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From "Marcel Reutegger (JIRA)" <j...@apache.org>
Subject [jira] Commented: (JCR-2633) Modified externally exception when modifying mixinTypes with single session
Date Thu, 20 May 2010 12:05:56 GMT

    [ https://issues.apache.org/jira/browse/JCR-2633?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12869577#action_12869577
] 

Marcel Reutegger commented on JCR-2633:
---------------------------------------

How is backward compatibility handled with persistMixinTypes.patch?

> Modified externally exception when modifying mixinTypes with single session
> ---------------------------------------------------------------------------
>
>                 Key: JCR-2633
>                 URL: https://issues.apache.org/jira/browse/JCR-2633
>             Project: Jackrabbit Content Repository
>          Issue Type: Bug
>          Components: jackrabbit-core
>    Affects Versions: 1.5.7, 2.0.0, 2.1.0
>         Environment: Bundle persistency manager based storage, Jackrabbit 1.5.7 but issue
also present in newer versions
>            Reporter: Berry van Halderen
>            Priority: Critical
>         Attachments: holdrefs.patch, issue.patch, persistMixinTypes.patch
>
>
> When first adding mixins and later removing all mixins, a system under heavy stress might
experience a modified externally exception even though there is only a single session into
the repository.
> The unit test with forced GC and shrinking of caches indicates the problem.
> The unit test itself is mostly trivial, the problem arrises when you add a
> mixin to a node, save it, do this again with another mixin and then remove
> both mixins and in the following save the shared item state cache is shrunk,
> and the garbage collectors hits at exactly the right time.  In the unit test a
> reference to the jcr:mixinTypes property must have been held.
> What will happen is that the ItemState of the jcr:mixinTypes property will get
> a modification count higher than 0 when addin the mixins.  Because a reference
> to the property is kept, it will not be evicted from the primary cache in the
> local item state manager.  When removing all mixins, an overlayed state will
> be created of this ItemState.  Because the state and overlayed state are
> linked, the ItemState won't be dropped from the primary cache of the shared
> item state manager.
> But is MAY be evicted from the secondary cache implementing the LRU/FIFO
> functionality.  If this is the case when while persisting the changes there is
> a small window where the overlayed state will be disconnected from the state.
> This happens just before collecting the changelog at:
>   o.a.j.c.state.ItemState.disconnect():211
>   o.a.j.c.state.SessionItemStateManager.disconnectTransientItemState():674
>   o.a.j.c.PropertyImpl.makePersistent():143
>   o.a.j.c.ItemImpl.persistTransientItems():609
>   o.a.j.c.ItemImpl.save():1087
>   o.a.j.c.SessionImpl.save():858
> Or in the when actually collecting the changelog in one of the methods
> Changelog.{modified(),deleted() or both.  I think the latter, but not really
> sure.
> In any case, this breaks the bondage that prevents the cached state in the
> shared item state manager.
> Now if the shared item state cache has been shrunk enough and the GC hits
> before o.a.j.c.state.SharedItemStateManager.Update.begin():650 when the
> disconnected state will be purged from the shared item state cache.  Just
> below line 650 the check for stale items will then cause a re-retrieval from
> the persisted store of the property.  Because that state will have a
> modification count of 0, it will conflict with the modification count of the
> mixin property type that is being persisted.
> It is true that the GC needs to go off at exactly the right time and the
> secondary cache needs to have shrunk to be able to evict the item.  This can
> however happen in extreme cases.  The patch that contains the unit test
> contains code that will force the purging of the item.
> There is still the matter why the modcount comes back at 0 when retrieving the
> property from the persistence manager, basically the assumption made in the
> code between session, local, and shared item state managers, their caches,
> etcetera is that the modification count in the shared item state is always
> incremented, and never reset.
> There is an apparent contract (partially documented) that the modification
> count is to be persisted.  Which is in fact the case for the InMemPersistence-
> Manager, but all bundle derived persistence managers do not persist the
> jcr:mixinType property at all, but just the mixintypes as part of the
> nodedefinition information.  This means that the jcr:mixinTypes property will
> always be re-created with modcount of 0, which leads to clashes.

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