04 - Vsam Esds And Rrds

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

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)

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ESDS Structure • Entry-Sequenced Data Set – records can be accessed • sequentially on RBA value • directly by supplying the RBA of desired record

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ESDS Structure

ESDS 100 byte records: Record 1 RBA = 0 Record 2 RBA = 100

Data Component Rec 1

Rec 2

Rec 3

0

100

200

………….

Rec n

Record 3 RBA = 200 etc. 4

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

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ESDS Control Interval

ESDS Rec

Rec

Rec

Rec

Rec

Cont

A

B

C

D

E

Info

Rec

Rec

Rec

Rec

Rec

Cont

F

G

H

I

J

Info

Rec

Rec

Rec

Rec

Rec

Cont

K

L

M

N

O

Info

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ESDS Control Area ESDS Control Area Rec

Rec

Rec

Rec

A

B

C

D

Rec

Rec

Rec

E

F

G

Rec

Rec

H

I

R R R C I D D D D F F F F C I D D D D F F F F

Free R R R Space

F S

Rec

Free

J

Space

R R C I D D D F F F C I D F

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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

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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

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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

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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

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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

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

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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.

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Procedure Division in ESDS Processing • • • • • •

OPEN statement READ statement WRITE statement REWRITE statement DELETE statement CLOSE statement 15

ESDS OPEN Statement • OPEN statement • Must open all files – Loading sequentially • OPEN OUTPUT esds-file.

– Retrieving sequentially • OPEN INPUT esds-file.

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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 19

ESDS WRITE Statement • WRITE statement (sequential) WRITE record-name [FROM area] [END-WRITE]

– Note: INVALID-KEY cannot be specified in ESDS write operation 20

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

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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

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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 26

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

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RRDS Structure

RRDS

Fixed-Length records: Record 1 in Slot 0 Dummy Rec in Slot 1

Data Component Rec 1 0

Rec 35 1

………….

Rec n

2

Record 35 in Slot 2 etc. 28

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 29

RRDS Control Interval

RRDS Rec

Dummy Dummy

E Dummy

Rec

Dummy Cont

B Rec

Rec

A

C

Dummy Dummy

Dummy

Info Rec

Cont

F

Info

Rec

Rec

Rec

Cont

H

G

D

Info

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RRDS Control Area RRDS Control Area Rec

Dummy

A Dummy

C I D D D F F F

Dummy R R

C Rec F

Dummy

Rec

Rec E

Dummy

Rec B

R R C I D D D F F F C I D D D F F F

Dummy Dummy R R

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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 32

RRDS Loading • VSAM – locates the desired slot – verifies that the slot does not contain an active record – write the record into that slot

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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 34

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 35

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 36

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 37

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

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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. 39

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 40

Procedure Division in RRDS Processing • • • • • •

OPEN statement READ statement WRITE statement REWRITE statement DELETE statement CLOSE statement 41

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.

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