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Date: Fri, 14 Sep 2007 01:58:32 -0700 (PDT)
From: "Marcel Reutegger (JIRA)" <jira@apache.org>
To: dev@jackrabbit.apache.org
Subject: [jira] Commented: (JCR-314) Fine grained locking in
 SharedItemStateManager
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    [ https://issues.apache.org/jira/browse/JCR-314?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12527415 ] 

Marcel Reutegger commented on JCR-314:
--------------------------------------

> What may be the consequences of allowing a reader to get a read lock in this situation even if the writer's changelog includes
> the VERSION_STORAGE_NODE_ID ?

That's probably a bad idea. It may happen that a reader sees inconsistent data.

But anyway, I think a versioning operation will modify only the version storage root node in rare cases. Merely when the version storage is nearly empty. As soon as there are enough versions created the version storage root node has all possible child nodes (named 00, 01, ... ff) and is not modified anymore.

> Fine grained locking in SharedItemStateManager
> ----------------------------------------------
>
>                 Key: JCR-314
>                 URL: https://issues.apache.org/jira/browse/JCR-314
>             Project: Jackrabbit
>          Issue Type: Improvement
>          Components: core
>    Affects Versions: 0.9, 1.0, 1.0.1, 1.1, 1.1.1, 1.2.1, 1.2.2, 1.2.3
>            Reporter: Marcel Reutegger
>         Attachments: FineGrainedISMLocking-v2.patch, FineGrainedISMLocking.patch, ISMLocking.patch, writes-per-second.jpg
>
>
> The SharedItemStateManager (SISM) currently uses a simple read-write lock to ensure data consistency. Store operations to the PersistenceManager (PM) are effectively serialized.
> We should think about more sophisticated locking to allow concurrent writes on the PM.
> One possible approach:
> If a transaction is currently storing data in a PM a second transaction may check if the set of changes does not intersect with the first transaction. If that is the case it can safely store its data in the PM.
> This fine grained locking must also be respected when reading from the SISM. A read request for an item that is currently being stored must be blocked until the store is finished.

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