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From "Stefan Guggisberg" <stefan.guggisb...@gmail.com>
Subject Re: Concurrent access problems
Date Thu, 28 Feb 2008 08:48:15 GMT
hi alexander

On Tue, Feb 26, 2008 at 12:22 PM, Foo Bar <mittspamkonto@gmail.com> wrote:
> Hi.
>
>  I have just made my first attempt to use Jackrabbit in a bigger
>  project, and I'm having problems with concurrent access.
>
>  I'm using jackrabbit 1.4 with the example repository.xml in JBoss 4.2.2.GA.
>
>  The first test I made is simple. I tried to change an implementation
>  that currently uses a filesystem to store files to use jackrabbit.
>  Users upload files, these get unique id:s and I save them in a flat
>  directory:
>
>  root
>  |
>  files
>  |-1
>  |-2
>  |-3
>  etc..
>
>  Each file-node is created according to the recipe at
>  http://wiki.apache.org/jackrabbit/ExamplesPage (jcr:content subnode
>  etc). The save is as simple as possible; I get the session, create the
>  file subnode with binary content and logout.
>  The id:s are fetched from a db (auto-increment), so they are unique
>  across transactions.
>
>  The saving works fine, until I start hitting the page with a few
>  (tenths of) simultaneous uploads. I then get validation exceptions,
>  telling me about conflicting changes:
>
>   javax.jcr.InvalidItemStateException:
>  6570ddc8-29a7-4335-a9ca-76dc98687d22: the item cannot be saved because
>  it has been modified externally.
>         at org.apache.jackrabbit.core.ItemImpl.getTransientStates(ItemImpl.java:378)
>         at org.apache.jackrabbit.core.ItemImpl.save(ItemImpl.java:1083)
>         at org.apache.jackrabbit.core.SessionImpl.save(SessionImpl.java:896)
>         at org.apache.jackrabbit.jca.JCASessionHandle.save(JCASessionHandle.java:178)
>
>  I've read the discussion here:
>  http://www.nabble.com/Problem-in-Multithreaded-Environment-td14779040.html
>  (nabble is down, google cache here:
>  http://66.102.9.104/search?q=cache:jqbwF-fsAjoJ:www.nabble.com/Problem-in-Multithreaded-Environment-td14779040.html)
>
>  This discussion, together with the issue at
>  http://issues.apache.org/jira/browse/JCR-584 basically says that it
>  should be ok to have several sessions concurrently adding subnodes to
>  a node, without any problems. The JIRA issue only explicitly says that
>  two sessions concurrently can add subnodes, and I assumed this to be
>  valid for any number of sessions. Is this a correct assumption?

there's a caveat when adding samename-sibling nodes. the current implementation
of NodeStateMerger doesn't merge concurrently added samename sibling nodes.
this only applies for the 1st entry, i.e. when 2 sessions try to add a
'foo[1]' child node;
see the @todo comment in o.a.j.core.state.NodeStateMerger#merge.
as a result you would see an Exception like the one mentioned above.

apart from that you might run into JCR-1359. i am currently working on
this issue.
i have made some progress but wasn't able to resolve it completely yet.

>
>  Is there anything I should keep in mind when trying to make this work?
>  Any properties who need to be set on nodes or in workspace? Any save
>  order that might cause problems?
>
>  I'm having trouble reproducing the problem in unit tests, so any hints
>  as to why this is happening would be greatly appreciated.
>
>  I don't really see serializing access as an option, since the plan is
>  to have many concurrent saves. I therefore ruled out locks. Is it true
>  that locks always imply serial access?

serial write access that is, yes.

cheers
stefan

>
>
>  Regards
>
>  Alexander
>

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