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From "Andy Jefferson (JIRA)" <j...@apache.org>
Subject [jira] Updated: (JDO-619) API required for enabling/disabling FOR UPDATE locking for SELECTs
Date Fri, 24 Apr 2009 19:36:31 GMT

     [ https://issues.apache.org/jira/browse/JDO-619?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel

Andy Jefferson updated JDO-619:

    Attachment: jdo619.patch

Updated patch. This time it includes
1). Update to @PersistenceCapable to include "serializeRead" defaulting to false.
2). Update to ComponentMetadata to include serializeRead property, using boolean
3). Addition of get/setSerializeRead on Query taking Boolean.
4). Addition of get/setSerializeRead on Transaction taking Boolean
5). Addition of DTD/XSD for all XML components of JDO2.3 for completeness (these include "serialize-read"
on <class>/<interface> with default of "false". The DTD/XSD for 'orm'/'jdoquery'/'jdoconfig'
aren't changed from 2.2, just copied for completeness since we need to update those for 'jdo'.

Thought it was best to have a default on the metadata (XML/annotations) since that is the
ultimate fallback so we get defined behaviour when nothing is specified. Query and Transaction,
since the override the metadata, use Boolean. Also thought that "false" was the best default
since it likely wouldn't be a good policy to just lock everything. Opinions may differ

> 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
>         Attachments: jdo619.patch
> 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
> 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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