Page 120 - Accounting Information Systems

CHAPT E R 2 Introduction to Transaction Processing 91

FI G U R E
2-43 TYPES OF POINTERS

Physical Address Pointer

CYL 121
Record 1 Surface 05 Next Record
Record 750
Physical Address

Relative Address Pointer

135 First record
•
•
•
Conversion Sequential
Routine Record 135
File
Pointer to 135th •
Record in the File •

Last Record

Logical Key Pointer

Key Hashing Record
9631 Algorithm 9631

A relative address pointer contains the relative position of a record in the file. For example, the
pointer could specify the 135th record in the file. This must be further manipulated to convert it to the
actual physical address. The conversion software calculates this by using the physical address of the begin-
ning of the file, the length of each record in the file, and the relative address of the record being sought.
A logical key pointer contains the primary key of the related record. This key value is then con-
verted into the record’s physical address by a hashing algorithm.

Batch Processing Using Sequential Files

The most basic computer-processing configuration is batch mode using sequential file structures. Figure
2-44 illustrates this method.
Each program in a batch system is called a run. In this example, there is an edit run, an AR file
update run, an inventory file update run, and two intermediate sort runs. The entire file or batch of records
is processed through each run before it moves to the next run. When the last run finishes processing the
batch, the session terminates.
A prominent feature of this system is the use of sequential files, which are simple to create and
maintain. Although sequential files are still used by organizations for backup purposes, their presence in
data processing is declining. This file structure is effective for managing large files, such as those used by
federal and state agencies that have a high activity ratio. The activity ratio of a file is defined as the per-
centage of records on the file that are processed each time the file is accessed. For example, a federal pay-
roll file has an activity ratio of 1:1. Each time the payroll file is accessed (payday), all the records on it
are processed because everyone gets a paycheck.