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Date: Mon, 10 Apr 2006 19:33:52 +0100 (BST)
From: "Christopher A. Larrieu (JIRA)" <jira@apache.org>
To: activemq-dev@geronimo.apache.org
Subject: [jira] Created: (AMQ-688) Avoid blocking producers
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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


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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