Run Units, Locks, and Database Transactions › Area Usage Modes

Area Usage Modes

Run units ready an individual area in a particular usage mode in order to define the scope of operations that can be performed against that area. The area usage modes, which are specified by the DML READY statement, are retrieval and update.

Retrieval

Retrieval specifies that the issuing run unit can perform only retrieval functions such as FIND, OBTAIN, and IF against records in that area. It cannot issue the STORE, MODIFY, ERASE, CONNECT, or DISCONNECT statements.

Update

Update specifies that the issuing run unit can modify as well as retrieve records in that area. That is, it can issue all available DML statements.

Ready Options

You can issue a ready option in conjunction with a usage mode to restrict retrieval or update of records in the specified area by other run units executing concurrently under the same central version. The ready options are:

• Protected indicates that other run units cannot ready the specified area in update usage mode and must wait until your run unit terminates. If you ready an area with the protected option while a concurrently executing run unit has readied the area in update mode, you will wait until the other run unit terminates.
• Exclusive indicates that other run units cannot ready the specified area in any usage mode and must wait until your run unit terminates. If you ready an area with the exclusive option while a concurrently executing run unit has readied the area, you will wait until the other run unit terminates.
• Shared indicates that more than one run unit running under the same central version can concurrently access the same area.

  Note: You cannot explicitly code SHARED in the READY statement (that is, READY UPDATE is functionally the same as READY SHARED UPDATE).

  Combinations of Usage Mode and Ready Options

The table below summarizes the effect that various combinations of usage modes and ready options have on concurrently executing run units.

The usage mode in which one run unit readies an area restricts the usage mode in which other run units executing under the same central version can ready that area. The following table shows in which usage modes two concurrent run units can ready an area. Y (yes) signifies that the second run unit can ready the area in the specified usage mode; N (no) signifies that it cannot.

                                         Run unit B
                   ┌────────────────────────────────────────────────────────────┐
                   │                                                            │
         ┌─────────┬───────┬─────────┬─────────┬──────────┬──────────┬──────────┐
         │         │SHARED │SHARED   │PROTECTED│PROTECTED │EXCLUSIVE │EXCLUSIVE │
         │         │UPDATE │RETRIEVAL│UPDATE   │RETRIEVAL │UPDATE    │RETRIEVAL │
      ┌──├─────────┼───────┼─────────┼─────────┼──────────┼──────────┼──────────┤
      │  │SHARED   │  Y    │   Y     │   N     │    N     │    N     │    N     │
      │  │UPDATE   │       │         │         │          │          │          │
      │  ├─────────┼───────┼─────────┼─────────┼──────────┼──────────┼──────────┤
      │  │SHARED   │  Y    │   Y     │   Y     │    Y     │    N     │    N     │
      │  │RETRIEVAL│       │         │         │          │          │          │
      │  ├─────────┼───────┼─────────┼─────────┼──────────┼──────────┼──────────┤
      │  │PROTECTED│  N    │   Y     │   N     │    N     │    N     │    N     │
Run   │  │UPDATE   │       │         │         │          │          │          │
unit  │  ├─────────┼───────┼─────────┼─────────┼──────────┼──────────┼──────────┤
A     │  │PROTECTED│  N    │   Y     │   N     │    Y     │    N     │    N     │
      │  │RETRIEVAL│       │         │         │          │          │          │
      │  ├─────────┼───────┼─────────┼─────────┼──────────┼──────────┼──────────┤
      │  │EXCLUSIVE│  N    │   N     │   N     │    N     │    N     │    N     │
      │  │UPDATE   │       │         │         │          │          │          │
      │  ├─────────┼───────┼─────────┼─────────┼──────────┼──────────┼──────────┤
      │  │EXCLUSIVE│  N    │   N     │   N     │    N     │    N     │    N     │
      │  │RETRIEVAL│       │         │         │          │          │          │
      └──└─────────┴───────┴─────────┴─────────┴──────────┴──────────┴──────────┘

Wait State

When a run unit cannot ready an area because a protected or exclusive restriction is already placed on that area by another run unit running under the same central version, it is placed in a wait state until the first run unit is finished.

Automatic Implicit Locking

The central version automatically maintains implicit record locks. These record locks are dependent on DBA specifications and on the area usage mode specified:

• Shared update- The central version always maintains record locks for run units executing in shared update usage mode.
• Shared retrieval-The central version maintains record locks for run units executing in shared retrieval mode only if specified by the DBA at system generation. If they are not maintained, a run unit with shared retrieval usage mode may yield unpredictable results if it accesses records being modified by a concurrently executing run unit with a shared update or protected update usage mode.
• Protected update-The central version maintains exclusive record locks for run units executing in protected update mode only if specified by the DBA at system generation. Shared record locks are never maintained for protected access because no other run unit can concurrently update records in the area.
• Protected retrieval, exclusive update, exclusive retrieval -The central version does not maintain implicit record locks for run units executing in these modes since the usage modes themselves prohibit concurrent update.

Default Usage Modes

Your DBA can assign default usage modes for subschema areas. The specified default determines the usage mode in which an area will automatically be readied for programs using that subschema. You do not have to code READY statements in programs that use such a subschema; however, if you issue a READY command for one area in the subschema, you must issue READY commands for all database areas to be accessed unless the FORCE option is specified for the default usage mode. Areas using the default usage mode combined with the FORCE option are automatically readied even if the run-unit already issued READY for other areas.

You can use the SUBAREA parameter of the IDMSRPTS utility to determine if the DBA has specified any default usage modes.