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Database generated version ID with WSJPA

Java Persistence API for WebSphere Application Server (WSJPA) has extended OpenJPA to work with database generated version IDs. These generated version fields, timestamp, or token, can be used to efficiently detect changes to a given row.

Trigger based version ID generation is supported for all databases that WAS supports. Support is based on two Version Strategies in JPA for WAS.

The Entity class is defined with the new Version Strategy annotation. The Entity has a surrogate version column. For example, 

@Entity(name="Item")
@VersionColumn(name="versionField")
@VersionStrategy("com.ibm.websphere.persistence.RowChangeTimestampStrategy")
public class Item implements Serializable
{	
	@Id
	private int id2;
	
	private String name;
	
	private double price;

   @OneToOne
   private Owner master;
}
The create table statement is as follows: 
CREATE TABLE ITEM
(ID2  INT  NOT NULL,
 NAME  VARCHAR(50),
 PRICE   DOUBLE,
 OWNER_ID INT,
             VERSIONFIELD GENERATED ALWAYS FOR EACH ROW ON 
              UPDATE AS ROW CHANGE TIMESTAMP
									PRIMARYKEY(ID2));
During any updates to Item, insert or update, the VersionColumn value is updated in the database. After the update, the value for VersionColumn is retrieved from the database and updated in the in-memory object. Thereby the objects in the data cache reflect the correct version value. Here the Entity is using the @VersionColumn, which produces a Surrogate Version ID rather than defining an explicit field in the entity.

The Entity could also use @Version annotation to define an explicit version field. The explicit version field could be of type Long or Timestamp corresponding to the @VersionStrategy. During any updates to Item, insert or update, the Version value is updated in the database. After the update, the value for Version is retrieved from the database and updated in the in memory object. Thereby the objects in the data cache reflect the correct version value.

This is an example where the Entity has a version field defined, and the type Timestamp matches the RowChangeTimestampStrategy in the @VersionStrategy. If the version field type is using type long, then the RowChangeVersionStrategy should be annotated to match: 

@Entity(name="Item")
@VersionStrategy("com.ibm.websphere.persistence.RowChangeTimestampStrategy")
public class Item implements Serializable
	
{
	@Id
	private int id2;
	
	private String name;
	
	private double price;
   
   @Version
   private Timestamp versionField;

   @OneToOne
   private Owner master;
}

Be aware of the following conditions when we use RowChangeVersionStrategy: