Slice allows any OpenJPA based application to work with data from multiple databases within the same transaction. Slice does not require any change to the existing OpenJPA application. The features offered in Slice can be available to any OpenJPA application by simply configuring a persistence unit in META-INF/persistence.xml.

Slice is activated via the following property settings:

The basic configuration property is ]]> This critical configuration activates a specialized factory class aliased as slice to create object management kernel that can work against multiple databases.

Each database slice is identified by a logical name unique within a persistent unit. The list of the slices is specified by slice.Names property. For example, specify three slices named "One", "Two" and "Three" as follows: ]]> This property is not mandatory. If this property is not specified then the configuration is scanned for logical slice names. Any property of the form slice.XYZ.abc will register a slice with logical name "XYZ". The order of the names can be significant if no slice.Master property is specified. Each database slice can be configured independently. For example, the following configuration will register two slices with logical name One and Two. ]]> Any OpenJPA specific property can be configured per slice basis. For example, the following configuration will use two different JDBC drivers for slice One and Two. ]]> Any property if unspecified for a particular slice will be defaulted by corresponding OpenJPA property. For example, consider following three slices ]]> In this example, Three will use slice-specific oracle.jdbc.Driver driver while slice One and Two will use the driver com.mysql.jdbc.Driver as specified by openjpa.ConnectionDriverName property value.

Slice needs to determine which slice will persist a new instance. The application can only decide this policy (for example, all PurchaseOrders before April 30 goes to slice One, all the rest goes to slice Two). This is why the application has to implement org.apache.openjpa.slice.DistributionPolicy and specify the implementation class in configuration ]]> The interface org.apache.openjpa.slice.DistributionPolicy is simple with a single method. The complete listing of the documented interface follows: slices, Object context); } ]]> While implementing a distribution policy the most important thing to remember is collocation constraint. Because Slice can not establish or query any cross-database relationship, all the related instances must be stored in the same database slice. Slice can determine the closure of a root object by traversal of cascaded relationships. Hence user-defined policy has to only decide the database for the root instance that is the explicit argument to EntityManager.persist() call. Slice will ensure that all other related instances that gets persisted by cascade is assigned to the same database slice as that of the root instance. However, the user-defined distribution policy must return the same slice identifier for the instances that are logically related but not cascaded for persist.

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