Characteristics of sequential file organization

Records of a sequentially organized file are always arranged logically in the order in which they were written to the file. Consequently,

• every record (except for the last) has a unique successor, and

• every record (except for the first) has a unique predecessor.

This relationship between predecessor and successor cannot be modified during the entire life of the file.

It is thus not possible to

• insert records,

• delete records, or

• change the position of a record within the specified order

in a sequential file.

However, sequential file organization does permit the

• updating of records already in existence
  (provided that their lengths remain the same and that the file is a disk storage file), and

• the appending of new records to the end of the file.

Individual records of a file cannot be directly (randomly) accessed; they can only be processed in the same order in which they are stored in the file.

To process sequential files, COBOL programs make use of the SAM access method, which is provided by DMS for this purpose. Further details on this topic are provided in “Introductory Guide to DMS” [4].

Sequential files can be set up on magnetic tape devices or direct access devices (disk storage units).