datastore identifier needed for improving replication of objects
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Key: JDO-620
URL: https://issues.apache.org/jira/browse/JDO-620
Project: JDO
Issue Type: New Feature
Reporter: Marco
Fix For: JDO 2 maintenance release 3
We are using multiple datastores (spread over multiple servers accross the
world) and have to replicate objects between them. Following the documentation
in http://www.datanucleus.org/products/accessplatform/guides/jdo/replication/
an object replication is done this way:
1) Detach an object graph from datastore A (using pm.detachCopy(...)).
2) Send the object graph to the destination (using Java native serialisation).
3) Mark the complete object graph dirty (recursively).
4) Attach the object graph to datastore B (using pm.makePersistent(...)).
Unfortunately, this imposes the following problems: The JDO implementation
optimizes write operations and thus normally only writes dirty fields to the
datastore. This is a great feature, but it requires step (3) - making all
fields dirty manually - to ensure that existing objects are updated completely.
The negative side effect of this is that the version of the object is
incremented. That means, the object has a higher version in datastore B as in
datastore A, even though the object was simply copied and thus should have the
same version number.
We therefore kindly request the following extension of the JDO standard - if
possible for the next release (2.3):
Every datastore should have an (optional) identifier. This datastore identifier
should be specified in the persistence.xml properties of the
PersistenceManagerFactory. The first time such a PersistenceManagerFactory is
started up, the datastore identifier should be persisted into the datastore
(maybe a configurable, optional operation). At every following startup, the
persistent datastore identifier should be compared with the one in the
persistence.xml. If they do not match, there should be different strategies
configurable in the persistence.xml:
* Throw an exception and do not startup the PMF.
* Copy the value from the persistence.xml into the datastore.
* Use the value from the persistence.xml without writing it into the datastore.
* Use the value from the datastore and ignore the setting in the
persistence.xml.
Please add an API method to the javax.jdo.PersistenceManagerFactory for getting
the datastore identifier:
String getDatastoreIdentifier()
Every detached object should know the datastore identifier of the datastore
where it was detached. It should be possible to access this value via the
JDOHelper. Thus, please add the following method to javax.jdo.JDOHelper:
String getDatastoreIdentifier(Object persistenceCapableObject)
When calling this method on an object which is currently connected to a
PersistenceManager, the method returns the same as would this code:
JDOHelper.getPersistenceManager(myObject).getPersistenceManagerFactory().getDatastoreIdentifier();
When the object has not yet been persisted (i.e. not detached, no
PersistenceManager assigned), the method JDOHelper.getDatastoreIdentifier(...)
should return null (just like JDOHelper.getVersion(...) does).
When there is no datastore identifier configured for the
PersistenceManagerFactory, the result should be an empty String rather than
null.
When attaching an object to a datastore, the JDO implementation should check,
whether this is the same datastore where the object was detached or whether it
is a different one - i.e. a replication is happening. If the object is
replicated, all fields should be written - no matter whether they're dirty or
not. If the object is attached to the same datastore where it was detached
before, only dirty fields should be written.
If there was no change (i.e. there is no dirty field), attaching should not
modify the object's version. This way, a replicated object should always have
the same version as the original.
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