Adding a column for collision detection

For EJB 2.x CMP entity beans, you can add a column in your relational database table for collision detection. This collision detection column is an additional database column reserved to determine if a record has been updated. To manage the collision detection column, you will need to provide your own database trigger implementation.

The following are the benefits of choosing to add a column for collision detection over the alternative optimistic concurrency control scheme of including attributes in a predicate for optimistic access intents:

• In the overqualified UPDATE statement, only the collision detection column and the primary key are required. Checking these two columns reduces the number of I/O operations over using multiple selected optimistic predicates.

• Use of collision detection column overcomes the limitation of optimistic predicate can not map to nullable columns or unsupported optimistic concurrency control data types such as BLOBs and CLOBs.

The following are restrictions of using the collision detection column:

• Any modifications or updates to the value of collision detection column is not supported.

• If you choose to use collision detection column, the use of the collision detection column will take precedence over the alternative optimistic concurrency control scheme of including attributes in a predicate for optimistic access intents. In addition, all the optimistic predicate settings will be ignored for optimistic access intent.

• Adding a column for collision detection is available for top-down and meet-in-the middle mapping. Bottom-up mapping¹ is not supported.

• Column names beginning with "DISCRIM" or "OCC_" are reserved names for the EJB Mapping tools.
  • Do not specify a CMP field name to begin with either the following strings: "DISCRIM" or "OCC_".

  • Do not associate a CMP with another CMP whose name begin with either the following strings: "DISCRIM" and "OCC_".

Select one of the following approaches to add a column for collision detection:

Adding a column for collision detection when generating a top-down mapping
This task describes how to add a column for collision detection when generating a top-down mapping.

Adding a column for collision detection when the CMP bean exists
This task describes how to add a column for collision detection using the Mapping editor.

 

Parent topic

Specifying optimistic concurrency control scheme for EJB 2.x CMP entity beans

¹ Bottom-up mapping is not