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As shown in the previous examples, you can use the @Version annotation to designate a field to be used by the EntityManager to perform optimistic locking for merge operations and concurrency management. Optimistic locking is a useful performance optimization that offloads work that would otherwise be required of the database. Databases typically offer a pessimistic locking service that allows the database client (in our case, the JPA EntityManager) to lock a row in a table to prevent another client from updating it while the EntityManager is applying some changes. This is an effective mechanism to ensure that two clients do not modify the same row at the same time, but requires expensive, low-level access checks inside the database. An alternative to pessimistic locking is to move concurrency control into a database client like the EntityManager, and employ an optimistic locking strategy. Using a dedicated @Version column, the EntityManager follows a couple of simple rules. Whenever it sends a modified entity out to the database, as during a commit or flush operation, it looks at the current value of the entity instance s @Version field, queries the current state of that entity s row from the database, and compares the version values. If they are the same, it increments the entity instance s @Version field (or whatever field is annotated @Version) and sends the change out to the database, causing an UPDATE statement to be executed. If the version values are different, that means that some other client modified the row between the time the row was last queried by the EntityManager and loaded into an entity instance, and the time that instance was flushed back out to the database. When such a difference is detected, we

Private Private Private Private Private Private _ModuleId As Integer _ItemId As Integer _PunchType As Integer _PunchUserID As Integer _PunchDate As DateTime _Punch_UserName As String

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call it a concurrency exception, and the EntityManager throws an exception and rolls back its transaction. The client of the EntityManager needs to anticipate that a concurrency exception might occur, and be prepared to resolve the conflict, typically by notifying the user of the conflict so that the entity can be refreshed. The use of a dedicated @Version column on an entity allows the EntityManager to perform optimistic locking simply by comparing the value of the @Version field stored in the entity instance with the value of the VERSION column in the database. If you don t specify an @Version field, the EntityManager has to walk through each field in the entity instance and compare its value to its corresponding, mapped column in the database, which is far more laborious. A declared @Version field will be autopopulated by the persistence framework and should not be updated by application code. The bottom line is that it s not a requirement to use an @Version field, but it s good practice to define an @Version field on your entities to allow the EntityManager to take advantage of this optimization.

Figure 4-5. The complete list of terms At this point we ve created a vocabulary and the terms that we will use to categorize content. We now have to identify which content types will use this vocabulary as a method for categorizing content, and configure our vocabulary so that it will appear on the content creation screens for those content types.

Now that the private variables have been changed, you need to change the get and set accessors that read and write these variables. They are in the Public Methods region. The ModuleId and ItemId properties are fine, since the variables did not change data type or name. Here is the new method section for the TimePunchInfo.vb file:

Support for Autogenerated Primary Key Values (@GeneratedValue)

In addition to built-in optimistic locking support through the @Version column, EJB 3 provides several convenient ways to autopopulate primary key columns when an entity is persisted. You can declare that a field s value should be populated using the following: An automatic mechanism maintained by the persistence framework (strategy=GenerationType.AUTO) A custom database sequence (strategy=GenerationType.SEQUENCE or GenerationType.IDENTITY) A custom database table, emulating a pseudo-sequence (strategy=GenerationType.TABLE) We found the autopopulated PK feature to be very high on the convenience scale, saving us from coding this up for each entity in our application. We recommend explicitly defining the sequence details, since even with the default setting strategy=AUTO, the persistence provider you are using will probably require you to install a sequence or table in the database schema that hosts the entity tables. Once installed, the AUTO case generates unique identifiers for any @Id field that is annotated @GeneratedValue or @GeneratedValue (strategy=GenerationType.AUTO), and at least leaves the entity class a little less cluttered. Listing 4-27 demonstrates usage of the default ID generation feature.

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