed?

No. Monitoring alerts are sent to the mobile number bound to the Alibaba Cloud account. The recipient cannot be changed.