[jira] Commented: (QPID-2921) c++ broker: Improvements to asynchronos completion

["Ken Giusti (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/QPID-2921?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12997848#comment-12997848 ] 

Ken Giusti commented on QPID-2921:
----------------------------------

I've submitted fixes for a sub-set of the issues described in this bug in time for the 0.10 release.  However, some issues remain.

Fixes have been submitted for:

i1) Remove the blocking of the polling thread by SessionState when pending for completion of outstanding async commands.
i2) Fix the enqueueAsync/enqueueComplete race condition that could result in multiple completions per single message
r1) allow clustering to hold off the completions of (various) commands
r2) provide a completion model that will allow performing completions from a thread that may not be the same as the initiating thread.

The following issues remain:

i3) do not allow an outbound message to satisfy a message.accept until the message is deleted from the queue's async store
i4) a message becomes eligible to satisfy a message.accept on a per-dequeue basis, not the last of all dequeues as it is currently done
i5) a message.accept references multiple messages - all must enforce rules i3 & i4 in order for the message.accept to complete.

Rather than leaving this issue unresolved for 0.10, I've cloned a new bug specifically for the remaining issues: 
https://issues.apache.org/jira/browse/QPID-3079


> c++ broker: Improvements to asynchronos completion
> --------------------------------------------------
>
>                 Key: QPID-2921
>                 URL: https://issues.apache.org/jira/browse/QPID-2921
>             Project: Qpid
>          Issue Type: Improvement
>          Components: C++ Broker
>    Affects Versions: 0.8
>            Reporter: Alan Conway
>            Assignee: Ken Giusti
>             Fix For: 0.9
>
>         Attachments: msg.patch, proposal1.diff
>
>
> ** Overview
> Asynchronous completion means that command execution is initiated in one thread
> (a client connection thread) and completed in a different thread.
> When the async store is loaded, message.transfer commands are
> completed by a store thread that does the async write.
> ** Issues with asynchronous completion code as of revision r1029686
> *** Not really asynchronous
> IncompleteMessageList::process blocks the connection thread till all
> outstanding async commands (messages) for the session are complete.
> With the new cluster, this could deadlock since it is blocking a Poller thread.
> *** Race condition for message.transfer
>     
> Sketch of the current code:
> // Called in connection thread 
> PersistableMessage::enqueueAsync { ++counter; } 
> // Called in store thread once message is written.
> PersistableMessage::enqueueComplete { if (--counter == 0) notifyCompleted(); }
> The intent is that notify be called once per message, after the
> message has been written to each queue it was routed to.
> However of a message is routed to N queues, it's possible for
> notifyCompleted to be called up to N times. The store thread could
> call notifyCompleted for the first queue before the connection thread
> has called enqueueAsync for the second queue, and so on.
> *** No asynchronous completion for message.accept
> We do not currently delay completion of message.accept until the
> message is deleted from the async store. This could cause duplicate
> delivery if the broker crashes after completing the message but 
> before it is removed from store.
> There is code in PersistableMessage to maintain a counter for dequeues
> analogous to to the async enqueue code but this is incorrect. 
> Completion of transfer is triggered when all enqueues for a message are complete.
> Completion of accept is triggered for *each* dequeue from a queue independently.
> Furthermore a single accept can reference many messages, so it can't be associated with a message.
> ** New requirements
> The new cluster design will need to participate in async completion, e.g.
> an accept cannot be comlpeted until the message is 
> - removed from store (if present) AND
> - replicated to the cluster (if present) as dequeued
> The new cluster also needs to asynchronously complete binding commands
> (declare, bind, delete) when they are replicated to the cluster.

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