Understanding EclipseLink, 2.7
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About Connections

A connection is an object that provides access to a data source by way of the driver you configure your application to use. Relational projects use JDBC to connect to the data source; EIS 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.

Typically, when using a server session, EclipseLink uses a different connection for both reading and writing. This lets you use nontransactional connections for reading and avoid maintaining connections when not required.

By default, an EclipseLink server session acquires connections lazily: that is, only during the commit operation of a persistence unit. Alternatively, you can configure EclipseLink to acquire a write connections at the time you acquire a client sessions.

Connections can be allocated from internal or external connection pools.