Java Data Objects

Architecture Within Application JVM

JDO supports a variety of architectures within the application’s JVM context. Your application can have one or multiple PersistenceManagers accessing the same or different datastores concurrently. Each PersistenceManager has its own persistent instance cache and its own associated Transaction instance, which manages a distinct transactional context. A JDO implementation may also maintain a shared cache of instances (not visible to applications) to optimize the application’s access of data in the datastore.

Single PersistenceManager

The simplest JDO application architecture has a single PersistenceManager, as illustrated in Figure 3-1. A PersistenceManager is the primary interface used by the application to access persistent services. It is an interface that is implemented by an instance of the JDO implementation. The persistent instances are managed in a cache, where they are used directly by the application. The JDO implementation manages the persistent instances both by using application control (e.g., using PersistenceManager and Query methods), and transparently (when the application accesses a field that is not loaded). The cache contains other artifacts, used to track the identity and state of the instances, but these artifacts are not visible to the application. Whenever we mention the cache, we are referring to the cache of persistent instances.

Figure 3-1. Application using a single PersistenceManager to access a datastore

The application ...