Spring Cloud Stream: spring.cloud.stream.default.consumer.concurrency property exactly meaning

The semantics of concurrency is dependent on the actual binder implementation. For example in the case of the Kafka binder, when you set concurrency, that value will be passed on to the underlying MessageListenerContainer. Lets say that your consumer application is consuming from a topic that has 3 partitions and you set the value of concurrency to 3. This will create 3 threads by the container in which each of them is handling a single partition from the topic (In fact, in the case of Kafka binder, 3 listener containers will be created behind the scenes). If there are more partitions on the topic than there are concurrent threads, then the partitions will be distributed across the various threads. The same semantics apply for the Rabbit binder as well although the actual implementation of it may vary slightly internally.