Nondurable Identity
Some datastores cannot provide a unique identity that can be used to locate a specific piece of data. This limitation can be common in log files, history files, and similar files, where performance is a primary concern and there is no need for the overhead associated with managing a durable identity for each datastore instance. Objects are typically inserted into the datastore with transactional semantics, but they are not accessed by key. They may have references to instances elsewhere in the datastore, but often they have no keys or indexes themselves. They might be accessed by other attributes, and they might be deleted in bulk. JDO defines a nondurable identity type for use when accessing instances in such datastores.
Multiple objects in the datastore might have the same values; we refer to them as duplicate objects. An application may want to treat the duplicate objects individually. For example, the application should be able to count the persistent instances to determine how many have the same values. In addition, if the application changes a single field of one duplicate instance, exactly one instance has its field changed in the datastore. If multiple duplicate instances are modified in memory, then instances in the datastore are modified to correspond with the instances modified in memory. Similarly, if an application deletes a specific number of duplicate objects, it should delete this same number of objects in the datastore.
As another example, a single ...
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