[jira] [Commented] (AMQ-5379) AMQP - allow setting prefetch size

["Robbie Gemmell (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/AMQ-5379?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14231394#comment-14231394 ] 

Robbie Gemmell commented on AMQ-5379:
-------------------------------------

Hi Dejan,

I think we need to change how the credit is observed after the initial flow, patch attached. As AMQPProtocolConverter could create the 'ActiveMQ consumer' with a 1000 msg prefetch (if we havent already also had a Flow before the Attach actually gets processed), it is likely it could buffer up to that many messages sends before we ever get a Flow, at which point the way the credit is currently observed (via getRemoteCredit()) wont actually give the amount of credit we received but rather what is currently believed to be left unused after accounting for buffered messages, leading us to underestimate the actual prefetch (it could even be negative if it already buffered sends for more messages than the actual received credit). I think getCredit() seems to be what to use instead.

Having said all that, I am now also thinking that a better way to go for now might be creating the 'ActiveMQ consumer' with 0 prefetch until the first flow arrives, and then giving it that much prefetch, since the AMQP consumer cant/wont actually be sent any messages over the wire until it gives the broker/proton credit anyway. This would help with multiple consumers etc by avoiding the 'ActiveMQ consumers' prefetching lots of messages they cant actually transmit to the AMQP consumer yet.

The above may possibly make the general issue underlying AMQ-5453 a little worse, since the 'ActiveMQ consumer' wouldnt have any 'advance credit' to use to get messages buffered ready to send before a 'drain' flow occurs. As it is currently almost entirely broken anyway, I'm not sure this would really be much more of an issue than already exists, which will still need a general fix either way. A possible saving grace in this specific case though is that I see that client is sending two flows, one with drain false then one with it true, which might still appear to work occasionally if messages get sent inbetween them.

I think the changes here mean that anyone using ClientAcknowledge with the JMS clients will find it isnt able to recieve more messages than the consumers configured prefetch size within a single ack window, since the desire for more messages is communicated by granting more credit, which would now have no effect. That in itself isnt necessarily worse than it was in ActiveMQ 5.10, which had an internal fixed window of 100 messages, the only difference is that if AMQP clients set their prefetch down below the old fixed 100 (e.g, to 1) they then wont be able to exceed that, whereas previously they would have apparently been able to since up to 99 messages would have actually been buffered awaiting the new credit. I guess that by using the new 'transport.prefetch' config to set a value will essentially force it to behave as it did previously, which may be a workaround in some cases.

When looking an issue relating to transactions, I came across the 'prefetch extension' concept in PrefetchSubscription. It currently looks to mainly use acknowlegement of specific messages to operate, but perhaps it could be modified to use credit (since we cant tell which specific messages, if any, that incoming credit may have been previously associated with) and help toward enabling things to work more along the lines of expectation for AMQP.

Sorry for writing another book size comment, I really dont mean to when I start :P

Robbie

> AMQP - allow setting prefetch size
> ----------------------------------
>
>                 Key: AMQ-5379
>                 URL: https://issues.apache.org/jira/browse/AMQ-5379
>             Project: ActiveMQ
>          Issue Type: Bug
>          Components: AMQP
>    Affects Versions: 5.10.0
>            Reporter: Dejan Bosanac
>            Assignee: Dejan Bosanac
>             Fix For: 5.11.0
>
>
> Currently the prefetch size is hardcoded to the value of 100



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