[QPID-2921] c++ broker: Improvements to asynchronos completion

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.8
• Fix Version/s: 0.9
• Component/s: C++ Broker
• Assignee: Ken Giusti
• Reporter: Alan Conway
• Created: 01/Nov/10 18:57
• Updated: 29/Jul/13 19:06
• Resolved: 22/Feb/11 15:25

Description

• • 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.

Issue Links

• is cloned by: QPID-3079 message.accept command should be completed on a per-dequeue basis
• relates to: QPID-2935 Support "best effort" producer flow control within the AMQP 0.10 implementation.