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COBOL by Example: VSAM Basics

COBOL 2002

Accessing VSAM (Virtual Storage Access Method) files on IBM mainframes, using ORGANIZATION IS INDEXED with dynamic access mode, implementing robust FILE STATUS error handling for production code, and working with Key Sequenced Data Sets for high-performance storage.

Code

       IDENTIFICATION DIVISION.
       PROGRAM-ID. VSAM-DEMO.
       
       ENVIRONMENT DIVISION.
       INPUT-OUTPUT SECTION.
       FILE-CONTROL.
           SELECT CUST-FILE ASSIGN TO "CUST.DAT"
           ORGANIZATION IS INDEXED
           ACCESS MODE IS DYNAMIC
           RECORD KEY IS CUST-KEY
           FILE STATUS IS WS-STATUS.
           
       DATA DIVISION.
       FILE SECTION.
       FD  CUST-FILE.
       01  CUST-REC.
           05  CUST-KEY    PIC 9(5).
           05  CUST-DATA   PIC X(50).
       
       WORKING-STORAGE SECTION.
       01  WS-STATUS       PIC 99.
       
       PROCEDURE DIVISION.
           OPEN I-O CUST-FILE.
           IF WS-STATUS NOT = "00"
               DISPLAY "VSAM Open Error: " WS-STATUS
               STOP RUN.
           
           *> Read a specific record
           MOVE 12345 TO CUST-KEY.
           READ CUST-FILE
               INVALID KEY DISPLAY "Record not found"
           END-READ.
           
           CLOSE CUST-FILE.
           STOP RUN.

Explanation

VSAM (Virtual Storage Access Method) is a high-performance file storage access method used on IBM mainframes. It is the backbone of legacy data processing, allowing for efficient storage and retrieval of vast amounts of data. While the COBOL syntax looks similar to standard Indexed Files, the underlying mechanism is different and highly optimized, often using Key Sequenced Data Sets (KSDS).

In this example, we use ACCESS MODE IS DYNAMIC, which allows the program to switch between sequential access (reading next record) and random access (reading by key) on the fly. This is a common requirement for browsing data or processing transactions where you might need to find a specific customer and then print a report of their subsequent transactions.

Robust error handling is critical when working with VSAM. The FILE STATUS clause is mandatory for production code. It captures the result of every I/O operation, allowing the program to react to conditions like "Record Locked," "Duplicate Key," or "File Full" (Status 34, 22, and 30 respectively), ensuring data integrity is maintained.

Code Breakdown

6
SELECT CUST-FILE. Maps the internal file handle to the external dataset "CUST.DAT".
7
ORGANIZATION IS INDEXED. Specifies that the file is accessed via an index key, typical for VSAM KSDS.
8
ACCESS MODE IS DYNAMIC. Enables both random access (READ key) and sequential access (READ NEXT).
9
RECORD KEY IS CUST-KEY. Defines the primary key field within the record structure used for indexing.
10
FILE STATUS IS WS-STATUS. Links the file operations to a status variable for error checking.
23
OPEN I-O CUST-FILE. Opens the file for both Input (reading) and Output (writing/updating).
24
IF WS-STATUS NOT = "00". Immediately checks if the file opened successfully. "00" indicates success.
29
MOVE 12345 TO CUST-KEY. Sets the key value we want to search for.
30
READ CUST-FILE. Attempts to retrieve the record matching the key.
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