The Application Server provides the following features to enhance performance or allow more fine-grained data checking. These features are supported only for entity beans with container managed persistence.
Use of any of these features results in a non-portable application.
The version consistency feature saves the bean state at first transactional access and caches it between transactions. The state is copied from the cache instead of being read from the database. The bean state is verified by primary key and version column values at flush for custom queries (for dirty instances only) and at commit (for clean and dirty instances).
Create the version column in the primary table.
Give the version column a numeric data type.
Provide appropriate update triggers on the version column.
These triggers must increment the version column on each update of the specified row.
Specify the version column.
Map the CMP bean to an existing schema.
Automatic schema generation is not supported for beans with version column consistency checking. Instead, work with your database administrator to create the schema and add the required triggers.
In many cases when an entity bean’s state is fetched from the database, its relationship fields are always accessed in the same transaction. Relationship prefetching saves database round trips by fetching data for an entity bean and those beans referenced by its CMR fields in a single database round trip.
To enable relationship prefetching for a CMR field, use the default subelement of the fetched-with element in the sun-cmp-mappings.xml file. By default, these CMR fields are prefetched whenever findByPrimaryKey or a custom finder is executed for the entity, or when the entity is navigated to from a relationship. (Recursive prefetching is not supported, because it does not usually enhance performance.) To disable prefetching for specific custom finders, use the prefetch-disabled element in the sun-ejb-jar.xml file.
Another feature that the Application Server provides is the read-only bean, an entity bean that is never modified by an EJB client. Read-only beans avoid database updates completely.
A read-only bean can be used to cache a database entry that is frequently accessed but rarely updated (externally by other beans). When the data that is cached by a read-only bean is updated by another bean, the read-only bean can be notified to refresh its cached data.
The Application Server provides a number of ways by which a read-only bean’s state can be refreshed. By setting the refresh-period-in-seconds element in the sun-ejb-jar.xml file and the trans-attribute element in the ejb-jar.xml file, it is easy to configure a read-only bean that is (a) always refreshed, (b) periodically refreshed, (c) never refreshed, or (d) programmatically refreshed.
Access to CMR fields of read-only beans is not supported. Deployment will succeed, but an exception will be thrown at runtime if a get or set method is invoked.
Read-only beans are best suited for situations where the underlying data never changes, or changes infrequently. For further information and usage guidelines, see Using Read-Only Beans.