Transcript 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)
2
ESDS Structure
• Entry-Sequenced Data Set
– records can be accessed
• sequentially on RBA value
• directly by supplying the RBA of desired record
3
ESDS Structure
ESDS
Data Component
100 byte records:
Record 1 RBA = 0
Rec
1
Rec
2
Rec
3
0
100
200
………….
Rec
n
Record 2 RBA = 100
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
5
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
6
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
R R R C
I
D D D D
Space F F F
F
Free
F
S
Rec
Free
J
Space
R R C
I
D D D
F F F
C
I
D
F
7
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
8
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
9
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
10
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
11
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
12
VSAM ESDS and COBOL
• Processing an ESDS is very similar to
processing a KSDS sequentially
13
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.
14
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.
16
ESDS OPEN Statement
• OPEN statement
• Must open all files
– Processing dynamically
• OPEN I-O esds-file.
– Only writing at end
• OPEN EXTEND esds-file.
17
ESDS READ Statement
• READ statement (sequential)
READ file-name RECORD [INTO area]
[AT END imperative-1]
NOT AT END imperative-2]
[END-READ]
18
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
21
ESDS REWRITE Statement
• REWRITE statement (sequential)
REWRITE record-name [FROM area]
[END-REWRITE]
22
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
23
ESDS DELETE Statement
• DELETE statement (sequential)
DELETE file-record RECORD
[END-DELETE]
24
ESDS CLOSE Statement
• CLOSE statement (sequential)
– list the ESDS that is to be closed
25
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
27
RRDS Structure
RRDS
Fixed-Length
records:
Record 1 in Slot 0
Data Component
Rec
1
0
Rec
35
1
………….
Rec
n
2
Dummy Rec in Slot 1
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
30
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
31
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
33
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
38
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.
42