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So ActiveMQ uses a prefetch limit on how many messages can be streamed to a consumer at any point in time. Once the prefetch limit is reached, no more messages are dispatched to the consumer until the consumer starts sending back acknowledgements of messages (to indicate that the message has been processed). The actual prefetch limit value can be specified on a per consumer basis.

Its a good idea to have large values of the prefetch limit if you want high performance and if you have high message volumes. If you have very few messages and each message takes a very long time to process you might want to set the prefetch value to 1 so that a consumer is given one message at a time. Specifying a prefetch limit of zero means the consumer will poll for more messages, one at a time, instead of the message being pushed to the consumer.

Pooled Connections and prefetch

Consuming messages from a connection pool can be problematic due to prefetch. Unconsumed prefetched messages are only released when a connection is closed, but with a pooled connection the connection close is deferred (for reuse) till the connection pool closes. This leaves prefetched messages unconsumed till the connection is reused. This feature can present as missing or out-of-sequence messages when there is more than one connection in the pool.
One solution is to use pooled connections for producers and a non-pooled connection for consumers. This might have performance impacts on the consumer side, if multiple threads try to consume messages at a fast rate. Alternatively, reduce the pool size to 1 for consumers. A third alternative is to reduce the prefetchSize to 1 or 0 with the pooled connection factory. When using Spring JMS and MessageDrivenPojo, you cannot use a prefetch of 0, so use 1 instead.