[jira] Commented: (JDO-620) datastore identifier needed for improving replication of objects

["Marco (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/JDO-620?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12656179#action_12656179 ] 

Marco commented on JDO-620:
---------------------------

Jörg, I thought quite a while about it and - unfortunately - was busy with other projects, thus the delay. Well, I think it is indeed necessary to serve both use cases. Especially, I think it's very likely that we will earlier or later face the situation that multiple organisations (=datastores) can modify the same object. That's why I think in combination with the datastore-identifier, we need a new versioning strategy: A version that combines the datastore idenfier of the last change AND the (long or timestamp - i.e. maybe 2 versioning strategies?!) version assigned by this datastore.

This way, a change being replicated preserves the version number (no increment during replication) and at the same time changes in multiple datastores are supported.

However, I still see a problems in this: Let's assume that an object has been replicated from datastore A to datastore B and has changed in datastore B. Thus, the versions are now: A.111 and B.2. Now, a new replication happens from A to B thus changing the version in B to be A.111, too. If we now modify the object in B again, it should be B.3, but the current version is A.111 - how do we know that the version was B.2 the last time that we modified it in B?

Hmmm... This seems to require some more thoughts. Anyone having any useful ideas?

> datastore identifier needed for improving replication of objects
> ----------------------------------------------------------------
>
>                 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 - http://www.jfire.org - 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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