Introduction to Data Management

Ensuring Database Consistency With TMF

7-4 15873 Tandem Computers Incorporated

Figure 7-2. TMF Audit Trails

S8020-022

CHANGE RECORDS

Server

R3R1

Before-images and

after-images of

records R1, R2, and R3

Status information

(in monitor or master

audit trail)

Audit

Trails

Database Files

R3R1

In the audit trail, the before-image shows the value of each field before the update,

while the after-image shows the value of each field following the update. TMF uses

the before-images to back out the changed information if a transaction fails. TMF uses

the after-images to reapply successful transactions to the database during recovery

from a system or disk failure, as discussed later in this section.

PATHWAY Environment Support

TMF provides a natural extension of the PATHWAY programming environment by

relating the idea of a database transaction to the code you supply in your application.

Thus, when you are writing a screen program to be run by a PATHWAY TCP, you

delimit the beginning and end of each transaction with two simple SCREEN COBOL

statements: BEGIN-TRANSACTION and END-TRANSACTION. These two

statements appear in the code shown in Figure 7-3.