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From adam <adam.suss...@gmail.com>
Subject Proposal to parallelize subscription recovery and retroactive playback
Date Tue, 14 Dec 2010 01:07:40 GMT

There is a pretty severe concurrency issue right now with retroactive
subscriptions and durable subscription playback.  Both operations lock
the topic such that it cannot accept new messages while a playback 
is occurring.  They also prevent multiple playbacks from happening
simultaneously.

This is a big problem if playback takes any significant amount of time.
In my application, stopping other consumers from seeing messages for 
more than a few seconds is unacceptable and I can easily cue up many
minutes worth of replay.

Additionally, if the recovery action requires locks or complex activity
that conflict with other connection/subscription oriented locks then
deadlock problems become likely.  See AMQ-3070 for an example.

So, I want to fix this, and I am willing to do the work, but I need
some guidance from you guys on the right thing to do.

Basically, I want to:

1) Remove dispatchValve from broker/region/Topic and move it to
the Subscription object instead in the form of a recovery specific
lock.  Topic.dispatch would ignore subscriptions in a recovery state.

2) Create an independent thread for running
SubscriptionRecoveryPolicy.recover

3) Create an independent thread for running Store.recoverSubscription

4) Implement the correct synchronization in #2 and #3 to allow the topic
to continue dispatching messages and allow recovery to not drop those
messages while recovering and to stop recovering at the right time.

#4 is of course the hard part.  The difficulties there explain the current
design.  I can see pretty well how to do this for retroactives, but I am
a little more fuzzy on durable recovery from the message store.

I would like to believe that I can just simply synchronize Topic.dispatch
with a simple read lock check on the subscription 'in recovery' lock
and call it done, but I am not sure about that.

If there are any thoughts you guys have on doing this properly, please let
me know.  I want to get it right and give you guys a patch.

thanks,

-adam

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