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From "Christopher A. Larrieu (JIRA)" <j...@apache.org>
Subject [jira] Commented: (AMQ-688) Avoid blocking producers
Date Wed, 12 Apr 2006 22:21:24 GMT
    [ https://issues.apache.org/activemq/browse/AMQ-688?page=comments#action_36039 ] 

Christopher A. Larrieu commented on AMQ-688:
--------------------------------------------

While experimenting with some code to force Queues to evict stored messages from memory, I
discovered that a bug in the Journal was causing messages that should have been evicted not
to be.  Somehow the list of listeners on the UsageManager was getting trampled, so that the
journal was never getting notification that memory utilization was getting high.  I updated
to the most recent revision of the source (r392487) and now persistent messages do get evicted
from Queues as expected, and producers don't get stalled or throttled by consumers.

> Avoid blocking producers
> ------------------------
>
>          Key: AMQ-688
>          URL: https://issues.apache.org/activemq/browse/AMQ-688
>      Project: ActiveMQ
>         Type: New Feature

>   Components: Broker
>     Versions: 4.0 RC 2
>     Reporter: Christopher A. Larrieu
>      Fix For: incubation

>
> Original Estimate: 8 weeks
>         Remaining: 8 weeks
>
> Our main goal
> is to avoid stalled producers by addressing the main culprit: too many undispatched messages
> in the broker's memory.  Our motivation is to handle significant --though temporary--
imbalances
> between production and consumption rates.
> Reaching this goal entails specific broker modifications:
> 1. When memory gets tight, start dropping undispatched non-persistent messages.  This
is the
> first-cut attempt to maintain throughput of persistent messages.
> Unlike the approach documented at http://docs.codehaus.org/display/ACTIVEMQ/Slow+Consumer+Handling,
> the message dropping will only occur after the UsageManager reaches capacity.  Non-persistent
> messages in dispatch lists will be dropped according to per-destination policy.  Subscriptions
> can purge their own messages triggered via callback from the UsageManager.
> 2. Evict messages if memory remains tight, to be fetched from backing store prior to
dispatch.
> ActiveMQ already supports this for persistent messages on Topics with durable subscriptions.
>  If a consumer's prefetch buffer is full, the splash-over messages remain as IndirectMessageReference
> objects in the dispatch list, with the actual message body loaded from store on demand.
 I
> believe we can extend this approach for Queues as well.  
> 3. Improve the efficiency with which evicted messages are loaded back into memory.  
> Currently, they are loaded one at a time as needed.  It would make sense to batch-load
message
> sets periodically.  This will require a significant shift in responsibilities between
objects,
> since an IndirectMessageReference doesn't know about other instances that can be loaded
in
> mass.
>  
> The goal will be to keep each subscription dispatch list stocked with a decent number
of messages
> in-memory to reasonably trade-off between it's consumer's performance and resource usage
in
> the broker.  As with everything else, we can implement this as a strategy class with
the first
> cut implementing a simple resource allocation strategy: divvy up available memory amongst
> all subscriptions and keep that memory filled with messages for dispatch.  I envision
a worker
> task assuming responsibility for keeping these lists filled.
> 4. Even with the above modifications, we still can't entirely avoid blocked producers,
so
> we'd like to add client-configurable time-outs to provide a bound for the time a producer
> can remain stalled.
> Maybe this should be a new attribute of ActiveMQConnection: maxProducerFlowControlWait.
 Calls
> to UsageManager.waitForSpace can take this quantity as an argument.  Failure to reach
sufficient
> space for the new message will throw an exception back up the stack and across the wire,
letting
> the producer know that the message was not delivered.
> 5. Finally, we need to extend disk support for Topics that have only non-durable subscribers,
> otherwise their dispatch lists can fill up with persistent messages.  In order to maintain
> compliance with JMS, it would be nice to provide some alternative to dropping persistent
messages.
> One possible first cut is to layer this on top of a DurableTopicSubscription by creating
an
> anonymous subscriber for every Topic that has only non-durable subscriptions.  When all
such
> subscriptions terminate, the broker can remove the anonymous subscriber.

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