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What is RabbitMQ Prefetch Count?

If you are just getting started you might be wondering what RabbitMQ prefetch count is? It is a common setting to come across and it is something you would probably want to be familiar with.

Answer: RabbitMQ prefetch count is a variable that limits the number of unacknowledged messages that can be consumed. This is used for QOS. It is defined using the prefetch_count variable. It is unlimited by default

Here is an example of how you could use this variable from a Python script:

channel.basic_qos(prefetch_count=1)

Not setting this variable could result in a large number of unacked messages.

This value will have no effect if auto-acks are enabled on the client.

Channel Prefetch Count vs Consumer Prefetch Count

The prefetch count can be set on a channel/connection basis or on a per consumer basis.

These are controlled using the ‘global’ variable.

global=False applied to individual consumers
global=True shared between all consumers on channel

The default value for the global variable is set to false in most APIs.

Setting the prefetch count on a per channel basis can be slow especially on clusters. It often makes sense to turn it off. You can do this by setting global to false.

If global is set to false then the limit will be applied to each individual consumer on the channel. If global is set to true then the limit will be shared between all consumers on that channel.

You can set the consumer prefetch count like this:

channel.basic_qos(prefetch_count=5, global=False)

You can set the channel prefetch count like this:

channel.basic_qos(prefetch_count=5, global=True)

You can set both of these values by including both of these lines:

channel.basic_qos(prefetch_count=5, global=False)
channel.basic_qos(prefetch_count=10, global=True)

Setting the Prefix Count

Some recommendations:

Few consumers with fast processing Use a larger value.
Many consumers with fast processing time Use a smaller value.
Long processing time Set the value to one.

References