SQL Statement Processing
When the program executes at runtime, the program load module and access module are loaded as necessary. The access module is loaded the first time the program calls the DBMS to access data in the database.
The DBMS attempts to validate the definition of a table to be accessed—that is, it verifies the table definition has not changed since the access module was created. If validation fails, the DBMS automatically recreates the access module if you have defined the access module to allow this.
Concurrency Control
When the application executes in a multiuser processing environment, the DBMS controls concurrent access to the same set of data by setting retrieval or update locks. The DBMS determines the type, level, and duration of the lock from the activities and the isolation level of the database transaction.
The CA IDMS defaults for locking favor the greatest possible concurrency that can be maintained while guaranteeing the integrity of the data. You can change the system defaults for locking by specifying a different isolation level and/or a different ready mode for an accessed area.
Note: For more information about specifying isolation level and ready mode, see Concurrency Control and Isolation Levels.
Execution Environments
CA IDMS application programs can execute in the DC/UCF region, a batch region, or other region such as a CICS region. Except for a local mode job, all processing of SQL statements occurs under the central version, the DC system component that manages multiuser, concurrent access to the database.
Local mode is a single-user batch processing environment that manages access to areas of the database independent of the central version. It is normally used for retrieval-only batch jobs and large-volume update applications that tend to monopolize an area of the database.
The central version performs automatic recovery for programs that end abnormally. No automatic recovery is performed for a local mode program.
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