View source for Introduction to Persistence Layer (ELUG)

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[[Special:Whatlinkshere/Introduction to Persistence Layer (ELUG)|Related Topics]]</div>This section provides the conceptual overview of the persistence layer of an EclipseLink application.

<span id="Persistence Layer Concepts"></span>
The purpose of your application's persistence layer is to use [[#transactions-sessions|a session]] at run time to associate [[Introduction to EclipseLink Application Development (ELUG)#Mapping Metatdata|mapping metadata]] and a [[#transactions-data-access|data source]] in order to create, read, update, and delete persistent objects using the [[#transactions-cache|EclipseLink cache]], [[#transactions-queries-and-expressions|queries and expressions]], as well as [[#transactions-transactions|transactions]].

This section introduces the following persistence layer concepts:
* [[#transactions-sessions|Sessions]]
* [[#transactions-data-access|Data Access]]
* [[#transactions-cache|Cache]]
* [[#transactions-queries-and-expressions|Queries and Expressions]]
* [[#transactions-transactions|Transactions]]



==<span id="transactions-sessions">Sessions</span>==
A session is the primary interface between the client application and the EclipseLink runtime, and represents the connection to the underlying data source.

For POJO projects, EclipseLink offers several different [[Introduction%20to%20EclipseLink%20Sessions%20(ELUG)|session types]], each optimized for different design requirements and architectures. The most commonly used session is the server session–a session that clients access on the server through a client session. The server session provides a shared cache and shared connection resources.

In JPA projects, sessions are used internally as follows: 
* an <tt>EntityManagerFactory</tt> wraps an <tt>org.eclipse.persistence.threetier.ServerSession</tt>;
* an <tt>EntityManager</tt> wraps an <tt>org.eclipse.persistence.sessions.UnitOfWork</tt> and an <tt>org.eclipse.persistence.threetier.ClientSession</tt>.



==<span id="transactions-data-access">Data Access</span>==
The login (if any) associated with a session determines how the EclipseLink runtime connects to the project's data source.

A login includes details of data source access, such as authentication, use of connection pools, and use of external transaction controllers. A login (an instance of <tt>Login</tt> interface) owns a data source platform.

A platform includes options, such as binding, use of native SQL, use of batch writing, and sequencing, that are specific to a particular data source. For more information, see [[Introduction%20to%20Data%20Access%20(ELUG)#Data Source Platform Types|Data Source Platform Types]].

For projects that do not persist to a data source, a login is not required. For projects that do persist to a data source, a login is always required.

For more information, see [[Introduction%20to%20Data%20Access%20(ELUG)|Introduction to Data Access]]



==<span id="transactions-cache">Cache</span>==
By default, an EclipseLink session provides an object level cache that guarantees object identity and enhances performance by reducing the number of times the application needs to access the data source. EclipseLink provides a variety of cache options, including locking, refresh, invalidation, isolation, and coordination. Using cache coordination, you can configure EclipseLink to synchronize changes with other instances of the deployed application. You configure most cache options at the session level. You can also configure cache options on a per-query basis, or on a descriptor to apply to all queries on the reference class.

For more information, see [[Introduction%20to%20Cache%20(ELUG)|Introduction to Cache]].



==<span id="transactions-queries-and-expressions">Queries and Expressions</span>==
EclipseLink provides several object and data query types, and offers flexible options for query selection criteria, including the following:
* EclipseLink expressions
* JP QL
* SQL
* Stored procedures
* Query by example

With these options, you can build any type of query. We recommend using predefined queries to define application queries. Predefined queries are held in the project metadata and referenced by name. This simplifies application development and encapsulates the queries to reduce maintenance costs.

Regardless of the architecture or persistent entity type, you are free to use any of the query options. The Workbench provides the simplest way to define queries. Alternatively, you can build queries in code, using the EclipseLink API.

For more information, see the following:
* [[Introduction%20to%20EclipseLink%20Queries%20(ELUG)|Introduction to EclipseLink Queries]]
* [[Introduction%20to%20EclipseLink%20Expressions%20(ELUG)|Introduction to EclipseLink Expressions]]



==<span id="transactions-transactions">Transactions</span>==
EclipseLink provides the ability to write transactional code isolated from the underlying database and schema by using a '''unit of work'''.

The unit of work isolates changes in a transaction from other threads until it successfully commits the changes to the database. Unlike other transaction mechanisms, the unit of work automatically manages changes to the objects in the transaction, the order of the changes, and changes that might invalidate other EclipseLink caches. The unit of work manages these issues by calculating a minimal change set, ordering the database calls to comply with referential integrity rules and deadlock avoidance, and merging changed objects into the shared cache. In a clustered environment, the unit of work also synchronizes changes with the other servers in the coordinated cache.

If an application uses EJB entity beans, you do not access the unit of work API directly, but you still benefit from its features: the integration between the EclipseLink runtime and the Java EE container automatically uses the unit of work.

For more information, see [[Introduction%20to%20EclipseLink%20Transactions (ELUG)|Introduction to EclipseLink Transactions]].



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