Latest revision as of 13:47, 29 May 2012

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Key API

• @Cacheable
• @Cache
• CacheIsolationType
• ReferenceMode

Shared, Isolated, Protected, Weak, and Read-only Cache

EclipseLink defines three cache isolation levels. The cache isolation level defines how caching for an entity is performed by the persistence unit and the persistence context.

The cache isolation levels are:

• Isolated - entities are only cached in the persistence context, not in the persistence unit.
• Shared - entities are cached both in the persistence context and persistence unit, read-only entities are shared and only cached in the persistence unit.
• Protected - entities are cached both in the persistence context and persistence unit, read-only entities are isolated and cached in the persistence unit and persistence context.

Isolated Cache

The isolated cache (L1) is the cache stored in the persistence context. It is a transactional or user session based cache. Setting the cache isolation to ISOLATED for an entity disables its shared cache. With an isolated cache all queries and find operations will access the database unless the object has already been read into the persistence context and refreshing is not used.

Use a isolated cache to do the following:

• avoid caching highly volatile data in the shared cache;
• achieve serializable transaction isolation;
• use the Oracle Virtual Private Database (VPD) feature (see Oracle Virtual Private Database (VPD)).

Each persistence context owns an initially empty isolated cache. The persistence context's isolated cache is discarded when the persistence context is closed, or the EntityManager.clear() operation is used.

When you use an EntityManager to read an isolated entity, the EntityManager reads the entity directly from the database and stores it in the persistence context's isolated cache. When you read a read-only entity it is still stored in the isolated cache, but is not change tracked.

The persistence context can access the database using a connection pool or an exclusive connection. The persistence unit property "eclipselink.jdbc.exclusive-connection.mode" can be used to use an exclusive connection. Using an exclusive connection provides improved user-based security for reads and writes. Specific queries can also be configured to use the persistence context's exclusive connection.

Shared Cache

The shared cache (L2) is the cache stored in the persistence unit. It is a shared object cache for the entire persistence unit. Setting the cache isolation to SHARED for an entity enables its shared cache. With a shared cache queries and find operations will resolve against the shared cache unless refreshing is used.

Use a shared cache to do the following:

• improve performance by avoiding database access when finding or querying an entity by Id or index;
• improve performance by avoiding database access when accessing an entity's relationships;
• preserve object identity across persistence contexts for read-only entities.

When you use an EntityManager to find a shared entity, the EntityManager first checks the persistence unit's shared cache. If the entity is not in the persistence unit's shared cache, it will be read from the database and stored in the persistence unit's shared cache, a copy will also be stored in the persistence context's isolated cache. Any query not by Id, and not by an indexed attribute will first access the database. For each query result row, if the object is already in the shared cache, the shared object (with its relationships) will be used, otherwise a new object will be built from the row and put into the shared cache, and a copy will be put into the isolated cache. The isolated copy is always returned, unless read-only is used. For read-only the shared object is returned as the isolated copy is not required.

The size and memory usage of the shared cache depends on the entities cache type. The JPA Cache and EclipseLink JpaCache can also be used to invalidate or clear the cache.

Protected Cache

The protected cache option allows for shared objects to reference isolated objects. Setting the cache isolation to PROTECTED for an entity enables its shared cache. The protected option is mostly the same as the shared option, except that protected entities can have relationships to isolated entities, where as shared can not.

Use a protected cache to do the following:

• improve performance by avoiding database access when finding or querying an entity by Id or index;
• improve performance by avoiding database access when accessing an entity's relationships to shared entities;
• ensure read-only entities are isolated to the persistence context;
• allow relationships to isolated entities.

Protected entities have the same life-cycle as shared entities, except for relationships, and read-only. Protected entities relationships to shared entities are cached in the shared cache, but their relationships to isolated entities are isolated and not cached in the shared cache. The @Noncacheable annotation can also be used to disable caching of a relationship to shared entities. Protected entities that are read-only are always copied into the isolated cache, but are not change tracked.

Cache Isolation Examples

Isolated cache annotation example

...
@Entity
@Cache(
  isolation=CacheIsolationType.ISOLATED
)
public class Employee {
  ...
}

Protected cache annotation example

@Entity
@Cache(
  isolation=CacheIsolationType.PROTECTED
)
public class Employee {
  @Id
  private long id;
  ...
  @OneToMany(mappedBy="manager")
  @Noncacheable
  private List<Employee> managedEmployees;
  ...
}

Isolated cache XML example

<?xml version="1.0"?>
<entity-mappings
	xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/orm"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://www.eclipse.org/eclipselink/xsds/persistence/orm http://www.eclipse.org/eclipselink/xsds/eclipselink_orm_2_4.xsd"
	version="2.4">
    <entity name="Employee" class="org.acme.Employee" access="FIELD">
        <cache isolation="ISOLATED"/>
    </entity>
</entity-mappings>

Protected cache XML example

<?xml version="1.0"?>
<entity-mappings
	xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/orm"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://www.eclipse.org/eclipselink/xsds/persistence/orm http://www.eclipse.org/eclipselink/xsds/eclipselink_orm_2_4.xsd"
	version="2.4">
    <entity name="Employee" class="org.acme.Employee" access="FIELD">
        <cache isolation="PROTECTED"/>
        <attributes>
            <id name= "id"/>
            <one-to-many name="managedEmployees" mapped-by="manager">
                <noncacheable/>
            </one-to-many>
        </attributes>
    </entity>
</entity-mappings>

Weak Reference Mode

EclipseLink offers a specialized persistence context cache for long-lived persistence contexts. Normally it is best to keep persistence contexts short-lived, such as creating a new EntityManager per request, or per transaction. This is referred to as a stateless model. This ensures the persistence context does not become too big, causing memory and performance issues. It also ensures the objects cached in the persistence context do not become stale or out of sync with their committed state.

Some two-tier applications, or stateful models require long-lived persistence contexts. EclipseLink offers a special weak reference mode option for these types of applications. A weak reference mode maintains weak references to the objects in the persistence context. This allows the objects to garbage collect if not referenced by the application. This helps prevent the persistence context from becoming too big, reducing memory usage and improving performance. Any new, removed or changed objects will be held with strong references until a commit occurs.

A weak reference mode can be configured through the "eclipselink.persistence-context.reference-mode" persistence unit property. The following options can be used:

• HARD - This is the default, weak references are not used. The persistence context will grow until cleared or closed.
• WEAK - Weak references are used. Unreferenced unchanged objects will be free to garbage collect. Objects that use deferred change tracking will not be free to garbage collect.
• FORCE_WEAK - Weak references are used. Unreferenced unchanged objects will be free to garbage collect. Unreferenced changed objects that use deferred change tracking will also be free to garbage collection, causing any changes to be lost.

Read-Only Entities

An entity can be configured as read-only using the @ReadOnly annotation or the read-only XML attribute. A read-only entity will not be tracked for changes and any updates will be ignored. Read-only entities cannot be persisted or removed. An read-only entity must not be modified, but EclipseLink does not currently enforce this, modification to read-only objects can corrupt the persistence unit cache.

Queries can also be configured to return read-only objects using the "eclipselink.read-only" query hint.

A SHARED entity that is read-only will return the shared instance from queries. The same entity will be returned from all queries from all persistence contexts. Shared read-only entities will never be copied or isolated in the persistence context. This improves performance by avoiding the cost of copying the object, and tracking the object for changes. This both reduces memory, reduces heap usage, and improves performance. Object identity is also maintained across the entire persistence unit for read-only entities, allowing the application to hold references to these shared objects.

An ISOLATED or PROTECTED entity that is read-only will still have an isolated copy returned from the persistence context. This gives some improvement in performance and memory usage from avoid tracking the object for changes, but it is not as significant as SHARED entities.

Oracle Virtual Private Database (VPD)

Oracle Database Server 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, an isolated cache should be used.

Any entity that maps to a table that uses VPD should have the descriptor configured as isolated.

When you use VPD, you typically should also use exclusive connections.

To support VPD, you are responsible for implementing session event handlers that the EclipseLink runtime invokes during the persistence context's 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).

VPD with Oracle Database Proxy Authentication

By using Oracle Database proxy authentication, you can set up VPD support entirely in the database. That is, rather than session event handlers to execute SQL, 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:

• postAcquireExclusiveConnection: used to perform VPD setup at the time EclipseLink allocates a dedicated connection to an isolated session and before the isolated session user uses the connection to interact with the database.
• preReleaseExclusiveConnection: used to perform VPD cleanup at the time the isolated session is released and after the user is finished interacting with the database.

In your implementation of these handlers, you can obtain the required user credentials from the associated session's properties.

Configuring Caching	Caching	Cache Type and Size
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