Implicit enqueuing on DL/I databases with DBCTL

IMS™ program isolation scheduling ensures that, when a task accesses a segment by a DL/I database call, it implicitly enqueues on all segments in the same database record as the accessed segment.

How long it is enqueued depends on the access method being used:

Direct methods (HDAM, HIDAM): If an ISRT, DLET, or REPL call is issued against a segment, that segment, with all its child segments (and, for a DLET call, its parent segments as well), remains enqueued upon until a DL/I TERM call is issued. The task dequeues from all other segments in the database record by accessing a segment in a different database record.
Sequential methods (HSAM, HISAM, SHISAM): If the task issues an ISRT, DLET, or REPL call against any segment, the entire database record remains enqueued upon until a DL/I TERM call is issued. If no ISRT, DLET, or REPL call is issued, the task dequeues from the database record by accessing a segment in a different database record.

The foregoing rules for program isolation scheduling can be overridden using the ‘Q’ command code in a segment search argument (this command extends enqueuing to the issue of a DL/I TERM call), or by using PROCOPT=EXCLUSIVE in the PCB (this macro gives exclusive control of specified segment types throughout the period that the task has scheduled the PSB).