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Choosing a Container

Version 2.0 introduced the DirectMessageListenerContainer (DMLC). Previously, only the SimpleMessageListenerContainer (SMLC) was available. The SMLC uses an internal queue and a dedicated thread for each consumer. If a container is configured to listen to multiple queues, the same consumer thread is used to process all the queues. Concurrency is controlled by concurrentConsumers and other properties. As messages arrive from the RabbitMQ client, the client thread hands them off to the consumer thread through the queue. This architecture was required because, in early versions of the RabbitMQ client, multiple concurrent deliveries were not possible. Newer versions of the client have a revised threading model and can now support concurrency. This has allowed the introduction of the DMLC where the listener is now invoked directly on the RabbitMQ Client thread. Its architecture is, therefore, actually “simpler” than the SMLC. However, there are some limitations with this approach, and certain features of the SMLC are not available with the DMLC. Also, concurrency is controlled by consumersPerQueue (and the client library’s thread pool). The concurrentConsumers and associated properties are not available with this container.

The following features are available with the SMLC but not the DMLC:

  • batchSize: With the SMLC, you can set this to control how many messages are delivered in a transaction or to reduce the number of acks, but it may cause the number of duplicate deliveries to increase after a failure. (The DMLC does have messagesPerAck, which you can use to reduce the acks, the same as with batchSize and the SMLC, but it cannot be used with transactions — each message is delivered and ack’d in a separate transaction).

  • consumerBatchEnabled: enables batching of discrete messages in the consumer; see Message Listener Container Configuration for more information.

  • maxConcurrentConsumers and consumer scaling intervals or triggers — there is no auto-scaling in the DMLC. It does, however, let you programmatically change the consumersPerQueue property and the consumers are adjusted accordingly.

However, the DMLC has the following benefits over the SMLC:

  • Adding and removing queues at runtime is more efficient. With the SMLC, the entire consumer thread is restarted (all consumers canceled and re-created). With the DMLC, unaffected consumers are not canceled.

  • The context switch between the RabbitMQ Client thread and the consumer thread is avoided.

  • Threads are shared across consumers rather than having a dedicated thread for each consumer in the SMLC. However, see the IMPORTANT note about the connection factory configuration in Threading and Asynchronous Consumers.

See Message Listener Container Configuration for information about which configuration properties apply to each container.