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From Stefan Guggisberg <stefan.guggisb...@day.com>
Subject Re: Modified externally exception due to eviction from shared ism cache
Date Wed, 19 May 2010 14:15:35 GMT
hi berry,


On Tue, May 18, 2010 at 12:22 PM, Berry van Halderen
<b.vanhalderen@1hippo.com> wrote:
> I've created an issue in Jira for the modified externally problem I've
> stumbled upon as I've now been able to reproduce it with a unit test.  Quite a
> large amount of investigation because a lot of factors must be in place.
> In the Jira issue the unit test, and patches are attached, so I won't have to
> post them here.
>
>  https://issues.apache.org/jira/browse/JCR-2633
>
> 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.

phew! ;) thanks a lot for sharing your very detailed and excellent analysis!
i know how difficult it is to track down those kind of issues...

>
> There are multiple approaches to fix this:
> - One could prevent the overlayed states being evicted by holding some
>  alternative reference to it (patch provided (holdrefs.patch));
> - modify the cache implementation at the shared item state manager such that
>  it will simply never release states from its secondary cache unless all
>  items are also gone from all local item state caches;
> - replace the modcount implementation, where instead of counting modifications
>  a state is marked dirty.  This is probably too much work, but has the
>  advantage of avoiding problems with a modcount which can actually reset
>  anyway (it being a short making it not unlikely);
> - ensure the modification count of the jcr:mixinTypes is stored by the
>  persistence manager (patch provided in jira issue (persistMixinTypes.patch).
>
> I would prefer the latter.  It holds more firm to the JCR model where
> meta-data as mixinTypes just properties, and it holds the apparent contract
> that modcount is to be persisted.  In my opinion it is unjust that the
> mixinTypes property wasn't persisted.  Apart from jcr:mixinTypes, only
> jcr:uuid and jcr:primaryType aren't persisted, but those are immutable
> therefor they are really in a different league.

i agree, i.e.  +1 for ensure the modification count of the
jcr:mixinTypes is stored by the
persistence manager

>
> The patches also contrain a small modification as suggestion such that the
> caching mechanism may be overridden.  Caching strategies are never a one-size
> fits all, therefor it is good to make it pluggable.

i tend to disagree ;) the 'cache' of the SISM is not a 'cache' in the
traditional sense
but has special semantics which are essential for maintaining data
integrity. i have
tried to explain this repeatedly in the past on this list. 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...

-1 for making the SISM cache 'pluggable'

cheers
stefan

> And a small modification
> to make jackrabbit-core compilable with Java5.
>
> \Berry
>

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