03 - VSAM KSDS and COBOL

VSAM KSDS and COBOL

Department of Computer Science

Northern Illinois University

August 2005

Some of the illustrations are from VSAM: Access Method Services and Programming Techniques by James Martin

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Processing a KSDS

• KSDS can be processed 3 ways– Sequentially

• Records accessed in key sequence

– Randomly• Records accessed via a specified key value

– Dynamically• Records accessed both sequentially and

randomly

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Sequentially Processing a KSDS

• Access records in ascending order on the file’s key field

• Usually begin with the first record and process to the end

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File-Control for Sequential KSDS

File-Control.SELECT file-name

ASSIGN to ddname

ORGANIZATION is INDEXED

ACCESS MODE is SEQUENTIAL

RECORD KEY is data-name-1

FILE STATUS is data-name-2.

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• File-Control.SELECT file-name

where file-name is the name used for the file throughout the

program

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• File-Control.ASSIGN to ddname

where ddname is the ddname in the JCL used to refer to the KSDS file

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• File-Control.ORGANIZATION is INDEXED

where INDEXED specifies that this file is a KSDS

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• File-Control.ACCESS MODE is SEQUENTIAL

where SEQUENTIAL specifies that the processing of the KSDS will be in order on the key field of the file

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• File-Control.RECORD KEY is data-name-1

where data-name-1 is the file’s key field and the records are going to be accessed ascending on this field

data-name-1 must appear in the file’s record description in the FILE SECTION

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• File-Control.FILE STATUS is data-name-2

where data-name-2 is a filed you specify in which VSAM provides information about each I/O operation

VSAM places a return code in data-name-2 for the application program to determine the result of the I/O operation

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Common File Status for KSDS

Code Meaning00 Successful completion

10 End of file reached

21 Sequence error

22 Duplicate key

23 Record not found

24 No more space

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FD Statement in KSDS

FD KSDS-file

LABEL RECORDS ARE [OMITTED

or STANDARD].

Treated as a comment but is required

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Procedure Division in KSDS Sequential Processing

• OPEN statement

• START statement

• READ statement

• WRITE statement

• REWRITE statement

• DELETE statement

• CLOSE statement

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KSDS OPEN Statement

• OPEN statement (sequentially)

• Must open all files– Loading sequentially

• OPEN OUTPUT ksds-file.

– Retrieving sequentially• OPEN INPUT ksds-file.

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KSDS START Statement

• START statement– Used to start sequential processing with a

record other than the first record in the file– Use with a file that is opened as input or

dynamically (I-O) and processing sequentially

– To use, place a value in the record key area

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KSDS START Statement

• START statementSTART vsam-file

[KEY IS {EQUAL TO=GREATER THAN>NOT LESS THANNOT <GREATER THAN OR EQUAL TO>= }

record-key][INVALID KEY imperative-1]

[NOT INVALID KEY imperative-2][END-START]

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KSDS READ Statement

• READ statement (sequential)– To retrieve records in key sequence, open

KSDS for either• INPUT or• I-O

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KSDS READ Statement

• READ statement (sequential)

READ file-name RECORD [INTO area]

[AT END imperative-1]

NOT AT END imperative-2]

[END-READ]

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KSDS READ Statement

• READ statement (sequential)

READ file-name [NEXT]

Specify NEXT on the READ statement to retrieve records sequentially when file is opened for I-O

Omit NEXT when file is opened for I-O and the records will be retrieved randomly based on the value in the RECORD KEY field

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KSDS READ Statement

• READ statement (sequential)– The key using dynamic access is knowing

how to switch from sequential to random access.

– The position for sequential retrieval is changed only by a START or a random READ statement.

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KSDS READ Statement

• READ statement (sequential) – A random READ statement can be used to

retrieve a specific record and then can be followed by a sequential READ statement (using the NEXT option)

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KSDS WRITE Statement

• WRITE statement (sequential)– KSDS must be opened for

• OUTPUT for file creation (loading of data)• I-O for file additions

– A value must be placed into the primary key field prior to the write

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• WRITE statement (sequential)

WRITE record-name [FROM area]

[INVALID-KEY imperative-1]

[NOT INVALID-KEY imperative-2]

[END-WRITE]

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• WRITE statement (sequential)– INVALID-KEY imperative-1 is executed when

• the value stored in the primary key field prior to issuing the WRITE is equal to that of a record already in the file

• an attempt is made to write beyond the boundaries of the file

• the primary key is not greater than the primary key of the previous record AND the file is opened for OUTPUT and sequential accessing is specified

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KSDS REWRITE Statement

• REWRITE statement (sequential)– the KSDS must be opened for I-O– the record to be rewritten needs to be the

last record read from the file • must read the record prior to rewriting it

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• REWRITE statement (sequential)

REWRITE record-name [FROM area]

[INVALID KEY imperative-1]

[NOT INVALID KEY imperative-2]

[END-REWRITE]

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• REWRITE statement (sequential)– [INVALID KEY imperative-1] is executed

when• the record to be changed does not exist in the

file• sequential access is specified and the value

placed in the primary key doesn’t equal the primary key fo the previously read record

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KSDS DELETE Statement

• DELETE statement (sequential)– KSDS opened as I-O– delete the record read in the last READ

statement– the space in KSDS is immediately

available for reuse

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• DELETE statement (sequential)

DELETE file-record RECORD



[END-DELETE]

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• DELETE statement (sequential)– [INVALID-KEY imperative-1] is executed when

• an attempt to delete a record that does not exist is made

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KSDS CLOSE Statement

• CLOSE statement (sequential)– list the KSDS that is to be closed

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KSDS Load Program in COBOL

• Go over handout

“COBOL JCL

& Source Review for Loading KSDS”

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KSDS Random Processing

• All I/O operations depend upon the record key

• Before READ a value must be in the record key

• Before WRITE a value must be in the record key

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File-Control for Random KSDS


ASSIGN to ddname


ACCESS MODE is RANDOM


FILE STATUS is data-name-2.

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File-Control for Random KSDS

File-Control.ACCESS MODE is RANDOM

The only difference in the SELECT statement for random processing is the word RANDOM access

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Procedure Division in KSDS Random Processing

• OPEN statement

• READ statement

• WRITE statement

• REWRITE statement

• DELETE statement

• CLOSE statement

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KSDS OPEN Statement

• OPEN statement (random)

• Must open all files– Retrieving randomly

• OPEN INPUT ksds-file.

– Writing randomly• OPEN OUTPUT ksds-file.

– Reading, rewriting and deleting randomly• OPEN I-O ksds-file.

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KSDS READ Statement

• READ statement (random)– To retrieve records based upon value in

the record key field, open KSDS for either• INPUT or• I-O

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KSDS READ Statement

• READ statement (random)– To retrieve records based upon value in

the record key field, open KSDS for either• INPUT or• I-O

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KSDS READ Statement

• READ statement (random)

READ file-name RECORD [INTO area]

[KEY IS data-name-1]


NOT INVALID KEY imperative-2]

[END-READ]

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KSDS READ Statement

• READ statement (random)


The key value of the record that is to be retrieved from the KSDS must be placed in data-name-1 prior to issuing the READ statement.

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KSDS READ Statement

• READ statement (random)– when invoked comparison is made

• between the value of the field specified in the RECORD KEY clause of the SELECT statement (known as “key of reference”) and

• the key values of the KSDS records

– search is done via the index– appropriate control interval is read into

memory and sequentially searched

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• WRITE statement (random)

WRITE record-name [FROM area]



[END-WRITE]

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• WRITE statement (random)– used to add records to a file– KSDS uses the RECORD KEY value to write the

record in its proper physical location

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KSDS REWRITE statement

• REWRITE statement (random)

REWRITE record-name [FROM area]


[NOT INVALID KEY imperative-2]

[END-REWRITE]

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KSDS REWRITE statement

• REWRITE statement (random)– used to update a record in its original physical

location– KSDS uses the RECORD KEY value to rewrite the

record in its proper physical location

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• DELETE statement (random)

DELETE file-record RECORD



[END-DELETE]

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• DELETE statement (random)– used to delete a record from its physical location– KSDS uses the RECORD KEY value to find the

record to delete

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KSDS CLOSE Statement

• CLOSE statement (random)– list the KSDS that is to be closed

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KSDS Dynamic Processing

• Processing can be a mix of sequential and random

• All the statements remain the same except the SELECT and the sequential READ

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• Using dynamic processing effectively is knowing how to switch from sequential to random access– position for sequential retrieval by a

• START or• random READ

– issue READ using NEXT for sequential processing

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• Note: – During dynamic processing issuing a

WRITE, REWRITE, or DELETE statement does not change file position

– Use START or random READ to change file position

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File-Control for Dynamic KSDS


ASSIGN to ddname


ACCESS MODE is DYNAMIC


FILE STATUS is data-name-2

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KSDS READ Statement

• READ statement (dynamic)

READ file-name [NEXT] RECORD [INTO area]



NOT INVALID KEY imperative-2]

[END-READ]

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KSDS READ Statement

• READ statement (dynamic)– NEXT is used in dynamic processing of

KSDS to retrieve records sequentially on the key

– if NEXT is omitted the records are retrieved randomly based on the value in the RECORD KEY field

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03 - VSAM KSDS and COBOL