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From "Stefan Guggisberg (JIRA)" <j...@apache.org>
Subject [jira] Commented: (JCR-2633) Modified externally exception when modifying mixinTypes with single session
Date Wed, 19 May 2010 14:30:57 GMT

    [ https://issues.apache.org/jira/browse/JCR-2633?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12869145#action_12869145
] 

Stefan Guggisberg commented on JCR-2633:
----------------------------------------

+1 in general for the persistMixinTypes.patch

-1 for making the SISM cache pluggable. 

the 'cache' of the SISM is not a 'cache' in the traditional sense
but has special semantics which are essential in maintaining data
integrity and are an integral part of jackrabbit's implementation of
isolation levels (read committed). making it 'pluggable' would
probably open the door for lots of new issues where people used 
some supposedly 'better' generic caching framework and are 
surprised when they encounter corrupted repositories... 

> 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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