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Taming wild EJBs: performance and scalability
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Figure 13.2 How optimistic locking strategy is implemented by a persistence provider. If the locking mode is set to WAIT, the User2 would wait until User1 releases the lock.
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timestamp to track changes. The version number approach is preferred and quite reliable, because it could be that two users may be trying to update the same data at the same instant (timestamp). Every time a specific database row is updated, the version column is incremented by the application. When a user reads the entity, the version column is retrieved and stored with the entity instance. After making changes in the entity object, as the user tries to save the entity to the database, the application compares the version attribute of the entity object with that of the database. If the version attributes are the same, then the user is allowed to hold a write lock on the object, saves the object to the database, increments the version attribute, and then releases the write lock. If the version attributes are different, then it is assumed the entity has since been updated by someone else and the update is rejected with an OptimisticLockingException. If you re building an application with JDBC, then you ll have to implement a locking strategy yourself. However, if you re using the EJB 3 JPA, the persistence provider makes your life much simpler. The EJB 3 JPA persistence providers are required to support the optimistic locking strategy using a version number column in the database table. Let s take a closer look at how this is done.
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Handling entity locking issues
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13.1.2 Optimistic locking and entity versioning
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If any of your entities are accessed concurrently in your applications, or detached entity instances are merged, then you must enable optimistic locking for those entities. For example, in the ActionBazaar application entities such as Item and Category are accessed concurrently. You must enable optimistic locking for these entities by adding a version attribute in these entities.
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The JPA specification does not mandate supporting optimistic locking without a version field on the entity.
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We can define a version attribute in the Entity item as follows:
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@Entity @Table(name = "ITEMS") public class Item implements Serializable { @Id @Column(name = "ITEM_ID") protected Long itemId; ... Specifies version property @Version @Column(name = "OPT_LOCK") private Long version; ... }
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Stores version in OPT_LOCK column
As this code shows, you can use @javax.persistence.Version on a persistence field or property. The persistence field should be a numeric field of type long, Long, int, or Integer, and it should be mapped to a database column on the primary table to which the entity is mapped. In our example, the version property is mapped to the OPT_LOCK column. You may see some resistance from your DBA to adding an extra column to store the version, but you will have to persuade her that you need this column to support optimistic locking in your applications. Applications cannot set the version attribute, as it is used directly by the persistence provider. The version column is automatically incremented when the persistence provider commits changes to the database. When you merge an entity instance to a persistence context, the persistence provider will check the version column to ensure that the detached entity has not become stale. If you are using TopLink Essentials, you can enable logging to see SQL statements when a detached entity instance is merged as follows:
--Assign return row DatabaseRecord(ITEMS.OPT_LOCK => 2) ... Connection ( ... )
Taming wild EJBs: performance and scalability
--UPDATE ITEMS SET ITEM_NAME = , OPT_LOCK = WHERE ((ITEM_ID = ) AND (OPT_LOCK = )) bind => [New Title: Vintage Cars from junkyard, 2, 902, 1]
In this code, the persistence provider is incrementing the value for the version attribute (ITEMS.OPT_LOCK) and updating the ITEMS table with the new value in the OPT_LOCK column. The WHERE clause includes the OPT_LOCK column and compares it with the old value. If the WHERE clause fails, then the persistence provider will throw an OptimisticLockException. Some persistence providers may choose to support additional mechanisms to check optimistic locking and, depending on their implementation, may render your applications less portable.
13.1.3 EntityManager and lock modes Although you will very rarely need it, the EntityManager provides a lock method to explicitly acquire a lock on an entity:
// begin transaction... entityManager.lock(item, LockModeType.READ); ... System.out.println(item.getInitialPrice());
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