When a Rabbit queue has multiple consumers, messages received by the queue are served in a round-robin fashion to the consumers. Each message is sent to only one consumer subscribed to the queue. Let’s say you had a queue named seed_bin and consumers Farmer Bob and Farmer Esmeralda subscribed to seed_bin. As messages arrive in seed_bin, the deliveries would look like this:
1 Message_A arrives in the seed_bin queue.
2 RabbitMQ sends Message_A to Farmer Bob.
3 Farmer Bob acknowledges receipt of Message_A.
4 RabbitMQ removes Message_A from the seed_bin queue.
5 Message_B arrives in the seed_bin queue.
6 RabbitMQ sends Message_B to Farmer Esmeralda.
7 Farmer Esmeralda acknowledges receipt of Message_B.
8 RabbitMQ removes Message_B from the seed_bin queue.
摘自<RabbitMQ in Action>
若存在多个consumer,每个consumer的负载可能不同，有些处理的快有些处理的慢，RabbitMQ并不管这些，只是简单的以round-robin的方式分配message，这可能造成某些consumer积压很多任务处理不完而一些consumer长期处于饥饿状态，可以使用prefetch_count=1的basic_qos方法可告知RabbitMQ只有在consumer处理并确认了上一个message后才分配新的message给他，否则分给另一个空闲的consumer。
来源：oschina
链接：https://my.oschina.net/u/2326611/blog/725659