There are several guidelines you can
use to determine which file organization and access mode to use in
an application.
Consider the following guidelines when choosing file
organization:
- If an application accesses records (whether fixed-length or
variable-length) only sequentially and does not insert records
between existing records, a QSAM or VSAM sequential file is the
simplest type.
- If you are developing an application for UNIX that
sequentially accesses records that contain only printable
characters and certain control characters, line-sequential files
work best.
- If an application requires both sequential and random access
(whether records are fixed length or variable length), a VSAM
indexed file is the most flexible type.
- If an application inserts and deletes records randomly, a
relative file works well.
Consider the following guidelines when choosing access mode:
- If a large percentage of a file is referenced or updated in an
application, sequential access is faster than random or dynamic
access.
- If a small percentage of records is processed during each run
of an application, use random or dynamic access.
Table 1. Summary of file organizations, access modes, and record formats of COBOL files| File organization |
Sequential access |
Random access |
Dynamic access |
Fixed length |
Variable length |
| QSAM (physical
sequential) |
X |
|
|
X |
X |
| Line sequential |
X |
|
|
X1 |
X |
| VSAM sequential
(ESDS) |
X |
|
|
X |
X |
| VSAM indexed (KSDS) |
X |
X |
X |
X |
X |
| VSAM relative (RRDS) |
X |
X |
X |
X |
X |
- The data itself is in variable format but can be read into and
written from COBOL fixed-length records.
|