Difference between revisions of "Introduction to Data Access (ELUG)"

Revision as of 11:32, 10 December 2007

One of the most important functions of a session is to provide access to a data source. This section explains session components specific to accessing a data source.

Data Access Concepts

This section describes concepts unique to EclipseLink data access, including the following:

• Externally Managed Transactional Data Sources
• Data Source Login Types
• Data Source Platform Types
• Authentication
• Connections
• Connection Pools

Externally Managed Transactional Data Sources

A EclipseLink transactional data source is externally managed if the connection pool is managed by a transaction service (such as an application server controlled transaction or a JTA transaction). A JTA managed data source or connection pool is commonly used in Java EE applications and normally required in EJB applications. Use an externally-managed connection pool as follows:

• Configure the session to use an ExternalTransactionController to integrate EclipseLink's unit of work with the external transaction service (see Integrating the Unit of Work with an External Transaction Service).
• Use the external-transaction-control option to specify the connection's login and inform EclipseLink that the connection is maintained by the external controller (see Configuring External Connection Pooling).
• You may need to configure the EclipseLink read connection pool or sequence connection pool to use a non-JTA connection pool in order to avoid transactional overhead (see Default (Write) and Read Connection Pools).

For more information on transactional data sources, see the following:

• JTA Controlled Transactions
• OTS Controlled Transactions

Refer to Introduction to EclipseLink Transactions for more information on EclipseLink transactions.

Data Source Login Types

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 owns a data source platform.

A data source platform includes options specific to a particular data source including such as binding, use of native SQL, use of batch writing, and sequencing. For more information about platforms, see 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.

In Workbench, the project type determines the type of login that the project uses, if applicable.

You can use a login in a variety of roles. A login's role determines where and how you create it. The login role you choose depends on the type of project you are creating and how you intend to use the login, as follows:

• POJO Session Role
• Development Role

There is a session login type for each project type that persists to a data source. The following are the types:

• DatabaseLogin
• EISLogin

Note that there is no XML login. EclipseLink XML projects are used for nonpersistent, in-memory object to XML data transformation and consequently there is no data source to log in to. For more information about persistent and nonpersistent projects, see Persistent and Nonpersistent Projects.

For additional information, see the following:

• Projects and Login
• Configuring Common Data Source Login Options

DatabaseLogin

If you are creating a project that accesses a relational database, you must configure the project with a DatabaseLogin. Your choice of DatabasePlatform further customizes your project for a particular type of database (see Database Platforms).

For more information, see Introduction to Database Login Configuration.

EISLogin

If you are creating a project that accesses a nonrelational data source using a JCA adapter, you must configure the project with an EISLogin. Your choice of EISPlatform further customizes your project for a particular JCA adapter and specifies what record type EclipseLink uses to exchange data with the EIS (see EIS Platforms).

For more information, see Introduction to EIS Login Configuration.

Data Source Platform Types

EclipseLink abstracts the details of your underlying data source using data source platform classes. EclipseLink provides the following data source platforms:

• Database Platforms
• EIS Platforms

A data source platform is owned by your project's Login. For more information about logins, see Data Source Login Types.

In general, Workbench provides minimal access to a platform API. To configure most platform options, you must use an amendment method (see Configuring Amendment Methods), or a preLogin event listener (see Managing Session Events with the Session Event Manager).

Database Platforms

EclipseLink interacts with databases using structured query language (SQL). Because each database platform uses its own variation on the basic SQL language, EclipseLink must adjust the SQL it uses to communicate with the database to ensure that the application runs smoothly.

The type of database platform you choose determines the specific means by which the EclipseLink runtime accesses the database, including the type of Java Database Connectivity (JDBC) driver to use. JDBC is an application programming interface (API) that gives Java applications access to a database. EclipseLink relational projects rely on JDBC connections to read objects from, and write objects to, the database. EclipseLink applications use either individual JDBC connections or a JDBC connection pool (see Connection Pools), depending on the application architecture.

EclipseLink provides a variety of database-specific platforms that let you customize your project for your target database.

Oracle Database platforms are located in org.eclipse.persistence.platform.database.oracle package and include the following:

• OraclePlatform
• Oracle8Platform
• Oracle9Platform
• Oracle10Platform
• Oracle11Platform

Non-Oracle Database platforms are located in org.eclipse.persistence.platform.database package and include the following:

• AccessPlatform for Microsoft Access databases
• AttunityPlatform for Attunity Connect JDBC drivers
• CloudscapePlatform
• DB2MainframePlatform
• DB2Platform
• DBasePlatform
• DerbyPlatform
• HSQLPlatform
• InformixPlatform
• JavaDBPlatform
• MySQL4Platform
• PointBasePlatform
• PostgreSQLPlatform
• SQLAnyWherePlatform
• SQLServerPlatform
• SybasePlatform
• TimesTen7Platform for TimesTen 7 database

Specify your database platform at the project level (see Configuring Relational Database Platform at the Project Level) for all sessions, or override this project-level configuration at the session level (see Configuring a Relational Database Platform at the Session Level).

If you set your database platform in Workbench, then Workbench manages the database platform configuration for you automatically.

EIS Platforms

EclipseLink interacts with an EIS data source indirectly by way of a JCA adapter. EclipseLink abstracts the details of an EIS data source using the org.eclipse.persistence.eis.EISPlatform class.

The type of EIS platform you choose determines the specific means by which the EclipseLink runtime accesses the EIS, including the type of JCA adapter to use. EclipseLink EIS projects rely on EIS connections to read objects from, and write objects to, the EIS. EclipseLink applications use individual EIS connections returned by the EIS connection factory specified by the EIS platform.

EclipseLink provides a variety of EISPlaform classes that let you customize your project for your target EIS.

EIS platforms for production are located in org.eclipse.persistence.eis.adapters package and include the following:

• org.eclipse.persistence.eis.adapters.aq.AQPlatform to access an EIS using Oracle Advanced Queuing messages.
• org.eclipse.persistence.eis.adapters.attunity.AttunityPlatform to access an EIS using an Attunity JCA adapter.
• org.eclipse.persistence.eis.adapters.jms.JMSPlatform to access an EIS using JMS messages.
• org.eclipse.persistence.eis.adapters.mqseries.MQPlatform to access an EIS using IBM MQSeries messages.

EIS platforms for testing are also located in org.eclipse.persistence.eis.adapters and include the following:

• org.eclipse.persistence.eis.adapters.blackbox.BlackBoxPlatform for testing your EIS project with the Sun BlackBox reference adapter using indexed records only.
• org.eclipse.persistence.eis.adapters.xmlfile.XMLFilePlatform for testing your EIS project with an EIS emulated as one or more XML files in the local file system using XML records.

Specify your EIS platform at the session level (see Configuring an EIS Login (ELUG)#BABEGEAA Configuring an EIS Data Source Platform at the Session Level).

If you set your platform in Workbench, then Workbench manages the EIS platform configuration for you automatically.

Authentication

Authentication is the means by which a data source validates a user's identity and determines whether or not the user has sufficient privileges to perform a given action.

For two-tier applications, simple JDBC authentication is usually sufficient (see Simple JDBC Authentication).

For three-tier applications, you can use simple JDBC authentication or, proxy authentication (see Oracle Database Proxy Authentication) when using the Oracle Call Interface (OCI) JDBC driver.

Authentication plays a central role in data security and user accountability and auditing (see Auditing).

Simple JDBC Authentication

When you configure a EclipseLink database login with a user name and password ( Configuring User Name and Password), EclipseLink provides these credentials to the JDBC driver that you configure your application to use (see Configuring Database Login Connection Options).

By default, EclipseLink writes passwords to and reads them from the sessions.xml file in encrypted form using JCE encryption. Optionally, you can configure a different encryption class (see Configuring Password Encryption).

Oracle Database Proxy Authentication

EclipseLink supports proxy authentication with the Oracle Database in JSE applications and JEE applications using OC4J native or managed data sources with Oracle JDBC driver release 10.1.0.2.0 or later and external connection pools (see External Connection Pools) only.

Oracle Database proxy authentication delivers the following security benefits:

• A limited trust model, by controlling the users on whose behalf middle tiers can connect, and the roles the middle tiers can assume for the user.
• Scalability, by supporting user sessions through Oracle Call Interface (OCI) and thick JDBC, and eliminating the overhead of reauthenticating clients.
• Accountability, by preserving the identity of the real user through to the database, and enabling auditing of actions taken on behalf of the real user.
• Flexibility, by supporting environments in which users are known to the database, and in which users are merely "application users" of which the database has no awareness.

For more information about authentication in an Oracle Database, see "Preserving User Identity in Multitiered Environments" in the Oracle Database Security Guide.

Configure your EclipseLink database login to use proxy authentication (see Configuring Oracle Database Proxy Authentication) to do the following:

• address the complexities of authentication in a three-tier architecture (such as client-to-middle-tier and middle-tier-to-database authentication, and client reauthentication through the middle -tier to the database)
• enhance database audit information (for even triggers and stored procedures) by using a specific user for database operations, rather than the generic pool user
• simplify VPD/OLS configuration (see Isolated Client Sessions and Oracle Virtual Private Database (VPD)) by using a proxy user, rather than setting user information directly in the session context with stored procedures

Auditing

Regardless of what type of authentication you choose, EclipseLink logs the name of the user associated with all database operations. This example shows the CONFIG level EclipseLink logs when a ServerSession connects through the main connection for the sample user "scott", and a ClientSession uses proxy connection "jeff".

EclipseLink Logs with Oracle Database Proxy Authentication

[EclipseLink Config]--ServerSession(13)--Connection(14)--Thread(Thread[main,5,main])--connecting(DatabaseLogin( platform=>Oracle9Platform   user name=> "scott" connector=>OracleJDBC10_1_0_2ProxyConnector datasource name=>DS))
[EclipseLink Config]--ServerSession(13)--Connection(34)--Thread(Thread[main,5,main])--Connected: jdbc:oracle:thin:@localhost:1521:orcl
User: SCOTT
[EclipseLink Config]--ClientSession(53)--Connection(54)--Thread(Thread[main,5,main])--connecting(DatabaseLogin(platform=>Oracle9Platform user name=> "scott" connector=>OracleJDBC10_1_0_2ProxyConnector datasource name=>DS))
[EclipseLink Config]--ClientSession(53)--Connection(56)--Thread(Thread[main,5,main])--Connected: jdbc:oracle:thin:@localhost:1521:orcl
User: jeff

For more information on configuring EclipseLink log level and log options, see Configuring Logging.

Your database server likely provides additional user auditing options. Consult your database server documentation for details.

Alternatively, you may consider using the EclipseLink unit of work in conjunction with your database schema for auditing purposes (see Implementing User and Date Auditing with the Unit of Work).

Connections

A connection is an object that provides access to a data source by way of the driver you configure your application to use (see Configuring Database Login Connection Options). Relational projects use JDBC to connect to the data source; EIS and XML projects use JCA. EclipseLink uses the interface org.eclipse.persistence.internal.databaseaccess.Accessor to wrap data source connections. This interface is accessible from certain events (see Descriptor Event Manager).

Typically, when using a server session, EclipseLink uses a a different connection for both reading and writing. This lets you use nontransactional connections for reading and avoid maintaining connections when not required. See Reading Through the Write Connection and Exclusive Write Connections for more information.

By default, a EclipseLink server session acquires connections lazily: that is, only during the commit operation of a unit of work. Alternatively, you can configure EclipseLink to acquire a write connections at the time you acquire a client sessions (see Lazy Connection Acquisition).

Connections can be allocated from internal or external connection pools (see Connection Pools).

Connection Pools

A connection pool is a service that creates and maintains a shared collection (pool) of data source connections on behalf of one or more clients. The connection pool provides a connection to a process on request, and returns the connection to the pool when the process is finished using it. When it is returned to the pool, the connection is available for other processes. Because establishing a connection to a data source can be time-consuming, reusing such connections in a connection pool can improve performance.

EclipseLink uses connection pools to manage and share the connections used by server and client sessions. This feature reduces the number of connections required and allows your application to support many clients.

You can configure your session to use internal connection pools provided by EclipseLink or external connection pools provided by a JDBC driver or Java EE container.

You can use connection pools in your EclipseLink application for a variety of purposes, such as reading, writing, sequencing, and other application-specific functions.

This section describes the following:

• Internal Connection Pools
• External Connection Pools
• Default (Write) and Read Connection Pools
• Sequence Connection Pools
• Application-Specific Connection Pools

Internal Connection Pools

For non-Java EE applications, you typically use internal connection pools. By default, EclipseLink sessions use internal connection pools.

Using internal connection pools, you can use Workbench to configure the default (write) and read connection pools (see Default (Write) and Read Connection Pools) and you can create additional connection pools for object identity (see Sequence Connection Pools), or any other purpose (see Application-Specific Connection Pools).

Using internal connection pools, you can optimize the creation of read connections for applications that read data only to display it and only infrequently modify data (see Configuring a Nontransactional Read Login).

For information on selecting the type of connection pool to use, see Configuring External Connection Pooling.

For more information on creating and configuring internal connection pools, see the following:

• Introduction to the Internal Connection Pool Creation
• Introduction to the Internal Connection Pool Configuration

External Connection Pools

For Java EE applications, you typically use external connection pools.

If you are using an external transaction controller (JTA), you must use external connection pools to integrate with the JTA (see Integrating the Unit of Work with an External Transaction Service).

Using external connection pools, you can use either Workbench or Java to configure the default (write) and read connection pools (see Default (Write) and Read Connection Pools) and create additional connection pools for object identity (see Sequence Connection Pools), or any other purpose (see Application-Specific Connection Pools).

For more information on selecting the type of connection pool to use, see Configuring External Connection Pooling.

Default (Write) and Read Connection Pools

A server session provides a read connection pool and a write connection pool. These could be different pools, or if you use external connection pooling, the same connection pool.

All read queries use connections from the read connection pool and all queries that write changes to the data source use connections from the write connection pool. You can configure attributes of the default read and write connection pools.

Whenever a new connection is established, EclipseLink uses the connection configuration you specify in your session's DatasourceLogin. Alternatively, when you use an external transaction controller, you can define a separate connection configuration for a read connection pool to avoid the additional overhead, if appropriate (see Configuring a Nontransactional Read Login).

For more information on configuring read and write connection pools, see Introduction to the Internal Connection Pool Configuration.

Sequence Connection Pools

An essential part of maintaining object identity (see Cache Type and Object Identity) is sequencing–managing the assignment of unique values to distinguish one instance from another. For more information, see Projects and Sequencing.

Sequencing involves reading and writing a special sequence resource maintained by your data source.

By default, EclipseLink includes sequence operations in a separate transaction. This avoids complications during the write transaction, which may lead to deadlocks over the sequence resource. However, when using an external transaction controller (such as a JTA data source or connection pool), EclipseLink cannot use a different transaction for sequencing. Use a sequence connection pool to configure a non-JTA transaction pool for sequencing. This is required only for table sequencing–not native sequencing.

In each server session, you can create one connection pool, called a sequence connection pool, that EclipseLink uses exclusively for sequencing. With a sequence connection pool, EclipseLink satisfies a request for a new object identifier outside of the transaction from which the request originates. This allows EclipseLink to immediately commit an update to the sequence resource, which avoids deadlocks.

You should use a sequence connection pool, if the following applies:

• You use table sequencing (that is, non-native sequencing). See Table Sequencing and Unary Table Sequencing for more information.
• You use external transaction controller (JTA).

You should not use a sequence connection pool, if the following applies:

• You do not use sequencing, or use the data source's native sequencing (see Native Sequencing with an Oracle Database Platform and Native Sequencing with a Non-Oracle Database Platform).
• You have configured the sequence table to avoid deadlocks.
• You use non-JTA data sources.

For more information, see the following:

• Introduction to the Internal Connection Pool Creation
• Introduction to the Internal Connection Pool Configuration

Application-Specific Connection Pools

When you use internal EclipseLink connection pools in a session, you can create one or more connection pools that you can use for any application purpose. These are called named connection pools, as you can give them any name you want and use them for any purpose.

Typically, use these named connection pools to provide pools of different security levels. For example, the "default" connection pool may only allow access to specific tables but the "admin" connection pool may allow access to all tables.

For more information, see the following:

• Introduction to the Internal Connection Pool Creation
• Introduction to the Internal Connection Pool Configuration
• How to Acquire a Client Session that Uses a Named Connection Pool

Data Access API

This section describes the following:

• Login Inheritance Hierarchy
• Platform Inheritance Hierarchy

Login Inheritance Hierarchy

This example illustrates the login types that are derived from abstract class org.eclipse.persistence.sessions.DatasourceLogin.

Login Inheritance Hierarchy

class org.eclipse.persistence.sessions.DatasourceLogin
    class org.eclipse.persistence.sessions.DatabaseLogin
    class org.eclipse.persistence.eis.EISLogin

Platform Inheritance Hierarchy

This example illustrates the platform type class hierarchy.

Platform Inheritance Hierarchy

org.eclipse.persistence.platform.database
   AccessPlatform
   AttunityPlatform
   CloudscapePlatform
   DatabasePlatform
   DB2MainframePlatform
   DB2Platform
   DBasePlatform
   DerbyPlatform
   HSQLPlatform
   InformixPlatform
   JavaDBPlatform
   PointBasePlatform
   PostgreSQLPlatform
   SQLAnyWherePlatform
   SQLServerPlatform
   SybasePlatform
   TimesTen7Platform
org.eclipse.persistence.platform.database.oracle
   Oracle8Platform
   Oracle9Platform
   OraclePlatform

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