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From "Jukka Zitting" <jukka.zitt...@gmail.com>
Subject Re: Enforcing constraints; Sessions and Threading
Date Mon, 05 Nov 2007 21:12:31 GMT

On 11/5/07, Dan Diephouse <dan.diephouse@mulesource.com> wrote:
> This is probably a horrendously stupid question, but I'm hoping someone
> can help me figure out how to enforce constraints nodes. I'm writing a
> JCR store for Abdera to store Atom entries. I'm creating a node for each
> entry and one of the properties on the node is the "resource name".
> (I.e. what's in the HTTP URL - /my_entry.atom). I don't ever want to
> have multiple nodes with the same resource name property. Whats the best
> way to enforce this so I can never have two threads create the same
> resource at the same time?

There's nothing like a UNIQUE constraint in JCR or Jackrabbit (though
it might be good to have at least a Jackrabbit feature request in Jira
for that), so the closest you can get probably to make the "resource
name" be the name of the entry node and have all the entries stored as
children of a parent node that doesn't allow same-name-siblings.

Alternatively you need to implement that constraint in your code.
Whenever creating a new node (or modifying the "resource name"
property) you could lock the subtree (or just a separate lock node)
and check that the constraint isn't being violated before saving the

> Also, can anyone point me to something which highlights how Sessions and
> threads are supposed to work? Are sessions single threaded? Can they be
> shared accross threads for reading? Should I pool them? Do I logout at
> app shut down or when I'm done reading data (it seems the former).

You should only access a Session from a single thread at a time.
There's some overhead to starting a new Session (and part of the
caching is Jackrabbit is session-bound), so if your application is
performance-sensitive then you may want to use a session pool.


Jukka Zitting

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