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Locking and Version [message #759244]

Mon, 28 November 2011 02:37

pop prem

Messages: 20
Registered: June 2010

Junior Member

Hi,

I need to clarify some of my doubts related to eclipselink locking.

How can i configure eclipselink that a row level lock should be held rather than a table level? I'm using spring transactions and havn't touched any configurations related to locking in eclipselink, so it uses default locking (optimistic). Please correct me if i'm wrong here.

I have added @Version column in my entity classes to support locking mechanism. Is this a good way or do i have to use the Locking API in code too like entityManager.lock or @Lock etc? Should i use these locks in code even when i have configured database transactions through spring? I'm confused with the usage of code level locking and declarative transaction management.

Thanks.

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Re: Locking and Version [message #759263 is a reply to message #759244]

Mon, 28 November 2011 07:11

Tom Eugelink

Messages: 817
Registered: July 2009

Senior Member

Actually it is quite simple; Eclipselink per default uses optimistic locking, this means that it actually does not use any explicit locking. All that happens is that if a record is updated, the version on which this applies is specified.
For example, suppose you have a record with version 5 and Eclipse link updates it:

UPDATE record SET somefield = somevalue, version = 6 WHERE somekey = anothervalue AND version = 5;

If one record is changed (Eclipselink check for that), all is fine. But if no record is updated, someone has already done an update (or delete) and in that case Eclipselink will throw an exception.

So Eclipselink does not do any explicit locking. Naturally this all happens in a transaction and the update will place a write lock during the transaction. But wheter that is a row, page or table level locking is configured in the database.

This behavior is pretty unrelated to transaction management. Transaction management configures how long a transaction is kept open (usually only during the request).

Tom

On 2011-11-28 03:37, popprem wrote:
> Hi,
>
> I need to clarify some of my doubts related to eclipselink locking.
>
> How can i configure eclipselink that a row level lock should be held rather than a table level? I'm using spring transactions and havn't touched any configurations related to locking in eclipselink, so it uses default locking (optimistic). Please correct me if i'm wrong here.
>
> I have added @Version column in my entity classes to support locking mechanism. Is this a good way or do i have to use the Locking API in code too like entityManager.lock or @Lock etc? Should i use these locks in code even when i have configured database transactions through spring? I'm confused with the usage of code level locking and declarative transaction management.
>
> Thanks.

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Re: Locking and Version [message #759280 is a reply to message #759263]

Mon, 28 November 2011 09:08

pop prem

Messages: 20
Registered: June 2010

Junior Member

Thanks a lot for your reply Tom.

So what i understood is, Locking machanism is handled by eclipselink and it is not related to spring transactions and the table/row level locking is based on the database configurations.

Is it enough only to have a version attribute in each entities and just enable transactions through Spring and go with persist/update/delete operations? If so, in what kind of situations should we use the locking API provided by Eclipselink (code level locking like entityManager.lock() etc)??

Thanks.

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Re: Locking and Version [message #759281 is a reply to message #759263]

Mon, 28 November 2011 09:08

pop prem

Messages: 20
Registered: June 2010

Junior Member

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Re: Locking and Version [message #759375 is a reply to message #759244]

Mon, 28 November 2011 13:48

James

Messages: 272
Registered: July 2009

Senior Member

Row versus table locks are up to your database, and can only be configured on your database. You can execute a native SQL query if you really want to lock an entire table, although not normally a good idea.

See,
http://en.wikibooks.org/wiki/Java_Persistence/Locking
--
James : http://wiki.eclipse.org/EclipseLink : http://en.wikibooks.org/wiki/Java_Persistence : http://java-persistence-performance.blogspot.com/

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Re: Locking and Version [message #759418 is a reply to message #759280]

Mon, 28 November 2011 15:46

Tom Eugelink

Messages: 817
Registered: July 2009

Senior Member

> So what i understood is, Locking machanism is handled by eclipselink and it is not related to spring transactions and the table/row level locking is based on the database configurations.

Correct.

> Is it enough only to have a version attribute in each entities and just enable transactions through Spring and go with persist/update/delete operations? If so, in what kind of situations should we use the locking API provided by Eclipselink (code level locking like entityManager.lock() etc)??

Yes. And I don't know any real world scenario where the EM.lock would be used. Maybe the Eclipselink guys can give an example.
I have one scenario in which I do some outside-of-Eclipse locking, but I fall back to regular SQL statements for that.

Tom

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