[rabbitmq-discuss] Producers hanging when reaching high memory watermark on 1.8.1

Alister Morton Alister.Morton at tradition.com
Mon Aug 16 14:08:25 BST 2010

threshold it, say, is only accepting messages at a rate of 0.01Hz. And
then, finally, it stops accepting messages altogether. Is that last,
minuscule change fundamentally different? I don't think so.

Yes, absolutely, it is a different state. It may appear to the client as indistinguishable but it is, nonetheless, a distinct and identifiably different state. If a broker was down to accepting single message per minute, is that a problem if the client only contributes one message per week? Whereas if the broker has signalled "I absolutely cannot accept any messages at all" then this is an identifiably different situation and may require handling in a different way.

You may decide that so long as the broker is accepting messages at all, however slowly, that you will live with it. That's a decision for the client logic. Equally, you may decide that if and only if a broker has stopped messages completely via flow control then you want to send them elsewhere.

If a system is slowed to a crawl you can't know that it suddenly isn't going to clear. If a system has issued a stop, you know for certain that until told otherwise, nothing is moving. Fundamentally different situations.

I remember being told years ago, never confuse an observed effect (i.e. the system /appears/ to have stopped) with what is defined to be the case (i.e. we have been told to stop sending).

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