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From "Craig Russell (JIRA)" <j...@apache.org>
Subject [jira] Commented: (JDO-619) API required for enabling/disabling FOR UPDATE locking for SELECTs
Date Fri, 12 Dec 2008 18:06:44 GMT

    [ https://issues.apache.org/jira/browse/JDO-619?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12656085#action_12656085
] 

Craig Russell commented on JDO-619:
-----------------------------------

> One way is to specify this statically in the JDO metadata for a persistence capable class:
e.g. add a boolean property useUpdateLock to the class metadata.

I agree this is a useful feature and it's good to have a standard name for the concept. SerializeRead
is a good crisp concept name, and we could use this for metadata as well as the API:

element class, element interface attribute serialize-read for metadata
Transaction and Query: setSerializeRead and getSerializeRead for API

One issue that I see is when using fetch plans. Which objects should be read with FOR UPDATE?
Only root objects, or the entire fetch plan object graph? 

Another issue is whether you can override the setting in metadata. If you have a class Foo
that in metadata has serialize-read, and your Transaction has SerializeRead false (presumably
the default) do you use serialize-read for Foo or not? I'd say yes. How do you describe this
behavior?



> API required for enabling/disabling FOR UPDATE locking for SELECTs
> ------------------------------------------------------------------
>
>                 Key: JDO-619
>                 URL: https://issues.apache.org/jira/browse/JDO-619
>             Project: JDO
>          Issue Type: New Feature
>            Reporter: Marco
>             Fix For: JDO 2 maintenance release 3
>
>
> We - http://www.jfire.org - have some code where it is essential that objects read from
the datastore are not manipulated by another transaction before they are modified and written
to the datastore. In SQL, you use "SELECT ... FOR UPDATE" for this purpose, which locks the
records included in the query result till the end of the transaction just like a write operation
does.
> In JDO, it is currently not yet possible to control whether reading causes read-locks
(simple SELECT) or write-locks (SELECT ... FOR UPDATE). There are, however, vendor-specific
solutions already. Thus, I'd like to first point out how DataNucleus solves this problem:
> 1) The page RDBMS persistence properties<http://www.datanucleus.org/products/accessplatform/rdbms/persistence_properties.html>
> describes the property "datanucleus.rdbms.useUpdateLock" which applies to all queries.
This would leave our code pure-JDO (not
> DataNucleus-dependent), but it's unfortunately not what we need: Most of the time a lock
is not required and this option would therefore
> unnecessarily slow down our application.
> 2) The page JDOQL<http://www.datanucleus.org/products/accessplatform/rdbms/jdoql.html>
shows this code snippet:
>     ((org.datanucleus.jdo.JDOTransaction)pm.currentTransaction()).setOption(
>         "transaction.serializeReadObjects", "true"
>     );
> This applies to all subsequent queries of one transaction. It works fine to enable/disable
the option back and forth during the same transaction. Obviously, this is the most useful
way to control the use of write-locks during read operations.
> 3) Additionally, the same page mentions that you can set "datanucleus.rdbms.query.useUpdateLock"
as a JDOQL extension. I assume
> that's simply code like this:
>     query.addExtension("datanucleus.rdbms.query.useUpdateLock", "true");
> In contrast to solution (2), this only affects the single explicit query and no implicit
queries which are used when accessing fields of the returned object(s).
> Having explained all this, I'd like to request the following feature for the next JDO
release (2.3):
> Please extend javax.jdo.Transaction and add 2 methods:
>   void setSerializeReadObjects(boolean)
>   boolean isSerializeReadObjects()
> This would make DataNucleus' solution (2) - see above - available via the JDO API.
> Additionally, please extend javax.jdo.Query and add 2 new method:
>   void setSerializeReadObjects(boolean)
>   boolean isSerializeReadObjects()
> This represents JDO-API for DataNucleus' solution (3) - see above.

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