ApsaraMQ for RocketMQ:Clustering consumption mode and broadcasting consumption mode

Background information

ApsaraMQ for RocketMQ is a messaging system that is based on the Publish-Subscribe pattern. In ApsaraMQ for RocketMQ, a consumer subscribes to a topic to obtain and consume messages.

In most cases, consumer applications are clustered and deployed in a distributed system. When you subscribe to a topic, you can specify the clustering consumption mode or the broadcasting consumption mode for your consumer clients based on your business requirements. Consumers in the same group consume messages based on the consumption mode that you specified.

Consumers that are identified by the same group ID belong to the same group and must use the same consumption logic and tags. For more information, see Subscription consistency.

Clustering consumption mode

Basic concept

Clustering consumption mode: In this mode, ApsaraMQ for RocketMQ delivers a message to only one of the consumers in the group.

Scenarios

Broadcasting consumption mode

Basic concept

Broadcasting consumption mode: In this mode, ApsaraMQ for RocketMQ broadcasts each message to all consumers in a group to ensure that the message is consumed by each consumer at least once.

Scenarios

Feature differences between the clustering consumption mode and the broadcasting consumption mode

The availability of features varies based on the consumption mode. The following table lists the features that are available in clustering consumption mode and the features that are available in broadcasting consumption mode.

Feature	Clustering consumption mode	Broadcasting consumption mode
TCP client SDKs
HTTP client SDKs
Ordered messages
Consumer offset resetting
Message retry
Accumulated message query and message accumulation alerting
Subscription query
Consumer progress	Consumer progress is maintained by brokers. The service reliability is higher in clustering consumption mode than that in broadcasting consumption mode. After a consumer is restarted, the consumer continues to consume messages based on the last consumption progress. Message retry is supported by brokers. For more information, see Message retry.	Consumer progress is maintained by consumers. The probability that a message is repeatedly consumed in broadcasting consumption mode is higher than that in clustering consumption mode. Each time a consumer is restarted, the consumer starts consumption from the latest message.

Usage notes

Make sure that you specify the same consumption mode for consumers in the same group. If you specify different consumption modes for consumers in the same group, the broadcasting consumption mode takes effect.

For example, you create a group with five consumers. If you specify the clustering consumption mode for two consumers in the group and the broadcasting consumption mode for the other three consumers in the group, the broadcasting consumption mode is used for all five consumers.

Clustering consumption mode

• In clustering consumption mode, each message is delivered to only one consumer in a group for processing. If a message needs to be processed by each consumer in the group, use the broadcasting consumption mode.

• In clustering consumption mode, a message may be routed to different consumers during message retries.

Broadcasting consumption mode

• In broadcasting consumption mode, ApsaraMQ for RocketMQ ensures that a message is delivered to each consumer at least once for consumption, but does not redeliver messages that fail to be consumed. You need to manually handle consumption failures.

• In broadcasting consumption mode, a consumer starts message consumption from the latest message each time the consumer is restarted. The consumer automatically skips the messages that are sent to the ApsaraMQ for RocketMQ broker when the consumer is stopped. Exercise caution when you use this mode.

• In broadcasting consumption mode, each message is repeatedly processed by a large number of consumers. We recommend that you use the clustering consumption mode in most cases.

Configuration methods

If you use a client SDK to subscribe to messages, you can configure the relevant parameter in the SDK to specify a consumption mode.

TCP client SDKs

• Clustering consumption mode

  // Set the MessageModel parameter to CLUSTERING. By default, the clustering consumption mode is used. 
  properties.put(PropertyKeyConst.MessageModel, PropertyValueConst.CLUSTERING);

• Broadcasting consumption mode

  // Set the MessageModel parameter to BROADCASTING. 
  properties.put(PropertyKeyConst.MessageModel, PropertyValueConst.BROADCASTING); 

HTTP client SDKs

HTTP client SDKs support only the clustering consumption mode. You do not need to specify a consumption mode on HTTP client SDKs. For more information, see Usage notes.