2. Salient Features

2.1. Transparency
2.2. Custom Distribution Policy
2.3. Heterogeneous Database
2.4. Parallel Execution
2.5. Distributed Query
2.6. Targeted Query
2.7. Distributed Transaction
2.8. Collocation Constraint

2.1. Transparency

The existing application or the persistent domain model requires no change to upgrade from a single database to a distributed database environment.

2.2. Custom Distribution Policy

User application decides how the newly persistent instances be distributed across the database slices. The data distribution policy across the slices may be based on the attribute of the data itself. For example, all Customer whose first name begins with character 'A' to 'M' will be stored in one slice while names beginning with 'N' to 'Z' will be stored in another slice.

This custom data distribution policy is specified by implementing org.apache.openjpa.slice.DistributionPolicy interface by the user application.

Slice tracks the original database for existing instances. When an application issues a query, the resultant instances can be loaded from different slices. This tracking is important as subsequent update to any of these instances is committed to the appropriate original database slice.


You can find the original slice of an instance pc by the static utility method SlicePersistence.getSlice(pc). This method returns the slice identifier string associated with the given managed instance. If the instance is not being managed then the method return null because any unmanaged or detached instance is not associated with any slice.


Currently, there is no provision for migrating an existing instance from one slice to another.

2.3. Heterogeneous Database

Each slice can be configured independently with its own JDBC driver and other connection parameters. Hence the target database environment can constitute of heterogeneous databases.

2.4. Parallel Execution

All database operations such as query, commit or flush operates in parallel across the database slices. The execution threading policy is configurable.

2.5. Distributed Query

The queries are executed across all slices and the results are merged into a single list. The query result that includes ORDER BY clause are sorted correctly by merging results from each individual slice.

The queries that specify an aggregate projection such as COUNT(), MAX(), MIN() and SUM() are correctly evaluated only if they return a single result.


The aggregate operation AVG() is not supported.

2.6. Targeted Query

You can target the query only to a subset of slices rather than all slices by setting a hint. The hint key openjpa.hint.slice.Target is set on any query and hint value is comma-separated list of slice identifiers. The following example shows how to target a query only to slice "One"

              EntityManager em = ...;
              String hint = "openjpa.hint.slice.Target";
              Query query = em.createQuery("SELECT p FROM PObject").setHint(hint, "One");
              List result = query.getResultList();
              // verify that each instance is originaing from the given slice
              for (Object pc : result) {
                 String sliceOrigin = SlicePersistence.getSlice(pc);
                 assertTrue ("One", sliceOrigin);

2.7. Distributed Transaction

The database slices participate in a global transaction provided each slice is configured with a XA-compliant JDBC driver, even when the persistence unit is configured for RESOURCE_LOCAL transaction.


If any of the configured slices is not XA-compliant and the persistence unit is configured for RESOURCE_LOCAL transaction then each slice is committed without any two-phase commit protocol. If commit on any slice fails, then atomic nature of the transaction is not ensured.

2.8. Collocation Constraint

No relationship can exist across database slices. In O-R mapping parlance, this condition translates to the limitation that the closure of an object graph must be collocated in the same database. For example, consider a domain model where Person relates to Adress. Person X refers to Address A while Person Y refers to Address B. Collocation Constraint means that both X and A must be stored in the same database slice. Similarly Y and B must be stored in a single slice.

Slice, however, helps to maintain collocation constraint automatically. The instances in the closure set of any newly persistent instance reachable via cascaded relationship is stored in the same slice. The user-defined distribution policy requires to supply the slice for the root instance only.