About this task

There are two forms of locking:

1.The implicit locking functions performed by CICS (or the access method) whenever your transactions issue a request to change data. These are described under:

v “Implicit locking for files”

v“Implicit enqueuing on logically recoverable TD destinations” on page 157

v“Implicit enqueuing on recoverable temporary storage queues” on page 157

v“Implicit enqueuing on DL/I databases with DBCTL” on page 158.

2.The explicit enqueuing function that you request by means of an EXEC CICS command. This is described under “Explicit enqueuing (by the application programmer)” on page 158.

Note: Locking (implicit or explicit) data resources protects data integrity in the event of a failure, but can affect performance if several tasks attempt to operate on the same data resource at the same time. The effect of locking on performance, however, is minimized by implementing applications with short units of work, as discussed under “Dividing transactions into units of work” on page 143.

Implicit locking for files

This section first describes the implicit locking provided while nonrecoverable files are being updated. It then describes the extended locking actions when recoverable files are being updated.

Nonrecoverable files

For BDAM files that are nonrecoverable (that is, LOG=NO is specified in the FCT entry), CICS does not lock records that are being updated.

By default, you get BDAM exclusive control, which operates on a physical block, is system wide, but lasts only until the update is complete. If a transaction reads a record for update under BDAM exclusive control, and the transaction subsequently decides not to change the data, it must release the BDAM exclusive control. To do this, issue an EXEC CICS UNLOCK command, which causes CICS to issue a RELEX macro.

If you don’t want BDAM exclusive control, specify SERVREQ=NOEXCTL on the file entry in the FCT.

For nonrecoverable VSAM files accessed in non-RLS mode, VSAM exclusive control locks the control interval during an update. For nonrecoverable VSAM files accessed in RLS mode, SMSVSAM locks the record during the update.

Figure 13 on page 155 illustrates the extent of locking for nonrecoverable files.

154CICS TS for z/OS 4.1: Recovery and Restart Guide

Page 166
Image 166
IBM SC34-7012-01 manual Implicit locking for files, Nonrecoverable files