What's the difference between SimpleMessageListenerContainer and DirectMessageListenerContainer in Spring AMQP? I checked both of their documentation pages, SimpleMessageListenerContainer has almost no explanation on inner workings, and DirectMessageListenerContainer has the following explanation:
The SimpleMessageListenerContainer is not so simple. Recent changes to the rabbitmq java client has facilitated a much simpler listener container that invokes the listener directly on the rabbit client consumer thread. There is no txSize property - each message is acked (or nacked) individually.
I don't really understand what these mean. It says listener container that invokes the listener directly on the rabbit client consumer thread. If so, then how does SimpleMessageListenerContainer do the invocation?
I wrote a small application and used DirectMessageListenerContainer and just to see the difference, I switched to SimpleMessageListenerContainer, but as far as I can see there was no difference on RabbitMQ side. From Java side the difference was in methods (SimpleMessageListenerContainer provides more) and logs (DirectMessageListenerContainer logged more stuff)
I would like to know the scenarios to use each one of those.
The SMLC has a dedicated thread for each consumer (concurrency) which polls an internal queue. When a new message arrives for a consumer on the client thread, it is put in the internal queue and the consumer thread picks it up and invokes the listener. This was required with early versions of the client to provide multi-threading. With the newer client that is not a problem so we can invoke the listener directly (hence the name).
There are a few other differences aside from txSize.
See Choosing a Container.