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Revision as of 22:42, 17 June 2010

Shared and Isolated Cache

Isolated Cache

This method always goes to the database for the initial read operation of an object whose descriptor is configured as isolated. By avoiding the shared session cache, you do not need to use the more complicated descriptor and query APIs to disable cache hits or always refresh. For more information about isolated client sessions, see Isolated Client Sessions. This is particularly useful for achieving serializable transaction isolation.

Isolated Client Sessions

An isolated client session is a special type of client session that provides its own session cache. This session cache is isolated from the shared session cache of its parent server session.

If in your EclipseLink project you configure all classes as isolated (see Configuring Cache Isolation at the Project Level), or one or more classes as isolated (see Configuring Cache Isolation at the Descriptor Level), then all client sessions that you acquire from a parent server session will be isolated client sessions.

This figure illustrates the relationship between a parent server session's shared session cache and its child isolated client sessions.

Isolated Client Sessions

Isolated Client Sessions

Each isolated client session owns an initially empty cache and identity maps used exclusively for isolated objects that the isolated client session accesses while it is active. The isolated client session's isolated session cache is discarded when the isolated client session is released.

When you use an isolated client session to read an isolated class, the client session reads the isolated object directly from the database and stores it in that client session's isolated session cache. When you use the client session to read a shared class, the client session reads the shared object from the parent server session's shared session cache. If the shared object is not in the parent server session's shared session cache, it will read it from the database and store it in the parent server session's shared session cache.

Isolated objects in an isolated client session's isolated session cache may reference shared objects in the parent server session's shared session cache, but shared objects in the parent server session's shared session cache cannot reference isolated objects in an isolated client session's isolated session cache.


Note: You cannot define mappings from shared classes to isolated classes.


Client sessions can access the data source using a connection pool, or an exclusive connection. To use an exclusive connection, acquire the isolated client session using a ConnectionPolicy (see How to Acquire a Client Session that Uses Exclusive Connections). Using an exclusive connection provides improved user-based security for reads and writes. Named queries can also use an exclusive connection (see Configuring Named Query Advanced Options).


Note: If an isolated session contains an exclusive connection, you must release the session when you are finished using it. We do not recommend relying on the finalizer to release the connection when the session is garbage-collected. If you are using an active unit of work in a JTA transaction, you do not need to release the client session–-the unit of work will release it after the JTA transaction completes.


Use isolated client sessions to do the following:

For more information, see the following:


Isolated Client Sessions and Oracle Virtual Private Database (VPD)

Oracle9i Database Server (and later) provides a server-enforced, fine-grained access control mechanism called Virtual Private Database (VPD). VPD ties a security policy to a table by dynamically appending SQL statements with a predicate to limit data access at the row level. You can create your own security policies, or use Oracle's custom implementation of VPD called Oracle Label Security (OLS). For more information on VPD and OLS, see the following:

http://www.oracle.com/technology/deploy/security/index.html.


To use the Oracle Database VPD feature in your EclipseLink-enabled application, use isolated client sessions.

Any class that maps to a table that uses VPD must have the descriptor configured as isolated (see Configuring Cache Isolation at the Descriptor Level).

When you use isolated client sessions with VPD, you typically use exclusive connections (see How to Acquire a Client Session that Uses Exclusive Connections).

To support VPD, you are responsible for implementing session event handlers that the EclipseLink runtime invokes during the isolated client session life cycle (see Isolated Client Session Life Cycle). The session event handler you must implement depends on whether or not you are using Oracle Database proxy authentication (see VPD with Oracle Database Proxy Authentication and VPD Without Oracle Database Proxy Authentication).

For information, see Configuring Exclusive Isolated Client Sessions for Virtual Private Database.


VPD with Oracle Database Proxy Authentication

If you are using Oracle Database proxy authentication ( Oracle Database Proxy Authentication), you must implement a session event handler for the following session events:

By using Oracle Database proxy authentication, you can set up VPD support entirely in the database. That is, rather than making the isolated client session execute SQL (see Using PostAcquireExclusiveConnection Event Handler and Using PreReleaseExclusiveConnection Event Handler), the database performs the required setup in an after login trigger using the proxy session_user.


VPD Without Oracle Database Proxy Authentication

If you are not using Oracle Database proxy authentication, you must implement session event handlers for the following session events:

In your implementation of these handlers, you obtain the required user credentials from the ConnectionPolicy associated with the session (see How to Acquire a Client Session that Uses Connection Properties).


Isolated Client Session Life Cycle

This section provides an overview of the key phases in the life cycle of an isolated session, including the following:

  • Setup required before using an isolated session
  • Interaction among isolated session objects
  • Clean-up required after using an isolated session

To enable the life cycle of an isolated session, use this procedure:

  1. Prepare VPD configuration in the database.
  2. Configure your project and session:
  3. Acquire an isolated session:
    • If you are using Oracle Database proxy authentication:
      Session myIsolatedClientSession = 
      server.acquireClientSession();

      Because you configured one or more descriptors as isolated, myIsolatedClientSession is an isolated session with an exclusive connection.
    • If you are not using Oracle Database proxy authentication:
      ConnectionPolicy myConnPolicy = (ConnectionPolicy)server.getDefaultConnectionPolicy().clone();
      myConnectionPolicy.setProperty("credentials", myUserCredentials);
      Session myIsolatedClientSession = server.acquireClientSession(myConnectionPolicy);
      


      Set the user's credentials as appropriate properties on myConnectionPolicy. Because you configured one or more descriptors as isolated, myIsolatedClientSession is an isolated session with an exclusive connection.
      The EclipseLink runtime raises a SessionEvent.PostAcquireExclusiveConnection event handled by your SessionEventListener (see Using PostAcquireExclusiveConnection Event Handler).
  4. Use myIsolatedClientSession to interact with the database.
    If the EclipseLink runtime raises a SessionEvent.NoRowsModified event, it is handled by your SessionEventListener (see Using NoRowsModifiedSessionEvent Event Handler).
  5. When you are finished using myIsolatedClientSession, release the isolated session:
    myIsolatedClientSession.release();
    

    The EclipseLink runtime prepares to destroy the isolated cache and to close the exclusive connection associated with this isolated session.
    The EclipseLink runtime raises a SessionEvent.PreReleaseExclusiveConnection event handled by your SessionEventListener (see Using PreReleaseExclusiveConnection Event Handler).
  6. Repeat steps #3 to #5 (as required) until the application exits.

Isolated Client Session Limitations

For the purposes of security as well as efficiency, observe the limitations described in the following section, when you use isolated client sessions in your EclipseLink three-tier application:


Mapping

Consider the following mapping and relationship restrictions when using isolated sessions with your relational model:

  • Isolated objects may be related to shared objects, but shared objects cannot have any relationships with isolated objects.
  • If a table has a VPD security policy associated with it, then the class mapped to that table must be isolated.
  • If one of the tables in a multiple table mapping is isolated, then the main class must also be isolated.

The EclipseLink runtime enforces these restrictions during descriptor initialization.


Inheritance

Aggregates and aggregate mappings inherit the isolated configuration of their parents.

If a class is isolated, then all inheriting classes should be isolated. Otherwise, if you relate a shared class to a shared superclass with isolated subclasses, it is possible that some of the isolated subclasses will lose object identity when the isolated session is released.

To give you the flexibility to mix shared and isolated classes, the EclipseLink runtime does not enforce these restrictions during descriptor initialization. If you wish to mix shared and isolated classes in your inheritance hierarchy, then you must be prepared to deal with this possible loss of object identity.


Caching and Cache Coordination

Isolated classes are never loaded into the shared cache of a parent server session. Isolated classes cannot be used with cache coordination.


Sequencing

We recommend that you do not configure a sequencing object or sequence table using VPD security. EclipseLink does not access sequencing objects using the isolated session's dedicated connection, and so VPD restricted sequence values are not available to the isolated session. Sequence objects not using VPD security are fine.

Transactions and JTA

We recommend that you explicitly release an isolated session when you are finished using it, rather than wait for the Java garbage collector to invoke the finalizer. The finalizer is provided as a last resort: waiting for the garbage collector may cause errors when dealing with a JTA transaction.


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