message.accept command may be completed on a per-dequeue basis
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Key: QPID-3079
URL: https://issues.apache.org/jira/browse/QPID-3079
Project: Qpid
Issue Type: Improvement
Components: C++ Broker
Affects Versions: 0.8
Reporter: Ken Giusti
Assignee: Ken Giusti
** 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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