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From "Ellis Pritchard (JIRA)" <j...@apache.org>
Subject [jira] Commented: (COCOON-2146) Using EventAware cache implementation breaks persistent cache restore on restart
Date Mon, 26 Nov 2007 20:25:43 GMT

    [ https://issues.apache.org/jira/browse/COCOON-2146?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12545589
] 

Ellis Pritchard commented on COCOON-2146:
-----------------------------------------

Thanks for your comment, Ard.

Your analysis may well explain some performance problems we were seeing on our system; unfortunately
I was not around at the time to analyze them, so I can't really be sure what caused them.
The action they took at the time was simply to cache less, i.e. only the expensive stuff,
rather than, well, basically everything!

Presumably though, if site-usage is fairly uniform, stuff that expires and is still referenced
from AbstractDoubleMapEventRegistry will get regenerated again at some fairly near point,
and replace the old entry with the new one, allowing it to be freed by the JVM? Only if you're
generating lots of unique pages or short-lived pages and caching them for a long time may
it become a 'real' problem?

I think that ehcache 1.4 (now in beta) may provide at least one solution to this problem,
with its new listeners functionality, which can get notification of the keys of entries removed
(for any reason) from the cache.

However, maybe there's a 'redesign the whole thing' kind of solution too?

As for whether we need it or not: according to "the powers that be" we do need a persistent
cache, so that when we bring the site up under load, on n instances (each with their own cache),
it doesn't hammer our database. The fact that was running without one for 8 months due to
this bug is not enough to convince them otherwise!  It's either that, or having to pre-warm
the cache during site-down time (and making site down time longer), or even worse, running
the whole site behind mod_cache with an expiry time of about an hour or so, to cover any lag
in getting it up again. A persistent EventCache just looks so attractive compared to those
options...



> Using EventAware cache implementation breaks persistent cache restore on restart
> --------------------------------------------------------------------------------
>
>                 Key: COCOON-2146
>                 URL: https://issues.apache.org/jira/browse/COCOON-2146
>             Project: Cocoon
>          Issue Type: Bug
>          Components: Blocks: Event Cache
>    Affects Versions: 2.1.10, 2.1.11-dev (Current SVN)
>            Reporter: Ellis Pritchard
>            Assignee: Grzegorz Kossakowski
>            Priority: Minor
>             Fix For: 2.1.11-dev (Current SVN)
>
>         Attachments: patch.txt
>
>
> In revision 412307 (Cocoon 2.1.10), AbstractDoubleMapEventRegistry and EventRegistryDataWrapper
were changed (without an informative SVN comment!) to use the commons MultiValueMap instead
of the MultiHashMap; I presume this was done in good faith because the latter map is deprecated
and will be removed from Apache commons-collections 4.0
> However, as a result, the persistent cache cannot be restored if the EventAware cache
implementation is used, since MultiValueMap is not Serializable! The old MultiHashMap was...
> Depending on whether StoreEventRegistryImpl or DefaultEventRegistryImpl is used, either
the event cache index is never written (ehcache doesn't store non-serializable objects on
disk), or a java.io.NotSerializableException is thrown (and caught, causing a full cache-clear)
when attempting to restore the event cache index.
> This is Major for us, since we use Event-based caching alot, and this is causing the
*entire* cache to no-longer persist across restarts (it's been like that for 8 months, since
I upgraded Cocoon to 2.1.10 in the last week I was working here, and now I'm back, they've
actually noticed!!)
> Work-around at the moment is to down-grade AbstractDoubleMapEventRegistry and EventRegistryDataWrapper
to the 2.1.9 versions (pre-412307), which works so long as Apache-commons 3.x is still in
use.

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