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From "Marco (JIRA)" <j...@apache.org>
Subject [jira] Commented: (JDO-619) API required for enabling/disabling FOR UPDATE locking for SELECTs
Date Wed, 27 May 2009 20:45:45 GMT

    [ https://issues.apache.org/jira/browse/JDO-619?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12713724#action_12713724
] 

Marco commented on JDO-619:
---------------------------

Adding an optional boolean parameter to pm.getObjectById(...) IMHO has the downside that already
quite a few overloaded getObjectById(...) methods exist, where one of them already has a boolean
parameter:

a) getObjectById(java.lang.Class cls, java.lang.Object key)
b) getObjectById(java.lang.Object oid)
c) getObjectById(java.lang.Object oid, boolean validate)

There's the question, to which of these getObjectById-methods do you want to add it? Adding
it as last parameter is obviously only possible for (a) and (c) leading to 2 new methods.
Alternatively you could add it at as first parameter and then have 3 new methods.

Is that helpful? Or confusing?

Any other opinions?


The PM.serializeRead(Object) is IMHO a really good idea. If I understand you correctly, that's
more or less the same as writing:

myTransaction.setSerializeRead(true);
pm.refresh(myPersistenceCapable);
myTransaction.setSerializeRead(null);

Right? Probably the only difference is that the above code might overwrite any previously
set value (that's why we encapsulate the Transaction.setSerializeRead(...) calls and employ
reference counting - we're using code like the above quite often).

When you add pm.serializeRead(Object), you'd need additionally pm.serializeReadAll(Collection)
to be somehow symmetric with all other methods (detach/detachAll, refresh/refreshAll etc.)
and to give the JDO implementation the ability for optimization.

> 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
>          Components: api2, specification, tck2
>            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
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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