Locking to provide program isolation

For all database organizations, the basic item locked is the database record.

The database record is locked when position is first obtained in it. The item locked is the root segment, or for HDAM or PHDAM, the anchor point. Therefore, for HDAM or PHDAM, all database records chained from the anchor are locked. The processing option of the PCB determines whether or not two programs can concurrently access the same database record. If the processing option permits updates, then no other program can concurrently access the database record. The database record is locked until position is changed to a different database record or until the program reaches a commit point.

When a program updates a segment with an INSERT, DELETE, or REPLACE call, the segment, not the database record, is locked. On an INSERT or DELETE call, at least one other segment is altered and locked.

Because data is always accessed hierarchically, when a lock on a root (or anchor) is obtained, IMS determines if any programs hold locks on dependent segments. If no program holds locks on dependent segments, it is not necessary to lock dependent segments when they are accessed.

The following locking protocol allows IMS to make this determination. If a root segment is updated, the root lock is held at update level until commit. If a dependent segment is updated, it is locked at update level. When exiting the database record, the root segment is demoted to read level. When a program enters the database record and obtains the lock at either read or update level, the lock manager provides feedback indicating whether or not another program has the lock at read level. This determines if dependent segments will be locked when they are accessed. For HISAM, the primary logical record is treated as the root, and the overflow logical records are treated as dependent segments.

These lock protocols apply when the PI lock manager is used; however, if the IRLM is used, no lock is obtained when a dependent segment is updated. Instead, the root lock is held at single update level when exiting the database record. Therefore, no additional locks are required if a dependent segment is inserted, deleted, or replaced.