VSAM ESDS and RRDS

Department of Computer Science

Northern Illinois University

September 2005

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

2

ESDS Structure

• Entry-Sequenced Data Set– cluster consists of data component only– records are stored in the order in which

they are entered into the data set– each record is reference by its relative byte

address (RBA)

3

ESDS Structure

• Entry-Sequenced Data Set– records can be accessed

• sequentially on RBA value• directly by supplying the RBA of desired record

4

ESDS Structure

ESDS

Data ComponentRec

1Rec

2Rec

3Rec

n………….0 100 200

100 byte records:

Record 1 RBA = 0

Record 2 RBA = 100

Record 3 RBA = 200

etc.

5

ESDS Control Interval

• Each CI is filled before records are written into the next control interval in sequence

• Control Areas are filled with CI’s that contain records

6

ESDS Control Interval

ESDS

Cont

Info

Rec

D

Rec

E

Rec

A

Rec

B

Rec

C

Cont

Info

Rec

I

Rec

J

Rec

F

Rec

G

Rec

H

Cont

Info

Rec

N

Rec

O

Rec

K

Rec

L

Rec

M

7

ESDS Control Area

ESDS Control Area

Rec

A

Rec

B

Rec

C

Rec

E

Rec

F

Rec

G

Free

Space

Rec

H

Rec

I

Free

Space

RDF

RDF

RDF

CIDF

RDF

RDF

RDF

CIDF

RDF

RDF

CIDF

CIDF

F S

Rec

J

Rec

D

8

ESDS Processing

• ESDS can be updated using REWRITE either– sequentially on RBA or – directly with given RBA

• ESDS deletion is only logical not physical– space is not reclaimed– VSAM just sets a code denoting logical

deletion

9

ESDS Loading

• Records are loaded in physical sequence

• VSAM fills each control interval with as many records as will fit

• No provision for distributed free space

• No index

10

ESDS Loading

• Any left over space at end of each CI becomes free space

• This free space is NOT usable in subsequent insertions because all records are added at the end of the data set

11

ESDS Retrieval

• For sequential processing VSAM– retrieves the records in the sequence in

which they are stored in the data set– can also be retrieved in descending order

starting with any record whose RBA is known

12

ESDS Retrieval

• For direct processing VSAM– must be given the RBA of the record to be

read

• RBAs can be kept somewhere, maybe in a table, as the ESDS is being loaded– does not allow for efficient processing of

records by key

13

VSAM ESDS and COBOL

• Processing an ESDS is very similar to processing a KSDS sequentially

14

File-Control for ESDS

File-Control.SELECT file-name

ASSIGN to ddname

ORGANIZATION is SEQUENTIAL

ACCESS MODE is SEQUENTIAL

FILE STATUS is data-name-2.

15

Procedure Division in ESDS Processing

• OPEN statement

• READ statement

• WRITE statement

• REWRITE statement

• DELETE statement

• CLOSE statement

16

ESDS OPEN Statement

• OPEN statement

• Must open all files– Loading sequentially

• OPEN OUTPUT esds-file.

– Retrieving sequentially• OPEN INPUT esds-file.

17

ESDS OPEN Statement

• OPEN statement

• Must open all files– Processing dynamically

• OPEN I-O esds-file.

– Only writing at end• OPEN EXTEND esds-file.

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ESDS 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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ESDS WRITE Statement

• WRITE statement (sequential)– new records cannot be added to an

existing ESDS by inserting a new record between two existing records

– new records are added only at the end– ESDS file must be opened for EXTEND

which automatically positions at end of the file in preparation

20

ESDS WRITE Statement

• WRITE statement (sequential)

WRITE record-name [FROM area]

[END-WRITE]

– Note: INVALID-KEY cannot be specified in ESDS write operation

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ESDS REWRITE Statement

• REWRITE statement (sequential)– Used to update existing records in an

ESDS in place– the ESDS 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

22

ESDS REWRITE Statement

• REWRITE statement (sequential)

REWRITE record-name [FROM area]

[END-REWRITE]

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ESDS DELETE Statement

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

statement– the space in ESDS is only marked as

deleted, space is not reclaimed

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ESDS DELETE Statement

• DELETE statement (sequential)

DELETE file-record RECORD

[END-DELETE]

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ESDS CLOSE Statement

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

26

RRDS Structure

• Relative Record Data Set– used in conjunction with randomizing or

hashing functions– a record can be accessed either

• sequentially in physical order OR• directly by supplying the relative record number

of the slot that contains the desired record

– processing can be combined in a program between physical sequential and direct

27

RRDS Structure

• Relative Record Data Set– consists of a data component only– contains fixed-length records only– each record is

• contained in a fixed-length slot• addressed by the relative record number of the

slot in which it is stored starting with number 0

28

RRDS Structure

RRDS

Data ComponentRec

1Rec35

Recn………….

0 1 2

Fixed-Length records:

Record 1 in Slot 0

Dummy Rec in Slot 1

Record 35 in Slot 2

etc.

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RRDS Control Interval

• Each CI is filled with fixed-length slots which contain either– an active record OR– a dummy record

• Slots containing dummy records are available for use in the addition of new records to the file

30

RRDS Control Interval

RRDS

Cont

Info

Rec

B

Rec

E

Dummy

Cont

Info

Rec

F

Rec

A

Rec

C

Cont

Info

Rec

G

Rec

D

Rec

H

Dummy Dummy

Dummy Dummy

DummyDummy

31

RRDS Control Area

RRDS Control Area

Rec

A

Rec

B

Rec

E

Rec

F

RDF

RDF

CIDF

RDF

RDF

CIDF

RDF

RDF

CIDF

Dummy

Dummy

Dummy

Dummy

Dummy

Dummy Dummy

Rec

C

32

RRDS Loading

• Records can be loaded either sequentially or directly

• It is more common to load directly

• Records are loaded using WRITE statement that specifies the relative record number of the slot into which the record is to be placed

33

RRDS Loading

• VSAM– locates the desired slot– verifies that the slot does not contain an

active record– write the record into that slot

34

RRDS Loading

• Free space that exists at the end of each control interval cannot be reused

• There may be available space after the load has been completed– these are dummy records into which no

active record has been loaded

• More space is allocated in an RRDS than other types of VSAM data sets

35

RRDS Loading

• When a randomizing routine is used, there is a possibility of synonyms– two key values hashing to the same slot

number

• An application must make provisions for handling synonyms– beyond the scope of this course

36

RRDS Retrieval

• For direct processing VSAM– the same hashing routine is used to

determine the slot number of the record needed

• The program should then check to make sure that the record key is the correct one

37

RRDS Updating

• RRDS records can be updated while processing the file either sequentially or directly

• VSAM performs a deletion by converting the slot into a dummy record

• VSAM performs an update by finding a dummy record an replacing it with a new active record

38

VSAM RRDS and COBOL

• In processing RRDS, an application must be able to – create an appropriate hashing function and – to handle synonyms

39

File-Control for RRDS

File-Control.SELECT file-name

ASSIGN to ddname

ORGANIZATION is RELATIVE

ACCESS MODE is RANDOM

RELATIVE KEY is data-name-1

FILE STATUS is data-name-2.

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File-Control for RRDS

File-Control.RELATIVE KEY is data-name-1

– relative key • is NOT in record description• is in Working-Storage Section• is where the slot number is placed prior to

directly accessing an RRDS record

41

Procedure Division in RRDS Processing

• OPEN statement

• READ statement

• WRITE statement

• REWRITE statement

• DELETE statement

• CLOSE statement

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Procedure Division in RRDS Processing

• We are not going into the specifics of the RRDS statements in this class.

• Please read the course notes for more information including pseudocode for processing an RRDS.