Prefetch

Scenario

Prefetch limits the number of unacknowledged messages. Once a consumer has more unacknowledged messages than the prefetch limit, the server stops sending messages to the consumer, unless at least one message is acknowledged. Prefetch is essentially a flow control measure on consumers.

Consider the following factors when setting prefetch:

• If the limit is too low, the performance may be affected, because RabbitMQ keeps waiting for the permission to send messages.
• If the limit is too high, a large number of messages may be transmitted to a consumer, leaving other consumers idle. In addition, you also need to consider consumer configurations. When processing messages, consumers save all messages in the memory. A high prefetch limit may affect consumer performance and may even crash the consumer.

For details about prefetch, see Consumer Prefetch.

What Is a Proper Prefetch Limit?

• If you have only one or a few consumers processing messages, it is recommended that you prefetch multiple messages at a time to keep the client busy. If your processing time and network status are stable, you can obtain an estimated prefetch value by dividing the total round-trip time by the processing time of each message on the client.
• If you have a large number of consumers and the processing time is short, a low prefetch limit is recommended. However, if the limit is too low, consumers will be idle after they have processed a batch of messages but the next batch has not yet arrived. If the limit is too high, a single consumer may be busy while other consumers are idle.
• If you have a large number of consumers and the processing time is long, set the prefetch value to 1 so that messages can be evenly distributed among all consumers.

Setting the Prefetch Value

The following example shows how to set the prefetch value to 10 for a single consumer on a Java client.

ConnectionFactory factory = new ConnectionFactory();

Connection connection = factory.newConnection();
Channel channel = connection.createChannel();

channel.basicQos(10, false);

QueueingConsumer consumer = new QueueingConsumer(channel);
channel.basicConsume("my_queue", false, consumer);

On a Java client, the default value of global is false. Therefore, the preceding example can be simply written as channel.basicQos(10).

The values of global are described as follows.