API required for enabling/disabling FOR UPDATE locking for SELECTs
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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
I've 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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