VSAM, A BRIEF INTRODUCTION
....................................................................... 1
Types of VSAM datasets ............................................................ 1
Internal Organization ............................................................... 2
Alternate Index .................................................................... 3
VSAM FILE IN CICS ................................................................ 3
USING VSAM FILES IN COBOL ...................................................... 6
Introduction
Introduction
VSAM, fully known as Virtual Storage Access Method is a data management system introduced
by IBM, as part of modern IBM operating systems like MVS in 1970s.VSAM software resides in
virtual storage along with the application program that needs it services for the manipulation of
data on the direct storage access device (DASD)
Types of VSAM datasets
VSAM provides three types of dataset organization:
  Key Sequenced Data Set (KSDS), where records are identified and sequenced on
a key field. Key field is a part of data record which uniquely identifies that record from all
other records in the dataset.
KSDS datasets are similar to Indexed Sequential Access Method (ISAM)
datasets, with many of the same characteristics, but also having distinct
advantages over ISAM.
A KSDS consists of two physical components on DASD.
  Data Component
  Index Component
Data component contains the records that hold the user data including the Key field. The
records in Data component are stored in ascending sequence of the key field. Index component
contains the key field and the pointer to the user data to which the key field belongs. These
components are jointly called base cluster.
The following figure
shows the layout of index component and the Data Component.
(Index Component)
Key
Pointer
field
(Data Component)
Key Field
Data Records
KSDS allows both random and sequential, retrieval and insertion of records in the
dataset. Insertion is achieved by providing free space between the records at the time of loading.
Excess of random
insertion/deletion of records may result in records becoming physically
out
of sequence but records remain in logical sequence when they are
accessed
through the index.
Physical sequencing of records help in efficient sequential retrieval of records and to
maintain that KSDS dataset needs occasional reorganization of the file.
  Entry Sequenced Data Set (ESDS), as the name depicts , the records are
sequenced in the order in which they are loaded in the dataset and identified by
specifying its physical location - the byte address of the first data byte of each record in
relationship to the beginning of the dataset.
A new insertion in the ESDS is always appended at the end of the dataset but allows
both random and sequential retrieval of the records.
Few Tips while using ESDS:  Even if ESDS allows variable length record, while updation the record length
cannot be changed.
  Records in ESDS cannot be physically deleted.
  ESDS are usually used by online programs as audit-trail or data entry file, which
can later be used batch programs to update the master files.
An ESDS only has the data component.
ESDS datasets are similar to Basic Sequential Access Method (BSAM) or Queued
Sequential Access Method (QSAM) datasets.
  Relative Record Data Set (RRDS), where each record is identified for access by
specifying its record number - the sequence number relative to the first record in the
dataset. RRDS datasets are similar to Basic Direct Access Method (BDAM) datasets.
Internal Organization
  Control Intervals
In VSAM, the smallest unit of data transferred between I/O buffers and dataset is defined
as a control interval. A control interval is basically a physical entity on disk and contains records,
control interval description field (CIDF 4 Bytes), Record description field (RDF 3 bytes) and (in the
case of KSDS clusters) possibly free space which may later be used to contain inserted
records.
With ESDS clusters, CI does not contain any free space as no insertion is made between
records. CI is completely filled before record is written into the next control interval in sequence.
With RRDS clusters, control intervals are filled with fixed-length slots, each containing
either an active record or a dummy record. Slots containing dummy records are available for use
when new records are added to the dataset.
  Control Areas
A control area consists of two or more control intervals. In case of KSDS clusters some
CI are left free for insertion purpose. For ESDS and RRDS clusters, control areas are filled with
control intervals that contain records.
 
Spanned Records:
When the length of record is more then the length of CI Size minus 7 then the records
are contained in more then one CI such a record is called
Spanned Record.
Free space left by spanned record is left unused.
Alternate Index:
An AIX is a file which allows access to a VSAM dataset by a key other than the
primary key. In case of KSDS, AIX contains primary key field of the base cluster and alternate
key field of the base cluster.
When the search on the alternate key is made then, AIX file is accessed.
The record with the same value of alternate index as given by user is found in AIX file.
This in turn gives the primary key corresponding to the record.
After this base cluster is searched randomly for the primary key and all the values
corresponding to primary key are retrieved.
Search on alternate keys are slower than the search on the primary key.
Even if there is no concept of primary key in ESDS it supports Alternate Key. In this case
Alternate cluster contains the Key field and RBA of the record. Alternate index may be on unique
or non-unique field.
VSAM FILE IN CICS
All Online Programs on Mainframe run in CICS. So, usage of VSAM files in CICS is an
important concept.
First of all there must be a FCT entry of the file that is required to be used
in CICS
program. Once there is a FCT entry, then a program can access the file using logical name.
There is always an interaction between logical and physical name of the file in JCL. Different
systems use different commands to show JCL. In VSE, typing LBL f3 gives the JCL indicating the
physical and logical name of the file. Once file has FCT entry, following functions can be used on
the file:  
BROWSING FUNCTIONS
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STARTBR
READNEXT
READPREV
RESETBR
ENDBR
STARTBR As the name indicates this means start browsing the file. In fact, this locates
the pointer on a particular record on the file from which reading of records has to begin
SYNTAX
EXEC CICS
STARTBR DATASET ( dataset-name)
RIDFLD ( key-field)
[ GTEQ, EQUAL]
END-EXEC.
Dataset-name: This is the Logical name of the file.
Key-Field
: This is the record field, which is to be located on the file.
GTEQ
: If specified, key is located on any record with key equal to or
greater than the key-field specified.
Default is
GTEQ if nothing is specified.
READNEXT This function reads the record already located by STARTBR
option.
Once located, records can be read in sequence.
SYNTAX
EXEC CICS
READNEXT DATASET (dataset-name)
INTO (Data-name1)
END-EXEC.
Data-name1: This indicates the field, which will contain the record that has been read.
READPREV This function is used to browse a file in backward direction. This also follows
STARTBR function.
SYNTAX
EXEC CICS
READPREV DATASET (dataset-name)
INTO (Data-name1)
END-EXEC.
Data-name1: This indicates the field, which will contain the record that has been read.
RESETBR This function is used to reset the browser once browsing is in progress.
SYNTAX
EXEC CICS
RESETBR DATASET (dataset-name)
RIDFLD (Data-name1)
END-EXEC.
ENDBR This option is used to end the browsing.
SYNTAX
EXEC CICS
ENDBR DATASET (dataset-name)
END-EXEC.
 
OTHER FUNCTIONS Other functions involved for datasets in CICS are
  READ DATASET
  WRITE DATASET
  REWRITE DATASET
  DELETE DATASET
READ DATASET: This statement is used to read a dataset in random mode.
SYNTAX
EXEC CICS
READ DATASET ( cics-file-name)
INTO ( cobol-data-record)
RIDFLD (key-field)
[GTEQ, EQUAL]
END-EXEC
WRITE DATASET This involves writing the record on to the file.
SYNTAX
EXEC CICS
WRITE DATASET (cics-file-name)
FROM (cobol-data-record)
RIDFLD (key-field)
END-EXEC
REWRITE DATASET This means replacing the contents of the record which has a key
matching with the key passed.
SYNTAX
EXEC CICS
REWRITE DATASET (cics-file-name)
FROM (cobol-data-record)
END-EXEC
DELETE DATASET This is used to delete record from the file which has same key as the
key passed.
SYNTAX
EXEC CICS
DELETE DATASET (cics-file-name)
RIDFLD (key-field)
END-EXEC
Using VSAM Files In Cobol
  We will consider SELECT statement for file. This declaration is made in the FileControl Section of the COBOL program.
SYNTAX
SELECT File-Name ASSIGN TO Assignment-name
ORGANIZATION
IS INDEXED
ACCESS MODE
IS ACCESS-MODE1
RECORD KEY
IS DATA-NAME1
FILE STATUS
IS DATA-NAME2
File-Name This is the logical name that will be referred in COBOL program. From now
we assume file name for our documentation as EMPLOYEE
Assignment-Name This is same as the DD name of the dataset in the JCL.
Organization is Indexed indicates that this dataset is a KSDS.
Access-mode This specifies the manner in which the file will be accesses. It can be
accessed either in Sequential mode or Random mode or Dynamic Mode.
RECORD KEY This is actually the primary key field of the KSDS. This is coded in the FD
clause. We will use EMP-REC as Record key for
documentation.
File Status This is equivalent to the return code after any access. Example: If you open a
dataset and get return code not equal to zeroes, this means that there has been some error in
opening of the dataset.
  After making Declaration in the FILE-CONTROL Section, we will consider the
declaration in the FILE SECTION of DATA DIVISION.
SYNTAX
FD
EMPLOYEE.
RECORDING MODE IS F
LABEL RECORDS ARE OMITTED
RECORD CONTAINS 80 CHARACTERS.
01
EMP-REC
PIC X (80).
It is mandatory to mention LABEL RECORDS in the FD Section.
  Processing Of File In Procedure Division
We can open file in either input or output or I-O or Extend mode.
We will discuss each of the modes: -
 
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 OUTPUT Mode This allows simply writing on a file. IF a file is used in this
mode, then all the existing records in the file will be deleted. This means that
Output mode places the pointer on the first position on the file. It does not
append records on the file.

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 I-O Mode This allows the user to add, delete and rewrite the records in the
dataset.
SYNTAX
EMPLOYEE INPUT.
INPUT Mode This will allow only Read or Browse function on the dataset.
EXTEND Mode This allows the user to append the records in the dataset.
Reading From VSAM Files In Sequential Mode
SYNTAX
READ EMPLOYEE [ INTO DATA-NAME1]
[AT END imperative statement]
Both INTO clause and AT END clause are optional. Imperative Statement
indicates the statements to be executed in case the end of file is reached.
 
Reading From VSAM Files In Random Or Dynamic Mode
SYNTAX
READ EMPLOYEE [ INTO DATA-NAME1]
[ INVALID KEY imperative statement ]
Imperative statement will be executed if the key is not found.
Important concept in reading files is that before any kind of read statement you must
move the record to be read in the RECORD KEY area in the FD statement. In our example value
must be move in EMP-REC before executing the read statement.
  Writing VSAM Files In Sequential Mode In case of writing records in VSAM file
accessed in sequential mode, it is necessary that the record, which is to be written has
primary key greater than the record with the highest primary key.
SYNTAX
WRITE EMPLOYEE [FROM DATA-NAME1]
[INVALID KEY imperative statement]
INVALID KEY may be the case in which either the record does not possess the highest
key (STATUS CODE 21) or the record already exists in the VSAM file.
  Writing In VSAM Files In Random Or Dynamic Mode In case of writing records in
VSAM file accessed in dynamic or random mode, it is not necessary that the record
which is to be written has primary key greater than the record with the highest primary
key.
SYNTAX
WRITE EMPLOYEE [FROM DATA-NAME1]
[INVALID KEY imperative statement]
INVALID KEY means record already exists in the VSAM file.
  REWRITING IN VSAM FILES
This enables the user to change the existing record. Normal procedure is: First read the
record, modify the contents and then write it back using REWRITE statement. The only
restriction is that Primary key cannot be modified.
If Access Mode is Sequential then it is necessary to read the record, in other case it is not
necessary to read the record.
SYNTAX
REWRITE EMPLOYEE [FROM DATA-NAME1]
[INVALID KEY imperative statement]
  Deleting From VSAM Files
Records can be physically deleted from VSAM files. This gets the space reclaimed and
provides the space for other records.
SYNTAX
DELETE EMPLOYEE
[INVALID KEY imperative statement]
  BROWSING VSAM FILES
Sometimes, it is necessary to browse VSAM files after a particular value i.e. not from
beginning to end but from some position till end. This is possible using combination of
START and READ NEXT statement.
START This actually positions the location from which reading is to be
done.
SYNTAX
START EMPLOYEE [KEY IS (= OR > OR <) DATA-NAME]
[INVALID KEY is imperative statement]
READ NEXT This starts reading the record in sequential manner including
record on which positioning is done using START statement.
SYNTAX
READ EMPLOYEE NEXT [INTO DATA NAME]
[AT END imperative statement]
the