rabbitmq 学习 之Consumer prefetch count（34）
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.
AMQP 0-9-1指定basic.qos方法，限制了通道（或连接）上未确认消息的数量（也称为“预取计数”）。不幸的是，通道不是理想的范围 - 因为单个通道可能消耗多个队列，所以通道和队列需要为发送的每个消息相互协调，以确保它们不超过限制。这在单台机器上很慢，而在跨群集消耗时非常慢。
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:
因此，RabbitMQ 在basic.qos方法中重新定义了 全局标志的含义：
| `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.
| `global` | AMQP 0-9-1中`prefetch_count`的含义 | RabbitMQ中`prefetch_count`的含义 |
|---|---|---|
| 假 | 在渠道上的所有消费者之间共享这个count | 单独应用于channel的每个新消费者 |
| 真 | 在连接上所有消费者共享这个count | 在渠道上的所有消费者之间共享这个count |
请注意，在大多数API中，global标志 的默认值为false。
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.