(Newbie)Optimistic Locking for Web Apps

[RogerV]

Hi

Could someone explain or point to a tutorial on how to implement optimistic locking correctly in a web-application. I've added the version column and annotation to my entity and I can see the version number incrementing in the database every time I perform an update. I've read the article at http://java.dzone.com/articles/dont-break-optimistic-locking but I can still overwrite changes.

The logic goes basically - Client A wants to update an entity. The web-app opens the entity manager, gets the entity, closes the entity manager so the entities are now detached and displays the entity to Client A in an HTML form. When client A submits the form, the web-app creates a new instance of the entity, opens the entity manager, merges the new entity and closes the entity manager again. At this point I see the version number increment in the database. However, if while Client A is thinking about changing the entity, client B selects the same entity, displays and submits his changes, they're being overwritten when client A submits their changes - I'm not getting an OptimisticLockingException being thrown and I see the version number increment by 2.

Since the entity manager is closed between getting and updating the entity, is it even possible to expect EclipseLink to deal with this?

[RogerV]

RogerV wrote:

Hi

Could someone explain or point to a tutorial on how to implement optimistic locking correctly in a web-application. I've added the version column and annotation to my entity and I can see the version number incrementing in the database every time I perform an update. I've read the article at http://java.dzone.com/articles/dont-break-optimistic-locking but I can still overwrite changes.

Please accept my apologies - I missed a crucial step from the example!

Regards

[tch]

Just for future people, why don't you let us know what you missed?

./tch



On Thu, Sep 10, 2009 at 8:16 AM, RogerV <roger.varley@...> wrote:

>
>
>
> RogerV wrote:
>>
>> Hi
>>
>> Could someone explain or point to a tutorial on how to implement
>> optimistic locking correctly in a web-application. I've added the version
>> column and annotation to my entity and I can see the version number
>> incrementing in the database every time I perform an update. I've read the
>> article at http://java.dzone.com/articles/dont-break-optimistic-locking
>> but I can still overwrite changes.
>>
>
> Please accept my apologies - I missed a crucial step from the example!
>
> Regards
> --
> View this message in context: http://www.nabble.com/%28Newbie%29Optimistic-Locking-for-Web-Apps-tp25381983p25382223.html
> Sent from the EclipseLink - Users mailing list archive at Nabble.com.
>
> _______________________________________________
> eclipselink-users mailing list
> eclipselink-users@...
> https://dev.eclipse.org/mailman/listinfo/eclipselink-users
>

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[RogerV]

tch wrote:

Just for future people, why don't you let us know what you missed?

I'm using the Struts 2 framework which will "automagically" create a new instance of the entity for me. I ommitted to include the version number as a hidden field on the form to return to the framework, so Struts 2 just gave it the default value which is insufficient to trigger the optimistic locking. Once I added the version attribute to the html form, optimistic locking works like a charm - it's actually embarassingly easy to implement!

So in summary;

1) Add the version number field to your database table.
2) Add the version annotation to your entity.java
3) Include the version number as a hidden field in your html data entry form and return it with the update request.
4) Create a new Entity instance with the returned data (this is the bit that Struts does for you)
5) EntityManager.merge(entity)

Regards