Disk Free Limit and Flow Control

[Malak Elflitty]

Hello,

While reading the documentation, some concepts were not clear to me. I have some questions about control flow in rabbitmq and the effect of disk free limit on control flow:

1- The documentation (Configuration — RabbitMQ)says :

Disk free space limit of the partition on which RabbitMQ is storing data. When available disk space falls below this limit, flow control is triggered.

Does that mean that in normal conditions flow control is not triggered? When available disk space exceeds the limit, values specified for credits are not used and there is no flow control?

2- The second question concerns the effect of disk space on flow control and is also related to the first question. The documentation (Free Disk Space Alarms — RabbitMQ) says:

When free disk space drops below a configured limit (50 MB by default), an alarm will be triggered and all producers will be blocked.

Are produced completely blocked? Or this case, flow control is triggered and producers  are not completely blocked, but they are given limited credits according to the values of credits parameters? 

Thanks in advance for your help.

Best Regards,

Malak

[Michal Kuratczyk]

Credit flow is always used (unless you set credits to 0 or you don't use publisher confirms, in which there is not much RabbitMQ can do to throttle the publisher). What the docs mean here is that publishers will be blocked when an alarm (disk or memory) is triggered. When publishers are blocked, they can't send any messages.

The goal is to give consumers a chance to consume some of the messages, which should free up the disk/memory and then the publishers get unblocked.

Best,

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Michał

RabbitMQ team