COBOL for the 21st Century

Chapter 13. Sequential File Processing

SYSTEMS OVERVIEW OF SEQUENTIAL FILE PROCESSING

Master Files

This unit will focus on master file processing where a master file is the major collection of data pertaining to a specific application. Companies will have master files in application areas such as payroll, accounts receivable, accounts payable, production, sales, and inventory. Batch processing is the most common way of processing these files.

In most companies, a master file will be stored on a magnetic disk. The features of magnetic media such as disk that make it ideally suited for storing master file data include the following:

1. Disks can store billions of characters.

2. Disk drives can read and write data very quickly.

3. Disk records can be any size.

Although tapes were once used for storing files, most companies today use disks because disks have greater versatility. About a decade ago, tapes were more often used for backing up a file in case the original disk file became unusable. Today, other forms of disk are used for backup, and tapes are no longer very common, even for backup.

A sequential file is one that is processed by reading the first record, operating on it, then reading the second record, operating on it, and so on. If a file is organized for sequential processing, it must always be read in sequence. When tapes were first introduced, they had to be processed sequentially, which, as you will see, is a serious limitation on their capabilities and one major reason why they have ...