Sequence grouping for container-managed persistence

After assembling an EJB module that contains container-managed persistence (CMP) beans, you can prevent certain types of database-related exceptions from occurring during application run time. Using sequence grouping, you can specify the order in which entity beans update relational database tables.

Eliminate exceptions resulting from referential integrity (RI) violationsSequence grouping is particularly useful for preventing violations of database referential integrity (RI). A database RI policy prescribes rules for how data is written to and deleted from the database tables to maintain relational consistency. Run-time requirements for managing bean persistence, however, can cause an EJB application to violate RI rules, which can cause database exceptions. These run-time requirements mandate that:

• Entity bean create and remove operations correlate to the database immediately upon method invocation.

• Entity bean changes are cached by the EJB container until either a finder method is called, or the transaction ends.

Consequently, the order in which entity beans update the database is unpredictable. That randomness translates into high risk of the application violating database RI. Although caching the operations for batch processing overrides these run-time requirements, it does not guarantee a bean persistence sequence that follows any given RI policy.

The only way to guarantee a persistence sequence that honors database RI is to designate the sequence, which you do in the EJB deployment descriptor editor of the assembly tool. Through the sequence grouping feature, you assign beans to CMP groups. Within each group you specify the order in which the persistence manager inserts bean data into the database to accomplish updates without violating RI.

See the Setting the run time for CMP sequence groups topic for detailed instructions on designating sequence groups. Consult your database administrator about the RI policy with which you need to synchronize.

Minimize exception risk for optimistic concurrency control schemesSequence grouping can also reduce the risk of transaction rollback exceptions for entity beans that are configured for optimistic concurrency control. In these concurrency control schemes, database locks are held for minimal amounts of time so that a maximum number of transactions consistently have access to the data. The relatively unrestricted state of the database can lead to transaction rollback exceptions for two common reasons:

• When concurrent transactions attempt to lock the same table row, database deadlock occurs.

• Transactions can occur in an order that violates application logic.

Use the sequence grouping feature to order bean persistence so that these scenarios are less likely to occur.