[jira] Commented: (JCR-314) Fine grained locking in SharedItemStateManager

["Oliver Zeigermann (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/JCR-314?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12488896 ] 

Oliver Zeigermann commented on JCR-314:
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Any idea how this could be achieved?

If you use blocking locks, it is very hard to avoid deadlocks. Of course you can use an ordered sequence of locks for all requests to avoid deadlocks, but that would mean you need a list of all resources to be locked before you even start doing anything. 

After my experience with the Slide server I can only recommend non blocking locks. Such locks would make a request fail if it is not able to acquire all locks necessay instead of letting it wait. This would require a notion of a transaction, however.

> 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
>
> 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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