https://www.rabbitmq.com/consumer-prefetch.html
Consumer prefetch is an extension to the consumer prefetch mechanism.
AMQP 0-9-1 specifies the basic.qos method to make it possible to limit the number of unacknowledged messages on a channel (or connection) when consuming (aka "prefetch count"). Unfortunately the channel is not the ideal scope for this - since a single channel may consume from multiple queues, the channel and the queue(s) need to coordinate with each other for every message sent to ensure they don't go over the limit. This is slow on a single machine, and very slow when consuming across a cluster.
Furthermore for many uses it is simply more natural to specify a prefetch count that applies to each consumer.
Therefore RabbitMQ redefines the meaning of the global flag in the basic.qos method:
| `global` | Meaning of `prefetch_count` in AMQP 0-9-1 | Meaning of `prefetch_count` in RabbitMQ |
|---|---|---|
| false | shared across all consumers on the channel | applied separately to each new consumer on the channel |
| true | shared across all consumers on the connection | shared across all consumers on the channel |
Note that the default value for the global flag is false in most APIs.
Single Consumer
The following basic example in Java will receive a maximum of 10 unacknowledged messages at once:
Channel channel = ...;
Consumer consumer = ...;
channel.basicQos(10); // Per consumer limit
channel.basicConsume("my-queue", false, consumer);
Independent Consumers
This example starts two consumers on the same channel, each of which will independently receive a maximum of 10 unacknowledged messages at once:
Channel channel = ...;
Consumer consumer1 = ...;
Consumer consumer2 = ...;
channel.basicQos(10); // Per consumer limit
channel.basicConsume("my-queue1", false, consumer1);
channel.basicConsume("my-queue2", false, consumer2);
Multiple Consumers Sharing the Limit
The AMQP 0-9-1 specification does not explain what happens if you invoke basic.qos multiple times with different global values. RabbitMQ interprets this as meaning that the two prefetch limits should be enforced independently of each other; consumers will only receive new messages when neither limit on unacknowledged messages has been reached.
For example:
Channel channel = ...;
Consumer consumer1 = ...;
Consumer consumer2 = ...;
channel.basicQos(10, false); // Per consumer limit
channel.basicQos(15, true); // Per channel limit
channel.basicConsume("my-queue1", false, consumer1);
channel.basicConsume("my-queue2", false, consumer2);
These two consumers will only ever have 15 unacknowledged messages between them, with a maximum of 10 messages for each consumer. This will be slower than the above examples, due to the additional overhead of coordinating between the channel and the queues to enforce the global limit.
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