IMS DB
Objectives
• To create awareness about the IMS DB technology and how it is used to perform data base operations.
• Target audience :- people who are relatively new to the IMS DB Technology.
Prerequisites• Knowledge of COBOL• Basic knowledge of data base management concepts
Course Outline1. An Introduction to DL/I Data Bases2. DL/I Programs and Control Blocks3. COBOL Basics for Processing a DL/I Data Base 4. Segment Search Arguments : How to use them5. Data retrieval from an IMS Data Base6. Adding and Updating Data to a Data Base7. Secondary Indexing8. Logical Data Bases9. Recovery and Restart10. DL/I Data Base Organizations11. Advanced DL/I features
References• IMS for the COBOL Programmer
Part 1: Data base processing with IMS/VS and DL/I DOS/VS
By Steve Eckols• IBM Redbooks : IMS Primer
By Rick Long, Mark Harrington, Robert Hain, Geoff Nicholls
• MVS Quick Ref Ver. 5.5
Module 1An Introduction to DL/I Data
BasesHierarchical StructuresWhy a Data Base Management System
Basic DL/I TerminologyBasic DL/I Data Base Processing
Hierarchical Structures• In a DL/I data base, data elements are organized in a hierarchical structure.• Some data elements are dependent on others.
Fig 1.1 A hierarchical structure
DL/I supports hierarchies that are difficult to implement with standard files.
Why a data base management system?
01 VENDOR-RECORD.05 VR-VENDOR-CODE PIC X(3).05 VR-VENDOR-NAME PIC X(30).05 VR-VENDOR-ADDRESS PIC X(30).05 VR-VENDOR-CITY PIC X(17).05 VR-VENDOR-STATE PIC XX.05 VR-VENDOR-ZIP-CODE PIC X(9).05 VR-VENDOR-TELEPHONE PIC X(10).05 VR-VENDOR-CONTACT PIC X(30).
Fig 1.2.a Record layout for the VENDORS data set01 INVENTORY-RECORD.
05 IR-ITEM-KEY.10 IR-VENDOR-CODE PIC X(3).10 IR-NUMBER PIC X(5).
05 IR-DESCRIPTION PIC X(35).05 IR-UNIT-PRICE PIC S9(5)V99 COMP-3.05 IR-AVG-UNIT-COST PIC S9(5)V99 COMP-3.05 IR-LOCATION-QUANTITY-DATA OCCURS 20 TIMES.
10 IR-LOCATION PIC X(3).10 IR-QUANTITY-ON-HAND PIC S9(7) COMP-3.10 IR-REORDER-POINT PIC S9(7) COMP-3.10 IR-QUANTITY-ON-ORDER PIC S9(7) COMP-3.10 IR-LAST-REORDER-DATE PIC X(6).
Fig 1.2.b Record layout for the Inventory Master data set
Fig 1.2 Record layouts that illustrate a hierarchical structure
Basic DL/I Terminology• Segment
– A grouping of data– The unit of data that DL/I transfers to and from your program in an I/O operation.– Consists of one or more fields
ADDRESSHouse
NumberStreetName
City State Country Zip Code
Fig 1.3 The ADDRESS segment with six fields
Segment TypeA category of dataThere can be a maximum of 255 segment types and 15 levels in one data base
Segment OccurrenceOne specific segment of a particular type containing user data
Note:- Within a data base there is only one of each segment type- it’s part of the data base’s definition-
but there can be an unlimited number of occurrences of each segment type. The word ‘segment’ is used to mean either ‘segment type’ or ‘segment occurrence’ and usually
the meaning is clear from the context
Basic DL/I Terminology (contd.)* 01 INVENTORY-VENDOR-SEGMENT. 05 IVS-VENDOR-CODE PIC X(3). 05 IVS-VENDOR-NAME PIC X(30). 05 IVS-VENDOR-ADDRESS PIC X(30). 05 IVS-VENDOR-CITY PIC X (17). 05 IVS-VENDOR-STATE PIC XX. 05 IVS-VENDOR-ZIP-CODE PIC X(9). 05 IVS-VENDOR-TELEPHONE PIC X(10).* 01 INVENTORY-ITEM-SEGMENT. 05 IIS-NUMBER PIC X(5). 05 IIS-DESCRIPTION PIC X(35). 05 IIS-UNIT-PRICE PIC S9(5)V99 COMP-3. 05 IIS-AVG-UNIT-COST PIC S9(5)V99 COMP-3.* 01 INVENTORY-STOCK-LOC-SEGMENT. 05 ISLS-LOCATION PIC X(3). 05 ISLS-QUANTITY-ON-HAND PIC S9(7) COMP-3. 05 ISLS-REORDER-POINT PIC S9(7) COMP-3. 05 ISLS-QUANTITY-ON-ORDER PIC S9(7) COMP-3.*Fig 1.5 Segment layouts for the Inventory data base
Vendor
Item
Stock Location
Fig 1.4 The hierarchical structure of the Inventory data base with three segment types
Basic DL/I Terminology (contd.)• Root Segment
– The segment type at the top of a hierarchy• Data base record
– Each occurrence of the root segment plus all the segment occurrences that are subordinate to it make up one data base record. Every data base record has one and only one root segment, although it may have any number of subordinate segment occurrences
Data base Record 1
Data base Record 2
Vendor 1
Item 1
Item 2
Loc 5Loc 4
Loc 3Loc 2
Loc 1
Loc 2Loc 1
Vendor 2
Item 1
Loc 2Loc 1
Fig 1.6 Two data base records from the Inventory data base
Basic DL/I Terminology (contd.)• Dependent Segment
– A segment other than the root segment in a data base record – Accessible only through one or more “parent” segments
• Parent Segment
– A segment that has one or more dependent segments• Child Segment
– Every dependent segment in a hierarchy• Twin Segment
– Two or more segment occurrences of the same type and with the same segment occurrence as their parent
• Path
– Series of segments leading from the root segment occurrence down to any specific segment occurrence
– Must be continuous- intermediate levels can’t be skipped
Basic DL/I Terminology (contd.)• Key or Sequence Field
– The field DLI uses to maintain segments in ascending sequence– Only a single field within a segment– Segments need not necessarily require a key field– If in a root segment, key field uniquely identifies the record
• Additional Search fields
– Used to search through the DB for particular values– Max 255 search fields in a segment
• Concatenated field
– Key formed to access a particular segment– Concatenation of keys of root segment and all successive children down to the accessed segment
• Undefined fields
– Fields not defined to IMS – Format determined by the program loading the DB
Basic DL/I Terminology (contd.)• Logical data bases
– Additional relationships within one physical data base
Fig 1.7 A logical relationship can connect two data bases
– In Fig 1.7, the line item segment is the logical child segment (or just logical child) of the item segment.
– Likewise, the item segment is the logical parent segment (or just logical parent) of the line item segment
Customer
Ship-to
Buyer Receivable
Payment Adjustment Line ItemStock Location
Item
Vendor
Basic DL/I Data Base Processing• Sequential Processing
– Top –> Down, Left -> Right– Position
• At any point, a program has a position in the data base.• Position reflects not only on retrieved segments, but on new segments inserted as well
Data base Record 1
Data base Record 2
Vendor 1
Item 1
Item 2
Loc 5Loc 4
Loc 3Loc 2
Loc 1
Loc 2Loc 1
Vendor 2
Item 1
Loc 2Loc 1
Fig 1.8 Sequential processing
Basic DL/I Data Base Processing (contd.)• Random (Direct) Processing
– Key (sequence) field required– Concatenated Key
• Completely identifies the path from the root segment to the segment you want to retrieve.
Concatenated Key:Vendor 2Item 1Location 1
Fig 1.9 Random Processing
Data base Record 1
Data base Record 2
Vendor 1
Item 1
Item 2
Loc 5Loc 4
Loc 3Loc 2
Loc 1
Loc 2Loc 1
Vendor 2
Item 1
Loc 2Loc 1
Module 2DL/I Programs and Control Blocks
The IMS Software EnvironmentHow DL/I relates to your application programs
Control BlocksDBDGENPSBGEN
IMS Processing OptionsACB & ACBGEN
Running an application program under DL/I
The IMS Software EnvironmentApplicationPrograms
IMS ControlBlocks
DL/I
OS
Data Base
IMS DC RemoteTerminal
Fig 2.1 The IMS Software Environment
How DL/I relates toyour application programsApplication
Program
Operating SystemAccess Method
(eg. VSAM)
FileData Set
Standard File Processing
ApplicationProgram
DL/I Data Base Processing
DL/I
Operating SystemAccess Method
(eg. VSAM)
Data BaseData Set
Fig 2.2 Standard file processing compared to DL/I data base processing
How DL/I relatesto your application programs (contd.)
• Standard file processing
– Standard COBOL statements (like READ / WRITE) invoke the appropriate access method (like VSAM)
– Format of the record as processed by the program should be the same as the format of the record in the file
• DL/I data base processing
– DLI - Interface between application program and the access method– CALL statement to invoke DL/I– Parameters passed by the CALL tell DL/I what operation to perform– DL/I invokes a standard access method- usually VSAM- to store data
base data on disk– Format of records in a data base data set need not match the layouts
of the segments that make up the data base– The way the program sees the data base is different from the way the
access method sees it.
Control Blocks• Physical structure of a DL/I data base isn’t specified in an application program• DL/I uses a set of control blocks(DBDs and PSBs) to define a data base’s structure• Data Base Descriptor (DBD)
– Describes the complete structure of a data base– A unique DBD for each DL/I data base
• Program Specification Block (PSB)
– Application program’s view of the Database– PSB Specifies
• Data bases (one or more) a program can access,• Data elements a program can “see” in those data bases• The processing a program can do with the data elements
– Application programs that have similar data base processing requirements can share a PSB
• Data Base Administrator (DBA) has to create DL/I control blocks• DBDGEN and PSBGEN Control Statements
SAMPLE DBDGEN (Explained in next slide)STMT SOURCE STATEMENT
1 PRINT NOGEN2 DBD NAME=INDBD,ACCESS=HIDAM3 DATASET DD1=IN,DEVICE=33804 **/ 3380 DISK STORAGE5 *6 SEGM NAME=INVENSEG, PARENT=0,POINTER=TB,BYTES=1317 LCHILD NAME=(INPXPNTR,INPXDBD),POINTER=INDX8 FIELD NAME=(INVENCOD,SEQ),BYTES=3,START=1,TYPE=C9 FIELD NAME=INVENNAM,BYTES=30,START=4,TYPE=C10 FIELD NAME=INVENADR,BYTES=30,START=34,TYPE=C11 FIELD NAME=INVENCIT,BYTES=17,START=64,TYPE=C12 FIELD NAME=INVENSTA,BYTES=2,START=81,TYPE=C13 FIELD NAME=INVENZIP,BYTES=9,START=83,TYPE=C14 FIELD NAME=INVENTEL,BYTES=10,START=92,TYPE=C15 FIELD NAME=INVENCON,BYTES=30,START=102,TYPE=C16 *17 SEGM NAME=INITMSEG,PARENT=INVENSEG,BYTES=4818 FIELD NAME=(INITMNUM,SEQ),BYTES=5,START=1,TYPE=C19 FIELD NAME=INITMDES,BYTES=35,START=6,TYPE=C20 FIELD NAME=INITMPRC,BYTES=4,START=41,TYPE=P21 FIELD NAME=INITMCST,BYTES=4,START=45,TYPE=P22 *23 SEGM NAME=INLOCSEG, PARENT=INITMSEG,BYTES=2124 FIELD NAME=(INLOCLOC,SEQ),BYTES=3,START=1,TYPE=C25 FIELD NAME=INLOCONH,BYTES=4,START=4,TYPE=P26 FIELD NAME=INLOCROP,BYTES=4,START=8,TYPE=P27 FIELD NAME=INLOCONO,BYTES=4,START=12,TYPE=P28 FIELD NAME=INLOCDAT,BYTES=6,START=16,TYPE=C29 *30 DBDGEN72 **/**************************************************************************73 **/ RECOMMENDED VSAM DEFINE CLUSTER PARAMETERS74 **/**************************************************************************75 **/* *NOTE276 **/* DEFINE CLUSTER (NAME(IN) NONINDEXED -77 **/* RECORDSIZE (2041,2041) -78 **/* COUNTERINTERVALSIZE (2048))79 **/* *NOTE2 - SHOULD SPECIFY DSNNAME FOR DD IN80 **/**************************************************************************162 **/***********SEQUENCE FIELD*************211 **/***********SEQUENCE FIELD*************325 FINISH326 END Fig 2.3 Assembler source listing for the Inventory data base DBDGEN
SAMPLE DBDGEN (contd.)• Explanation of Fig 2.3
– First macro – DBD – identifies the data base and specifies the DL/I access method
– Second macro – DATASET- identifies the file that would contain the data base
– Symbolic name (IN) identifies the data set in the JCL at execution time
– Segment types are defined using the SEGM macro– Segment hierarchical relationships are specified by the PARENT
parameter on a SEGM macro• PARENT= 0 or absence of PARENT parameter specifies root segment
– POINTER parameter and LCHILD macro are needed for HIDAM Databases
– Only search fields need be specified in the DB
DBDGEN (contd.)– FIELD macro defines a field in the DB• START position of field within segment• NAME name of the field• LENGTH length of the field• TYPE data type of the field
FIELD Macro TYPE Codes Data Type
C CharacterP Packed decimalZ Zoned decimalX HexadecimalH Half word BinaryF Full word Binary
SEQ parameter specifies a sequence fieldsegment occurrences are added in sequence by values in these fields
Fig 2.4 FIELD macro TYPE parameter codes
SAMPLE PSBGENSTMT SOURCE STATEMENT1 PRINT NOGEN2 PCB TYPE=DB,DBDNAME=INDBD,PROCOPT=LS3 SENSEG NAME=INVENSEG4 SENSEG NAME=INITMSEG,PARENT=INVENSEG5 SENSEG NAME=INLOCSEG,PARENT=INITMSEG6 PSBGEN PSBNAME=INLOAD,LANG=COBOL87 END
Fig 2.5 Assembler source listing for the Inventory data base load program’s PSBGEN
• Explanation of Fig 2.5
– PCB (Program Communication Block) refers to one data base.
– One PCB macro for each database accessed– Segment Level Sensitivity
• A program’s access to parts of the data base identified at the segment level• Within sensitive segments, the program has access to all fields
– Field level sensitivity• When the program accesses that segment, only sensitive fields are presented
PSBGEN (contd.)– DBDNAME parameter on the PCB macro specifies the name of
the DBD – KEYLEN parameter specifies the length of the longest
concatenated key the program can process in the data base– PROCOPT parameter specifies the program’s processing options– For each PCB macro, subordinate SENSEG macros identify the
sensitive segments in the data base – Names specified in the SENSEG macros must be segment names
from the DBDGEN for the data base named in the DBDNAME parameter of the PCB macro
– PSBGEN macro• Indicates that there are no more statements in the PSBGEN job• PSBNAME parameter specifies the name to be given to the output PSB module• LANG parameter specifies the language in which the related application program will be
written.
IMS Processing Options• Indicates to IMS the type of access allowed for a sensitive segment (SENSEG)• Commonly used Processing Options
– PROCOPT=G means only read-only access– PROCOPT=R means read/replace access– PROCOPT=I means insert access allowed– PROCOPT=D means Read/Delete access– PROCOPT=A means all the above options present– For GSAM DBs PROCOPT=LS for output and GS (Get Sequential) for input– PROCOPT=L allows a 'load' into the DB. If VSAM DB, it should be empty prior to the
load• The PROCOPT given for a Sensitive segment would override the one given for
the DB– Example : - PCB TYPE=DB,NAME=LDB42F,PROCOPT=G, KEYLEN=200 SENSEG NAME=SEGL4201, PARENT=0,PROCOPT=A – WARNING : Indiscriminate use of PROCOPTS can lead to inexplicable results !
ACB & ACBGEN• ACB(Application Control Blocks) : It is created by merging and expanding PSB’s and
DBD’s into an IMS internal format when an application program is scheduled for execution.
• ACBGEN : The process of building ACB is called Block Building and is done by means of ACBGEN.
• IMS can build ACB’s either dynamically or it can prebuild them using ACB maintenance utility.
• ACB’s cannot be prebuilt for GSAM DBD’s.• ACB’s can be prebuild for PSB’s that reference GSAM databases.• ACB’s save instruction, execution and direct-access wait time and improves
performance in application scheduling.• ACB’s are maintained in IMS.ACBLIB library.
Running an application program under DL/I
• Batch program does not access IMS directly• JCL invokes the DL/I ‘batch initialization module’ DFSRRC00 which loads
the application program and the required DL/I modules• The program and DL/I modules execute together• Sample JCL :
//JOBNAME JOB (ACCT),'PGMR NAME',// CLASS=J, // MSGCLASS=Z, // NOTIFY=&SYSUID//JOBLIB DD DSN=YOUR.PROGRAM.LOAD.LIBRARY,// DISP=SHR// DD DSN=YOUR.SYSTEM.RESLIB.LIBRARY, // DISP=SHR//PROC EXEC PROCNAME, SYMBOLIC PARAMETERS//*********************************************************//PROCNAME PROC//********************************************************//* THIS PROC LOADS AN IMS VSAM DATABASE //* A PROGRAM 'LOAD' IS USED FOR THIS PURPOSE//* THE PSB USED FOR LOADING IS LOADPSB //********************************************************//LOAD EXEC PGM=DFSRRC00, // PARM='DLI,LOAD,LOADPSB'
SAMPLE JCL (Contd.)//DFSRESLB DD DSN=YOUR.DFRESLIB.LIBRARY, // DISP=SHR //IMS DD DSN=YOUR.DBD.LIBRARY, // DISP=SHR // DD DSN=YOUR.PSB.LIBRARY, // DISP=SHR //IMSLOGR DD DSN=YOUR.IMSRLOG.DATASET, // DISP=SHR //IEFRDER DD DSN=YOUR.IEFRDER.DATASET, // DISP=OLD//* DD NAMES ARE AS SPECIFIED IN THE DATABASE//DATA DD DSN=VSAMDB.DATA.PART,DISP=SHR //INDEX DD DSN=VSAMDB.INDEX.PART,DISP=SHR//INPUT DD DSN=FILE.USED.FOR.LOADING, // DISP=SHR//DFSVSAMP DD DSN=IMSVS.PROCLIB(DFSVSAMP), // DISP=SHR//CPXMOPTS DD DSN=PARMLIB.LIBRARY(LOAD),// DISP=SHR//CPXMRPTS DD SYSOUT=* //SYSOUT DD SYSOUT=* //SYSPRINT DD SYSOUT=* //SYSUDUMP DD SYSOUT=* //IMSERR DD SYSOUT=* //IMSPRINT DD SYSOUT=*
Module 3COBOL Basics for Processing a
DL/I Data BaseThe ENTRY and GO BACK Statements
The DL/I CallThe PCB Mask
ENTRY andGO BACK Statements
ENTRY ‘DLITCBL’ USING PCB-name1[PCB-name2...]
Fig 3.1 Format of the DL/I ENTRY Statement
• Application program is invoked under the control of the batch initialization module• DLITCBL => ‘DL/I to COBOL’ is the entry point to the program • DL/I supplies the address of each PCB defined in the program’s PSB• PCBs must be defined in the Linkage Section• Linkage Section definition of a PCB is called a ‘PCB Mask’• Addressability to PCBs established by listing the PCB Masks on the ENTRY statement• PCB masks should be listed on the ENTRY statement in the same sequence as they appear in your
program’s PSBGEN• GO BACK Statement
– When a program ends, it passes control back to the DL/I – DL/I reallocates resources and closes the data base data sets– Use GO BACK and not a STOP RUN statement
The DL/I Call• CALL statements are used to request DL/I services• Parameters you code on the CALL statement specify, among other things, the
operation you want DL/I to perform
CALL ‘CBLTDLI’ USING DLI-functionPCB-masksegment-io-area[segment-search-argument(s)]
Fig 3.2 Format of the DL/I call
• CBLTDLI => ‘COBOL to DL/I’, is an interface module that is link edited with your program’s object module
• PLITDLI, ASMTDLI are other options
The DL/I Call (contd.)• The DL/I Function
– First parameter coded on any DL/I call– Four character working storage field containing the function code
01 DLI-FUNCTIONS. 05 DLI-GU PIC X(4) VALUE ‘GU ’. 05 DLI-GHU PIC X(4) VALUE ‘GHU ’. 05 DLI-GN PIC X(4) VALUE ‘GN ’. 05 DLI-GHN PIC X(4) VALUE ‘GHN ’. 05 DLI-GNP PIC X(4) VALUE ‘GNP ’. 05 DLI-GHNP PIC X(4) VALUE ‘GHNP’. 05 DLI-ISRT PIC X(4) VALUE ‘ISRT’. 05 DLI-DLET PIC X(4) VALUE ‘DLET’. 05 DLI-REPL PIC X(4) VALUE ‘REPL’. 05 DLI-CHKP PIC X(4) VALUE ‘CHKP’. 05 DLI-XRST PIC X(4) VALUE ‘XRST’. 05 DLI-PCB PIC X(4) VALUE ‘PCB ’.
Fig 3.3 DL/I function codes
• COBOL doesn’t allow coding literals in a CALL statement
The DL/I Call (contd.)– Get functions
• First six 05-level items in Fig 3.3• Used to retrieve segments from a DL/I data base• GU ‘get unique’ function causes DL/I to retrieve a specific segment occurrence based on field values that you
specify• GN ‘get next’ function used to retrieve segment occurrences in sequence• GNP ‘get next within parent’ function lets you retrieve segment occurrences in sequence, but only
subordinate to an established parent segment• The three get function codes that contain an H are ‘get hold functions’ and are used to specify an intent to
update a segment after you retrieve it• GHU or the ‘get hold unique’ function corresponds to GU• GHN or the ‘get hold next’ function corresponds to GN• GHNP or the ‘get hold next within parent’ function corresponds to GNP
– Update functions• Used to change data in the data base• ISRT or the ‘insert’ function is used to add a new segment occurrence to a data base– whether it be change an
existing data base or to load a new one• DLET or the ‘delete’ function is used to remove a segment from a data base• REPL or the ‘replace’ function is used to replace a segment occurrence
The DL/I Call (contd.)
– Other functions• Functions CHKP (the ‘checkpoint’ function) and XRST (the ‘restart’
function) are used in programs to take advantage of IMS’s recovery and restart features
• Function PCB is used in CICS programs• Function SYNC is used for releasing resources that IMS has locked for the
program (applicable only in a BMP)• Function INIT allows an application to receive status codes regarding deadlock and data availability (from DB PCBs)
The DL/I Call (contd.)• PCB mask
– Second parameter on the DL/I call– The name of the PCB mask defined in the program’s Linkage Section– ENTRY statement establishes a correspondence between PCB masks in
the Linkage Section and the PCBs within the program’s PSB– After each DL/I call, DL/I stores a status code in the PCB mask, which
the programmer can use to determine whether the call succeeded or failed
• Segment I/O Area
– Third parameter on the DL/I call– Name of the working storage field into which DL/I will return retrieved
data or from which it will get data for an update operation
The DL/I Call (contd.)• Segment search argument
– Optional parameter on the DL/I call– Identifies the segment occurrence you wish to access– Multiple SSAs on a single DL/I call– Two kinds of SSAs– unqualified and qualified– An unqualified SSA
• Supplies the name of the next segment type that you want to operate on• If you issue a GN call with an unqualified SSA, DL/I will return the next occurrence of the
segment type you specify
– A qualified SSA• Combines a segment name with additional information that specifies the segment occurrence
to be processed• A GU call with a qualified SSA might request a particular occurrence of a named segment type
by providing a key value
The PCB Mask• For each data base your program accesses, DL/I maintains an area of storage
called the program communication block (PCB)• Masks are defined for those areas of storage in the Linkage Section of your
program01 INVENTORY-PCB-MASK. 05 IPCB-DBD-NAME PIC X(8). 05 IPCB-SEGMENT-LEVEL PIC XX. 05 IPCB-STATUS-CODE PIC XX. 05 IPCB-PROC-OPTIONS PIC X(4). 05 FILLER PIC S9(5) COMP. 05 IPCB-SEGMENT-NAME PIC X(8). 05 IPCB-KEY-LENGTH PIC S9(5) COMP. 05 IPCB-NUMB-SENS-SEGS PIC S9(5) COMP. 05 IPCB-KEY PIC X(11).
Fig 3.4 PCB mask for an Inventory data base
The PCB Mask (contd.)• Data base name
– The name of the data base being processed• Segment level
– Specifies the current segment level in the data base– After a successful call, DL/I stores the level of the segment just processed in this field
• Status code
– Contains the DL/I status code– When DL/I successfully completes the processing you request in a call, it indicates
that to your program by moving spaces to the status code field in the PCB– If a call is unsuccessful or raises some condition that isn’t normal, DL/I moves some
non-blank value to the status code field– It is good programming practice to evaluate the status code after you issue a DL/I call
• Processing options (would be elaborated later)
– Indicates the processing a program is allowed to do on the data base• Segment name feedback area
– The name of the segment is stored by DL/I in this field after each DL/I call.
The PCB Mask (contd.)• Key length feedback area
– The field DL/I uses to report the length of the concatenated key of the lowest level segment processed during the previous call
– Used with the key feedback area• Number of sensitive segments
– Contains the number of SENSEG macros subordinate to the PCB macro for this data base
• Key feedback area
– Varies in length from one PCB to another– As long as the longest possible concatenated key that can be used with
the program’s view of the data base– After a data base operation, DL/I returns the concatenated key of he
lowest level segment processed in this field, and it returns the key’s length in the key length feedback area
Module 4Segment Search Arguments
Types of SSAsBasic Unqualified SSA
Basic Qualified SSACommand Codes
The Null Command CodePath Call
Multiple Qualifications
Types of SSAs• SSA identifies the segment occurrence you want to access• It can be either
– Qualified– Unqualified
• An unqualified SSA simply names the type of segment you want to use• A qualified SSA specifies not only the segment type, but also a specific occurrence of it
– Includes a field value DL/I uses to search for the segment you request
– Any field to which the program is sensitive to can be used in an SSA
• Because of the hierarchical structure DL/I uses, you often have to specify several levels of SSAs to access a segment at a low level in a data base
• You can code as many SSAs on a single call as you need• You can combine qualified and unqualified SSAs on a single call
Basic Unqualified SSA
01 UNQUALIFIED-SSA.* 05 UNQUAL-SSA-SEGMENT-NAME PIC X(8). 05 FILLER PIC X VALUE SPACE.*
Fig 4.1 A basic unqualified SSA
• A basic unqualified SSA is 9 bytes long• The first eight bytes contain the name of the segment you want to process• If the segment name is less than eight characters long, you must pad it on the right with blanks• The ninth position of a basic unqualified SSA always contains a blank
– The DL/I uses the value in position 9 to decide what kind of SSA you are providing
Basic Unqualified SSA (contd.)– To access a particular segment type, you must modify the segment
name during program execution, by moving an appropriate eight-character segment name to the field UNQUAL-SSA-SEGMENT-NAME
– For example,
MOVE ‘INVENSEG’ TO UNQUAL-SSA-SEGMENT-NAME MOVE ‘INITMSEG’ TO UNQUAL-SSA-SEGMENT-NAME
• Alternatively, you can code the segment name as a literal when you define a qualified SSA
– For example, 01 UNQUAL-VENDOR-SSA PIC X(9) VALUE ‘INVENSEG ’.* 01 UNQUAL-ITEM-SSA PIC X(9) VALUE ‘INITMSEG ’.* 01 UNQUAL-STOCK-LOC-SSA PIC X(9) VALUE ‘INVENSEG ’.
Basic Qualified SSA
01 VENDOR-SSA.* 05 FILLER PIC X(9) VALUE ‘INVENSEG(’. 05 FILLER PIC X(10) VALUE ‘INVENCOD =’. 05 VENDOR-SSA-CODE PIC X(3). 05 FILLER PIC X VALUE ‘)’.* Fig 4.2 A basic qualified SSA
• A qualified SSA lets you specify a particular segment occurrence based on a condition that a field within the segment must meet
• The first eight characters of a basic qualified SSA is the eight character segment name• The ninth byte is a left parenthesis • Immediately following the left parenthesis in positions 10 through 17 is an eight character field
name
Basic Qualified SSA (contd.)• After the field name, in positions 18 and 19, you code a two-character relational operator to indicate the kind
of checking DL/I should do on the field in the segment
– The qualified SSA relational operators are shown below ( stands for a single blank space)
Equal to EQ = =Not equal to NE <>Greater Than GT > >Greater than or Equal to GE >= =>Less Than LT < <Less than or Equal to LE <= =<
• After the relational operator, you code a variable field into which you move the search value you want to use for the call
• The length of the search value field can vary depending on the size of the field in the segment– it is the only part of a basic qualified SSA that doesn’t have a fixed length
• The last character in the qualified SSA is a right parenthesis
Command Codes
Fig 4.4 Qualified SSA format with a single command code
• Command are used in SSAs for three purposes
– To extend DL/I functionality– To simplify programs by reducing the number of DL/I calls– For performance improvement resulting from the reduced number of DL/I calls
Fig 4.3 Unqualified SSA format with a single command code
Command Codes (contd.)• To use command codes, code an asterisk in position 9 of the SSA• Then code your command codes starting from position 10.• When DL/I finds an asterisk in position 9, it knows command codes will follow• From position 10 onwards, DL/I considers all characters to be command codes until
it encounters a space (for an unqualified SSA) or a left parenthesis (for a qualified SSA)
• It is unusual to use more than one command code in a single SSA• A basic unqualified SSA with a single variable command code is shown below
01 UNQUALIFIED-SSA.* 05 UNQUAL-SSA-SEGMENT-NAME PIC X(8). 05 FILLER PIC X VALUE “*”. 05 UNQUAL-SSA-COMMAND-CODE PIC X. 05 FILLER PIC X VALUE SPACE.*
Command Codes (contd.)Command Code Meaning
C Concatenated KeyD Path CallF First OccurrenceL Last OccurrenceN Path Call IgnoreP Set ParentageQ Enqueue SegmentU Maintain position at this level
V Maintain position at this and all superior levels
– Null command code
Fig 4.5 SSA Command Codes
The Null Command Code• Value is a hyphen (–)• Although command code position is present, DL/I ignores it• Particularly useful if you would like to use the same SSA with and without
command codes• An SSA with the null command code is shown below
01 UNQUALIFIED-SSA.* 05 UNQUAL-SSA-SEGMENT-NAME PIC X(8). 05 FILLER PIC X VALUE “*”. 05 UNQUAL-SSA-COMMAND-CODE PIC X VALUE “-”. 05 FILLER PIC X VALUE SPACE.*
Path Call• A DB call with an SSA that includes the 'D' Command code is a "PATH CALL“ . It’s a facility
where in we can retrieve an entire path of the segment• Consider a sample GU call CALL 'CBLTDLI' USING DLI-GU INVEN-PCB-MASK INVEN-STOCK-LOC-SEG VENDOR-SSA ITEM-SSA STOCK-LOC-SSA
Normally, DL/I operates on the lowest level segment that is specified in an SSA(STOCK-LOC-SSA in the above E.g.)
• In case if we need data from not just from the lowest level but from other levels as well we normally have to give 3 separate GU calls.This will reduce the efficiency of the program
• Such a call operates on two or more segments rather than just one segment.• If a program has to use "Path call" then "P" should be one of the values specified in the
PROCOPT parameter of the PCB in the programs PSBGEN.• If path call is not explicitly enabled in the PSBGEN job there will be an 'AM' status code.
Multiple Qualifications• There are two cases in which you would use multiple qualification
– When you want to process a segment based on the contents of two or more fields within it
– When you want to process a segment based on a range of possible values for a single field
• To use multiple qualification, you connect two or more qualification statements (a field name, a relational operator, and a comparison value) within the parentheses of the SSA.
• To connect them, you use the Boolean operators AND and OR• Either of the two symbols shown in the table below may be used for AND or OR• The independent AND operator is used for special operations with secondary indexes and will
be discussed later
Multiple Qualifications (contd.)01 VENDOR-SSA.* 05 FILLER PIC X(9) VALUE ‘INVENSEG(’. 05 FILLER PIC X(10) VALUE ‘INVENCOD>=’. 05 VENDOR-SSA-LOW-CODE PIC X(3). 05 FILLER PIC X VALUE ‘&’. 05 FILLER PIC X(10) VALUE ‘INVENCOD<=’. 05 VENDOR-SSA-HIGH-CODE PIC X(3). 05 FILLER PIC X VALUE ‘)’.
• The above SSA, which uses multiple qualifications can be used to retrieve vendor segments whose vendor codes fall within a certain range
– The first qualification statement specifies that the vendor code field must be greater than or equal to a particular value; that is the low end of the range
– The second qualification statement specifies that the vendor code field must be less than or equal to a particular value; that is the high end of the range
– To retrieve segments that fall within this range, you would first move values for low and high ends of the range to VENDOR-SSA-LOW-CODE and VENDOR-SSA-HIGH-CODE
– Then you would execute GN calls that include VENDOR-SSA
Module 5Retrieving Data from a Data Base
The GU CallThe GN Call
The GNP CallStatus Codes Expected during Sequential Processing
Using Command Codes with Retrieval CallsMultiple Processing
The GU Call• Used for random processing• Applications of random processing
– When a relatively small number of updates are posted to a large data base
– To establish position in a data base for subsequent sequential retrieval
• You know what data you want to retrieve and you want to get to it directly• Independent of the position established by the previous calls
CALL ‘CBLTDLI’ USING DLI-GUINVENTORY-PCB-MASKINVENTORY-STOCK-LOC-SEGMENTVENDOR-SSAITEM-SSASTOCK-LOCATION-SSA.
• A typical GU call like the one above, wherein a complete set of qualified SSAs to retrieve a segment, includes one for each level in the hierarchical path to the segment you want to retrieve is called a ‘fully qualified call’
The GU Call (contd.)• Usually, GU processing is based on sequence (key) fields with unique values• However, for some applications you may find it necessary to either
– Access a segment whose sequence field allows non-unique values– Access a segment based on a field that is not the segment’s key field– In the above cases, DL/I returns the first segment occurrence with the
specified search value• Special considerations for GU calls without a full set of qualified SSAs
1.When you use an unqualified SSA in a GU call, DL/I accesses the first segment occurrence in the data base that meets the criteria you specify
2. If you issue a GU call without any SSAs, DL/I returns the first occurrence of the root segment in the data base
3. If you omit some SSAs for intermediate levels in a hierarchical path, the action DL/I takes depends on your current position and on the SSAs that are missing
• DL/I either uses the established position or defaults to an unqualified SSA for the segment• Recommended style of coding
– Code a qualified or unqualified SSA for each level in the path from the root segment to the segment you want to retrieve
The GU Call (contd.)• Status codes you can expect during random processing with GU calls
– Only two status code values need to be considered– spaces and GE
– Spaces means the call was successful and the requested segment was returned in your program’s segment I/O area
– A GE status code indicates that DL/I couldn’t find a segment that met the criteria you specified in the call
The GN CallCALL ‘CBLTDLI’ USING DLI-GN
INVENTORY-PCB-MASKINVENTORY-STOCK-LOC-SEGMENTSTOCK-LOCATION-SSA.
• Used for basic sequential processing• After any successful data base call, your data base position is immediately before the next segment
occurrence in the normal hierarchical sequence• Before your program issues any calls, position is before the root segment of the first data base
record• The GN call moves forward through the data base from the position established by the previous call• If a GN call is unqualified (that is, if it does not employ an SSA), it returns the next segment
occurrence in the data base regardless of type, in hierarchical sequence• If a GN call includes SSAs– qualified or unqualified– DL/I retrieves only segments that meet
requirements of all SSAs you specify• If you include an unqualified SSA or omit an SSA altogether for a segment type, DL/I allows any
occurrence of that segment type to satisfy the call• But when you specify a qualified SSA, DL/I selects only those segment occurrences that meet the
criteria you specify
The GNP CallCALL ‘CBLTDLI’ USING DLI-GNP
INVENTORY-PCB-MASKINVENTORY-STOCK-LOC-SEGMENTUNQUALIFIED-SSA.
• Used for sequential processing within parentage• Works like the GN call, except it retrieves only segments that are subordinate to the currently
established parent• To establish parentage, your program MUST issue either a GU call or a GN call, and the call
must be successful
– Parentage is never automatically established, in spite of the hierarchical structure of the data base
• The segment returned by the call becomes the established parent• Subsequent GNP calls return only segment occurrences that are dependent on that parent• When there are no more segments within the established parentage DL/I returns GE as the
status code
The GNP Call (contd.)Established Parent
Vendor 1
Item 1
Item 2
Loc 5Loc 4
Loc 3Loc 2
Loc 1
Loc 2Loc 1
Established Parent
Vendor 1
Item 1
Item 2
Loc 5Loc 4
Loc 3Loc 2
Loc 1
Loc 2Loc 1
Fig 5.1 Sequential retrieval with GNP call
Status Codes you can expect during
Sequential Processing
Using Command Codes with Retrieval Calls
• The F command code
– When you issue a call with an SSA that includes the F command code, the call processes the first occurrence of the segment named by the SSA, subject to the call’s other qualifications
– Can be used when you are doing sequential processing and you need to back up in the data base, or in other words, the F command code can be used for sequential retrieval using GN and GNP calls
– Meaningless with GU calls, because GU normally retrieves the first segment occurrence that meets the criteria you specify
• The L command code
– When you issue a call with an SSA that includes the L command code, the call processes the last occurrence of the segment named by the SSA, subject to the call’s other qualifications
• The D command code
– Used to retrieve more than one segment occurrence using just one call– Normally DL/I operates on the lowest level segment you specify in an SSA, but in many
cases, you want data not just from the lowest level in the call, but from other levels as well– Makes it easy to retrieve an entire path of segments
Using Command Codes with Retrieval Calls
(contd.)– The usage of the D command code is illustrated below
01 VENDOR-SSA. 05 FILLER PIC X(11) VALUE “INVENSEG*D(”. 05 FILLER PIC X(10) VALUE “INVENCOD =”. 05 VENDOR-SSA-CODE PIC X(3). 05 FILLER PIC X VALUE “)”.* 01 ITEM-SSA. 05 FILLER PIC X(11) VALUE “INITMSEG*D(”. 05 FILLER PIC X(10) VALUE “INITMNUM =”. 05 ITEM-SSA-NUMBER PIC X(5). 05 FILLER PIC X VALUE “)”.* 01 LOCATION-SSA. 05 FILLER PIC X(11) VALUE “INLOCSEG*D(”. 05 FILLER PIC X(10) VALUE “INLOCLOC =”. 05 LOCATION-SSA-CODE PIC X(3). 05 FILLER PIC X VALUE “)”.* 01 PATH-CALL-I-O-AREA. 05 INVENTORY-VENDOR-SEGMENT PIC X(131). 05 INVENTORY-ITEM-SEGMENT PIC X(48). 05 INVENTORY-STOCK-LOC-SEGMENT PIC X(21).*
. . .*
CALL “CBLTDLI” USING DLI-GUINVENTORY-PCB-MASKPATH-CALL-I-O-AREAVENDOR-SSAITEM-SSA LOCATION-SSA.
Using Command Codes with Retrieval Calls
(contd.)• The C command code
– If you are developing a program that retrieves just lower-level segment occurrences from a data base, you don’t have to code separate SSAs for each level in the hierarchical path
– Instead you can use a single SSA with the C command code– Then, rather than coding a field name, relational operator, and search value, you
specify the concatenated key for the segment you are interested in– An illustration of the use of the C command code is shown below
* 01 LOCATION-SSA.*
05 FILLERPIC X(11) VALUE ‘INLOCSEG*C(‘. 05 LOCATION-SSA-VENDOR PIC X(3). 05 LOCATION-SSA-ITEM PIC X(5). 05 LOCATION-SSA-LOCATION PIC X(3). 05 FILLERPIC X VALUE ‘)’.
* . . .
CALL ‘CBLTDLI’ USING DLI-GUINVENTORY-PCB-MASKINVENTORY-STOCK-LOC-SEGMENTLOCATION-SSA.
Using Command Codes with Retrieval Calls
(contd.)• The P command code
– When you issue a GU or GN call, DL/I normally establishes parentage at the lowest level segment that is retrieved
– However, if you want to override that and cause parentage to be established at a higher-level segment in the hierarchical path, you can use the P command code in its SSA
• The U command code
– When you use an unqualified SSA that specifies the U command code in a GN call, DL/I restricts the search for the segment you request to dependents of the segments with the U command code
– Has the same effect as a call which contains a qualified SSA for the current position– Is ignored if used with a qualified SSA
• The V command code
– Effect is same as coding the U command code at that level and all levels above it in the hierarchy
– Is ignored if used with a qualified SSA
Using Command Codes with Retrieval Calls
(contd.)• The Q command code
– This command code is used to enqueue, or reserve for exclusive use, a segment or path of segments
– You only need to use the Q command code in an interactive environment where there is a chance that another program might make a change to a segment between the time you first access it and the time you are finished with it
Multiple Processing• Multiple processing is a general term that means a program can have more than one position in a single physical
data base at the same time• DL/I lets the programmer implement multiple processing in two ways
1.Through multiple PCBs2.Through multiple positioning
• Multiple PCBs
– The DBA can define multiple PCBs for a single data base– Then, the program has two (or more) views of the data base– As with PCBs for different data bases, each has its own mask in the
Linkage Section and is specified in the ENTRY statement– It is up to the program’s logic to decide when to use a particular PCB to
access the data base– This method for implementing multiple processing, though flexible, is
inefficient because of the overhead imposed by the extra PCBs
Multiple Processing (contd.)• Multiple positioning
– Lets a program maintain more than one position within a data base using a single PCB– To do that, DL/I maintains a distinct position for each hierarchical path the program
processes– Most of the time, multiple positioning is used to access segments of two or more
types sequentially at the same time
Fig 5.2 Two data base records to illustrate multiple positioning
A1
B13B12
B11
Data base Record 1
C13C12
C11A2
B22
B21
Data base Record 2
C22
C21
Multiple Processing (contd.)MOVE ‘SEGB ’ TO UNQUAL-SSA-SEGMENT-NAME.CALL ‘CBLTDLI’ USINGDLI-GN
SAMPLE-DB-PCBSEGMENT-B-I-O-AREAUNQUALIFIED-SSA.
MOVE ‘SEGC ’ TO UNQUAL-SSA-SEGMENT-NAME.CALL ‘CBLTDLI’ USING DLI-GN
SAMPLE-DB-PCBSEGMENT-C-I-O-AREA UNQUALIFIED-SSA.
– When you use multiple positioning, DL/I maintains its separate positions based on segment type– As a result you include an unqualified SSA in the call that names the segment type whose
position you want to use– It is the DBA who decides whether single or multiple positioning will be in effect in the
program’s PSB– As a result multiple positioning is not the characteristic of the data base but instead, it’s how
DL/I allows a program to view a data base– The same program can be processed with either single or multiple positioning by different
programs– The technique a program uses is determined by the program’s PSB
Module 6Adding and Updating Data
to a Data BaseThe ISRT Call
The Get Hold CallsThe REPL CallThe DLET Call
Common IMS Status CodesIMS Abends
Sample IMS Program
The ISRT Call• The ISRT call is used to add a segment occurrence to a data base, either during update
processing of an existing data base or during load processing of a new data base• Before an ISRT call is issued, you should first build the segment occurrence by moving data
to the fields of the segment description• After formatting the segment, you issue the ISRT call with at least one SSA: an unqualified
SSA for the segment type you want to add• Consider the example below
CALL ‘CBLTDLI’ USING DLI-ISRTINVENTORY-PCB-MASKINVENTORY-STOCK-LOC-SEGMENTUNQUALIFIED-SSA.
• Here UNQUALIFIED-SSA specifies the segment name• Because the SSA is unqualified, DL/I tries to satisfy the call based on the current position in
the data base• As a result, you need to be careful about position when you issue an ISRT call that specifies
only a single unqualified SSA
The ISRT Call (contd.)• A safer technique is to specify a qualified SSA for each hierarchical level above the one
where you want to insert the segment, as illustrated below
CALL ‘CBLTDLI’ USING DLI-ISRTINVENTORY-PCB-MASKINVENTORY-STOCK-LOC-SEGMENTVENDOR-SSAITEM-SSAUNQUALIFIED-SSA.
• If SSAs for vendor and item are initialized with the proper key values, DL/I inserts the new segment occurrence in the correct position in the data base
• When you issue a fully qualified ISRT call like this, DL/I returns a status code of GE if any segment occurrence you specify in an SSA isn’t present in the data base
• As a result, you can issue an ISRT call with qualified SSAs instead of first issuing GU calls to find out if higher-level segments in the path are present
• By issuing one call instead of two (or more), you can save system resources
The ISRT Call (contd.)• Where inserted segments are stored
– If the new segment has a unique sequence field, as most segment types do, it is added in its proper sequential position
– However, some lower-level segment types in some data bases have non-unique sequence fields or don’t have sequence fields at all
– When that’s the case, where the segment occurrence is added depends on the rules the DBA specifies for the data base
– For a segment without a sequence field, the insert rule determines how the new segment is positioned relative to existing twin segments
• If the rule is “first”, the new segment is added before any existing twins• If the rule is “last”, the new segment is added after all existing twins• If the rule is “here”, it is added at the current position relative to existing twins, which may be first, last, or anywhere in
the middle
– For a segment with non-unique sequence fields, the rules are similar, but they determine where the new segment is positioned relative to existing twin segments that have the same key value
The ISRT Call (contd.)• Status codes you can expect during insert processing
– GE When you use multiple SSAs and DL/I cannot satisfy the call with the specified path
– I I When you try to add a segment occurrence that is already present in the data base
– For load processing you might get status codes LB, LC, LD or LE.
• In most cases they indicate that you are not inserting segments in exact hierarchical sequence
• That means there is an error in your program or the files from which you are loading the data base contain incorrect data
The Get Hold Calls• There are three get hold functions you can specify in a DL/I call:
1.GHU (Get hold unique)2.GHN (Get hold next), and,3.GHNP (Get hold next within parent)
• These calls parallel the three retrieval calls earlier discussed• Before you can replace or delete a segment, you must declare your intent to do
so, by retrieving the segment with one of these three calls• Then you must issue the replace or delete call before you do another DL/I
processing in your program
The REPL Call• After you have retrieved a segment with one of the get hold calls, you can make changes to the data in
that segment, then issue an REPL call to replace the original segment with the new data• There are two restrictions on the changes you can make:
1.You can’t change the length of the segment2.You can’t change the value of the sequence field (if the
segment has one)• Never code a qualified SSA on an REPL call: if you do, the call will fail• An example of a typical replace operation is shown belowCALL ‘CBLTDLI’ USING DLI-GHU
INVENTORY-PCB-MASKINVENTORY-STOCK-LOC-SEGMENTVENDOR-SSAITEM-SSALOCATION-SSA.
ADD TRANS-RECEIPT-QTY TO ISLS-QUANTITY-ON-HAND.SUBTRACT TRANS-RECEIPT-QTY FROM ISLS-QUANTITY-ON-ORDER.CALL ‘CBLTDLI’ USING DLI-REPL
INVENTORY-PCB-MASKINVENTORY-STOCK-LOC-SEGMENT.
The REPL Call (contd.)• Status codes you can expect during replace processing
– If you try to use a qualified SSA on an REPL call, you will get an AJ status code
– If your program issues a replace call without an immediately preceding get hold call, DL/I returns a DJ status code
– If your program makes a change to the segment’s key field before issuing the REPL call, DL/I returns a DA status code
The DLET Call• The DLET call works much like REPL• You must first issue a get hold call to indicate that you intend to make a change to the segment
you are retrieving• Then you issue a DLET call to delete the segment occurrence from the data base• For example, to delete a stock location that is no longer active, you’d code a series of statements
like the ones belowCALL ‘CBLTDLI’ USING DLI-GHU
INVENTORY-PCB-MASKINVENTORY-STOCK-LOC-SEGMENTVENDOR-SSAITEM-SSALOCATION-SSA.
CALL ‘CBLTDLI’ USING DLI-DLETINVENTORY-PCB-MASKINVENTORY-STOCK-LOC-SEGMENT.
• Notice that the DLET call does not include any SSAs• There is one important point you must keep in mind whenever you use the DLET call– when you
delete a segment, you automatically delete all segment occurrences subordinate to it• The status codes you might get after a DLET call are the same as those you can get after an REPL
call
Common IMS Status Codes• Returned by IMS after each DB call
– Field STATUS-CODE X(02) in the PCB-MASK definition– Acceptable and unacceptable status codes– ‘GE’ – record occurrence not found– ‘GB’ – End of DB reached
• Status codes relate to the type of IMS call
• GHN, GHNP, GHU, GU – AB, AK, GE, GB
– AK – Invalid field name in SSA
• ISRT – AB, AC, AD, AJ, AK, II
– AC – Segment not found– AD – Wrong PCB used– II -- Segment occurrence already exists in the DB
• REPL -- AB, AC, AD, DJ
• DLET – AB, AJ, DJ
– AB – I/O area not specified in the call– AJ – Invalid SSA format (invalid command code etc)– DJ – Segment not in ‘HOLD’ status
IMS Abends• U0456 -- PSB stopped• U0456 -- IMS Compile option ‘DLITCBL’ not set to ‘Y’• U0458 -- DB Stopped• U0844 -- DB being updated is full• S013 -- Error opening the DB
A few tips on resolving IMS abends: • Confirm that the Abend is caused by IMS – check the job log for IMS return code• Check the JCL – if modified from another JCL, verify that changes are correct• Check the SYSOUT dump for IMS diagnostic messages• Use MVS/QW to get further information on the abend
Sample IMS ProgramIDENTIFICATION DIVISION. PROGRAM-ID. PATGET2. ENVIRONMENT DIVISION. CONFIGURATION SECTION. SOURCE-COMPUTER. IBM-370. OBJECT-COMPUTER. IBM-370. INPUT-OUTPUT SECTION. FILE-CONTROL. DATA DIVISION. FILE SECTION. WORKING-STORAGE SECTION.
77 TOP-PAGE PIC X VALUE '1'. 77 GET-UNIQUE PIC X(4) VALUE 'GU'.
01 HOSPITAL-SSA. 05 FILLER PIC X(19) VALUE 'HOSPITAL(HOSPNAME ='. 05 HOSPNAME-SSA PIC X(20). 05 FILLER PIC X VALUE ')'. 01 WARD-SSA. 05 FILLER PIC X(19) VALUE 'WARD (WARDNO ='. 05 WARDNO-SSA PIC X(04). 05 FILLER PIC X VALUE ')'. 01 PATIENT-SSA. 05 FILLER PIC X(19) VALUE 'PATIENT (PATNAME ='. 05 PATNAME-SS PIC X(20). 05 FILLER PIC X VALUE ')'. 01 UNQUAL-HOSPITAL-SSA PIC X(9) VALUE 'HOSPITAL '. 01 UNQUAL-WARD-SSA PIC X(9) VALUE 'WARD '. 01 UNQUAL-PATIENT-SSA PIC X(9) VALUE 'PATIENT '.
01 WS-ISRT PIC X(4) VALUE 'ISRT'. 01 WS-GHU PIC X(4) VALUE 'GHU '. 01 HOSP-I-O-AREA. 05 HOSP-NAME PIC X(20). 05 HOSP-ADDRESS PIC X(30). 05 HOSP-PHONE PIC X(10).
01 WARD-I-O-AREA. 03 WARD-NO PIC X(04). 03 TOT-ROOMS PIC 9(03). 03 TOT-BEDS PIC XXX. 03 BEDAVAIL PIC X(3). 03 WARD-TYPE PIC X(20).
Sample Program (contd.)01 PATIENT-I-O-AREA. 03 PATIENT-NAME PIC X(20). 03 PATIENT-ADDRESS PIC X(30). 03 PATIENT-PHONE PIC X(10). 03 BEDINDENT PIC X(4). 03 DATEADMT PIC X(8). 03 PREV-STAY-FLAG PIC X. LINKAGE SECTION. 01 PCB-MASK. 02 DBD-NAME-1 PIC X(8). 02 SEG-LEVEL-1 PIC XX. 02 STATUS-CODE-1 PIC XX. 02 PROCESS-OPTIONS-1 PIC X(4). 02 KEY-LENGTH PIC S9(5) COMP. 02 SEG-NAME-FDBK-1 PIC X(8). 02 LENGTH-FB-KEY-1 PIC S9(5) COMP. 02 NUMB-SENS-SEGS-1 PIC S9(5) COMP. 02 KEY-FB-AREA-1 PIC X(26).
PROCEDURE DIVISION. ENTRY 'DLITCBL' USING PCB-MASK. PERFORM INSERT-HOSP-PARA THRU INSERT-HOSP-EXIT. PERFORM INSERT-WARD-01-PARA THRU INSERT-WARD-01-EXIT. PERFORM INSERT-PATIENTS-PARA THRU INSERT-PATIENTS-EXIT. GOBACK.INSERT-HOSP-PARA. MOVE 'MACNEAL ‘ TO HOSP-NAME. MOVE 'ABC DDDD' TO HOSP-ADDRESS. MOVE '12345' TO HOSP-PHONE.
CALL 'CBLTDLI' USING WS-ISRT PCB-MASK HOSP-I-O-AREA UNQUAL-HOSPITAL-SSA.
Sample Program (contd.)IF STATUS-CODE-1 NOT EQUAL SPACES
EXIT.INSERT-HOSP-EXIT.
EXIT.INSERT-WARD-01-PARA. MOVE '01' TO WARD-NO. MOVE 10 TO TOT-ROOMS. MOVE 20 TO TOT-BEDS. MOVE '03' TO BEDAVAIL MOVE 'INTENSIVE' TO WARD-TYPE. CALL 'CBLTDLI' USING WS-ISRT PCB-MASK WARD-I-O-AREA UNQUAL-HOSPITAL-SSA UNQUAL-WARD-SSA. IF STATUS-CODE-1 NOT EQUAL SPACES EXIT.INSERT-WARD-01-EXIT.EXIT.INSERT-PATIENTS-PARA. MOVE 'MACNEAL' TO WARDNO-SSA. MOVE 'JOHN SMITH' TO PATIENT-NAME. MOVE '123 HAMILTON STR' TO PATIENT-ADDRESS. MOVE '12345 ' TO PATIENT-PHONE. MOVE '1111' TO BEDINDENT. MOVE '02021999' TO DATEADMT. MOVE 'N' TO PREV-STAY-FLAG. CALL 'CBLTDLI' USING WS-ISRT PCB-MASK PATIENT-I-O-AREA
HOSPITAL-SSA WARD-SSA
UNQUAL-PATIENT-SSA.
IF STATUS-CODE-1 NOT EQUAL SPACES EXIT. INSERT-PATIENTS-EXIT.EXIT.
Module 7Secondary Indexing
The Need for Secondary IndexingA Customer Data Base
Secondary IndexesSecondary Keys
Secondary Data StructuresDBDGEN Requirements for Secondary IndexesPSBGEN Requirements for Secondary Indexing
Indexing a Segment based on a Dependent SegmentThe Independent AND Operator
Sparse SequencingDuplicate Data Fields
The Need for Secondary Indexing• Often you need to be able to access a data base in an order other than its primary
hierarchical sequence• Or, you may need to access a segment in a data base directly, without supplying its
complete concatenated key• With secondary indexing both are possible
A Customer Data BaseCustomer
Ship-to
Buyer Receivable
Payment Adjustment Line Item
Fig 7.1 The customer data base
The Customer Data Base (contd.) 01 CUSTOMER-SEGMENT. 05 CS-CUSTOMER-NUMBER PIC X(6). 05 CS-CUSTOMER-NAME PIC X(31). 05 CS-ADDRESS-LINE-1 PIC X(31). 05 CS-ADDRESS-LINE-2 PIC X(31). 05 CS-CITY PIC X(18). 05 CS-STATE PIC XX. 05 CS-ZIP-CODE PIC X(9).* 01 SHIP-TO-SEGMENT. 05 STS-SHIP-TO-SEQUENCE PIC XX. 05 STS-SHIP-TO-NAME PIC X(31). 05 STS-ADDRESS-LINE-1 PIC X(31). 05 STS-ADDRESS-LINE-2 PIC X(31). 05 STS-CITY PIC X(18). 05 STS-STATE PIC XX. 05 STS-ZIP-CODE PIC X(9).* 01 BUYER-SEGMENT. 05 BS-BUYER-NAME PIC X(31). 05 BS-TITLE PIC X(31). 05 BS-TELEPHONE PIC X(10).* 01 RECEIVABLE-SEGMENT. 05 RS-INVOICE-NUMBER PIC X(6). 05 RS-INVOICE-DATE PIC X(6). 05 RS-PO-NUMBER PIC X(25). 05 RS-PRODUCT-TOTAL PIC S9(5)V99COMP-3. 05 RS-CASH-DISCOUNT PIC S9(5)V99COMP-3. 05 RS-SALES-TAX PIC S9(5)V99COMP-3. 05 RS-FREIGHT PIC S9(5)V99COMP-3. 05 RS-BALANCE-DUE PIC S9(5)V99COMP-3.*
Fig 7.2 Segment Layouts for the Customer Data Base (Part 1 of 2)
The Customer Data Base (contd.) 01 PAYMENT-SEGMENT. 05 PS-CHECK-NUMBER PIC X(16). 05 PS-BANK-NUMBER PIC X(25). 05 PS-PAYMENT-DATE PIC X(6). 05 PS-PAYMENT-AMOUNT PIC S9(5)V99 COMP-3.* 01 ADJUSTMENT-SEGMENT. 05 AS-REFERENCE-NUMBER PIC X(16). 05 AS-ADJUSTMENT-DATE PIC X(6). 05 AS-ADJUSTMENT-TYPE PIC X. 05 AS-ADJUSTMENT-AMOUNT PIC S9(5)V99 COMP-3.* 01 LINE-ITEM-SEGMENT. 05 LIS-ITEM-KEY. 10 LIS-ITEM-KEY-VENDOR PIC X(3). 10 LIS-ITEM-KEY-NUMBER PIC X(3). 05 LIS-UNIT-PRICE PIC S9(5)V99 COMP-3. 05 LIS-QUANTITY PIC S9(7) COMP-3.*
Fig 7.2 Segment Layouts for the Customer Data Base (Part 2 of 2)
Secondary IndexesCustomer
Ship-to
Buyer Receivable
Payment Adjustment Line Item
Prefix DataRec. Seg. Addr.
InvoiceNo.
IndexPointerSegment
Invoice number index data base
Secondary IndexData BaseCustomer Data Base
Indexed Data Base
Index TargetSegment
Index SourceSegment
Fig 7.3 Secondary Indexing Example in which the Index Source Segment and the Index Target Segment are the same
Secondary Indexes (contd.)• DL/I maintains the alternate sequence by storing pointers to segments of the indexed data
base in a separate index data base• A secondary index data base has just one segment type, called the index pointer segment• The index pointer segment contains two main elements– a prefix element and a data element• The data element contains the key value from the segment in the indexed data base over which
the index is built, called the index source segment• The prefix part of the index pointer segment contains a pointer to the index target segment–
the segment that is accessible via the secondary index• The index source and target segments need not be the same• After a secondary index has been set up, DL/I maintains it automatically as changes are made to
the indexed data base– though the index is transparent to application programs that use it
– So, even if a program that is not sensitive to a secondary index updates a data base record in a way that would affect the index, DL/I automatically updates the index
– That can also result in performance degradation
Secondary Indexes (contd.)• If multiple access paths are required into the same data base, the DBA can define
as many different secondary indexes as necessary– each stored in a separate index data base
– In practice, the number of secondary indexes for a given data base is kept low because each imposes additional processing overhead on DL/I
Secondary Keys• The field in the index source segment over which the secondary index is built is
called the secondary key• The secondary key need not be the segment’s sequence field– any field can be
used as a secondary key• Though usually, a single field within the index source segment is designated as the
secondary key for a secondary index, the DBA can combine as many as five fields in the source segment to form the complete secondary key
– These fields need not even lie adjacent to each other
• Secondary key values do not have to be unique
Secondary Data Structures• A secondary index changes the apparent hierarchical structure of the data base• The index target segment is presented to your program as if it were a root segment,
even if it isn’t actually the root segment• As a result, the hierarchical sequence of the segments in the path from the index target
segment to the root segment is inverted: those segments appear to be subordinate to the index target segment, even though they are actually superior to it
• The resulting rearrangement of the data base structure is called a secondary data structure
Customer
Receivable
Ship-to Payment Adjustment Line Item
BuyerFig 7.4 Secondary Data Structure for the Secondary Index
Secondary Data Structures (contd.)• Secondary data structures don’t change the way the data base segments are
stored on disk
– They just alter the way DL/I presents those segments to application programs
• When you code an application program that processes a data base via a secondary index, you must consider how the secondary data structure affects your program’s logic
DBDGEN Requirements forSecondary Indexes• Because a secondary index relationship involves two data bases, two DBDGENs are
required– one for the indexed data base and the other for the secondary index data base
Fig 7.5 Partial DBDGEN output for the customer data base showing the code to implement the secondary index
DBDGEN Requirements forSecondary Indexes (contd.)
Fig 7.6 DBDGEN output for the Secondary Index Data Base
• In the DBDGEN for the indexed data base, an LCHILD macro relates an index target segment to its associated secondary index data base
• In the DBDGEN for the secondary index data base, an LCHILD macro relates the index pointer segment to the index target segment
DBDGEN Requirements forSecondary Indexes (contd.)
• ACCESS=INDEX in the DBD macro in Fig 7.6 tells DL/I that an index data base is being defined
• The INDEX parameter of the LCHILD macro in Fig 7.6 specifies the name of the secondary key field– CRRECXNO
• The XDFLD macro in Fig 7.5 supplies a field name (CRRECXNO) that is used to access the data base via the secondary key
– This key field does not become a part of the segment
– Instead, its value is derived from up to five fields defined within the segment with FIELD macros
• The SRCH parameter defines the field(s) that constitute the secondary index
PSBGEN Requirements forSecondary Indexing• Just because a secondary index exists for a data base doesn’t mean DL/I will
automatically use it when one of your programs issues calls for that data base• You need to be sure that the PSBGEN for the program specifies the proper processing
sequence for the data base on the PROCSEQ parameter of the PSB macro• If it doesn’t, processing is done using the normal hierarchical sequence for the data
base• For the PROCSEQ parameter, the DBA codes the DBD name for the secondary index
data base that will be used
Fig 7.7 PSBGEN Output
PSBGEN Requirements forSecondary Indexing (contd.)
• The SENSEG macros in Fig 7.7 reflect the secondary data structure imposed by the secondary index
• When the PROCSEQ parameter is present, processing is done based on the secondary index sequence
• If a program needs to access the same indexed data base using different processing sequences, the program’s PSBGEN will contain more than one PCB macro, each specifying a different value for the PROCSEQ parameter
Indexing a Segmentbased on a Dependent Segment
Fig 7.8 Secondary Indexing Example in which the Index Source Segment and the Index Target Segment are different
Customer
Ship-to
Buyer Receivable
Payment Adjustment Line Item
Prefix DataCust. Seg. Addr.
ItemNo.
IndexPointerSegment
Invoice number index data baseSecondary Index Data Base
Customer Data Base
Indexed Data Base
Index TargetSegment
Index SourceSegment
Indexing a Segmentbased on a Dependent Segment
(contd.)• The Index Source Segment and the Index Target Segment need not be the same• Some applications require that a particular segment be indexed by a value that is derived from a dependent segment
– In such a case, the Index Target Segment and the Index Source Segment are different
– For example, in Fig 7.8, you can retrieve customers based on items they have purchased
– In other words, the SSA for a get call would specify an item number, but the call would retrieve a customer segment
• The only restriction you need to be aware of here is that the Index Source Segment must be a dependent of the Index Target Segment
– Thus, in the example shown in Fig 7.8, it wouldn’t be possible to index the buyer segment based on values in the line item segment, because the line item segment isn’t dependent on the buyer segment
– Similarly , you couldn’t index the line item segment based on the customer segment, because the customer segment is superior to the line item segment
The Independent AND Operator• When used with secondary indexes, AND ( * or & ) is called the dependent AND operator• The independent AND (#) lets you specify qualifications that would be impossible with the dependent
AND• This operator can be used only for secondary indexes where the index source segment is a dependent
of the index target segment• Then, you can code an SSA with the independent AND to specify that an occurrence of the target
segment be processed based on fields in two or more dependent source segments• In contrast, a dependent AND requires that all fields you specify in the SSA be in the same segment
occurrence• An SSA that uses the independent AND operator is shown below
01 ITEM-SELECTION-SSA.* 05 FILLER PIC X(9) VALUE ‘CRCUSSEG(’. 05 FILLER PIC X(10) VALUE ‘CRLINXNO =’. 05 SSA-ITEM-KEY-1 PIC X(8). 05 FILLER PIC X VALUE ‘#’. 05 FILLER PIC X(10) VALUE ‘CRLINXNO =’. 05 SSA-ITEM-KEY-2 PIC X(8). 05 FILLER PIC X VALUE ‘)’.
Sparse Sequencing• When the DBA implements a secondary index data base with sparse sequencing (also called
sparse indexing), it is possible to omit some index source segments from the index• Sparse sequencing can improve performance when some occurrences of the index source
segment must be indexed but others need not be• DL/I uses a suppression value, a suppression routine, or both to determine whether a segment
should be indexed (either when inserting a new segment or processing an existing one)• If the value of the sequence field(s) in the index source segment matches a suppression value
specified by the DBA, no index relationship is established (for an insert) or expected (for any other call)
• The DBA can also specify a suppression routine that DL/I invokes to determine the index status for the segment
• The suppression routine is a user-written program that evaluates the segment and determines whether or not it should be indexed
• Note:
– When sparse indexing is used, its functions are handled by DL/I
– You don’t need to make special provisions for it in your application program
Duplicate Data Fields• For some applications, it might be desirable to store user data from the index source segment in
the index pointer segment• When the DBA specifies that some fields are duplicate data fields, this is possible• Up to five data fields can be stored in the index data base, and DL/I maintains them
automatically• Duplicate data fields are useful only when the index data base is processed as a separate data
base• Note:
– Duplicate data fields impose extra DL/I overhead and require extra DASD storage
– It is the DBA’s responsibility to decide whether the advantages of using duplicate data fields outweigh the extra DL/I overhead and DASD storage requirements mentioned above
Module 8Logical Data Bases
Introduction to Logical Data BasesLogical Data Base Terminology
DBDGENs for Logical Data Bases
An Introduction to Logical Data Bases
• Inter related databases.
Inter related databases A logical child segment has 2 parent segmentsOne physical parent and one logical parent
DB1
DB2
Logical Parent
Physical Parent
Logical relationship
Virtual Logical Child
C2
SEG-1
LP
VLC
Real Logical Child
SEG-a SEG-b RLC
PP
Logical Data Base Terminology• Real Logical Child
– The child under consideration
• Physical Parent
– Original parent of the child
• Logical Parent
– The parent in the other data base
• Virtual Logical Child
– The child as seen from the other data base
• Three types of Logical data bases
– Unidirectional.• The child accesses logical parent’s data but the reverse is not allowed.
– Bi-directional virtual.• Accesses in both the directions, but the child exists only in the physical DB.
– Bi-directional physical.• Accesses in both the directions, but the child exists both in the physical DB as well as the logical DB.
DBDGENs for a Logical Data Base******DBD1******...6 SEGM NAME=RLC,7 PARENT=(PP,PTR), (LP,DBD2), 8 POINTER=(TWIN,LTWIN), RULES=(LLV,LAST),BYTES=169 FIELD NAME=********************************10 FIELD NAME=********************************...
******DBD2*******...6 SEGM NAME=LP, PARENT=SEG-1, BYTES=487 LCHILD NAME= (RLC,DBD1), POINTER=PTR, PAIR=VLC8 FIELD NAME=********************************9 FIELD NAME=********************************10 FIELD NAME=********************************
Module 9Recovery and Restart
Introduction to Data Base RecoveryIntroduction to Checkpointing
Types of CheckpointingExtended Restart
Database Image Copy
Introduction to Data Base Recovery
• The process of recovering the data base in case of application program failure• Back out changes made by the abended program, correct the error and rerun the program.• Types of recoveries
– Forward recovery– Backward recovery
• Forward Recovery
– Data base changes for a time period is accumulated – A copy of the data base is created– The changes are applied to this data base copy– DL/I uses change-data stored in DL/I logs for forward recovery– Used when a data base is physically damaged
• Backward Recovery
– Data base changes due to the failed program is reverted directly in the data base– Program log records are read backwards and their effects are reversed in the data base– When back out is complete data base is in the former state that was before the failure– Normally applied when the program ends in a controlled fashion and no data base damage
Introduction to Checkpointing• Synonyms: synchronization point, sync point, commit point and point of integrity• Program execution point at which the DB changes are complete and accurate• DB changes made before the most recent checkpoint are not reversed by recovery• Normally the start of the pgm is considered as a default checkpoint• In case of a number of DB updates, explicit checkpoints can be specified• Explicit checkpoints can be established using checkpoint call(CHKP) inside the
program• CHKP creates a checkpoint record on DL/I log which prevents recovery before that
point
Types of Checkpointing• Types of checkpointing
– Basic checkpointing– Symbolic checkpointing
• Basic checkpointing
– Simple form of checkpointing.– Issues checkpoint calls that the DL/I recovery utilities use during recovery
processing • Symbolic checkpointing
– More advanced type of checkpointing– Used in combination with extended restart– Programs resume from the point following the checkpoint, in case of a failure– Store program data and CHKP records and retrieve them at the time of restart– Along with symbolic CHKP call you must use the XRST (Extended Restart) call too.
Extended Restart (XRST)• The XRST call is used in connection with the symbolic checkpoint call• It is used to restart your program• The XRST call precedes a symbolic checkpoint call• The XRST call must be issued only once• It should be issued early in the execution of the program• It must precede any CHKP call• The program is restarted from a symbolic CHKP taken during a previous execution of the
program• The CHKP used to perform the restart can be identified by entering the checkpoint ID• CHKP ID can be specified in 2 ways
– In the I/O area pointed to by the XRST call – Specifying ID in the CKPTID= field of EXEC statement
in the program's JCL
Database Image Copy• Job which is run to take backup copies of IMS database datasets at periodic intervals
– Traditionally, batch cycle starts at 7 pm and ends at 7 am– Image Copy jobs are usually run before and after a batch cycle– If abend occurs, revert to the DB generated by image copy job
and rerun– Commonly used image copy utility is BMC Software’s
ICPUMAIN– Database and Image copy DD names specified in the ICPSYSIN
card– Advantage : Simple, Fast, Automated procedure
Module 10 DL/I Data Base Organizations
DL/I Organizations & Access Methods Hierarchical Sequential Organization
Hierarchical Direct OrganizationAdditional IMS Access Methods
DL/I Organizations & Access Methods
• File Organization is a description of how a file is processed & Access Method is the software used to implement that processing.
• DL/I provides two basic data base organizations :
– Hierarchic Sequential: In this the segments that make up the database record are related to one another by their physical locations.
– Hierarchic Direct : In this the segment occurrences include prefixes that contain direct pointers to related segments.
Hierarchic Sequential Organizations Access Methods
• HS Organizations provide four types of Access Methods
– HSAM ( Hierarchic Sequential Access Method) : The program in HSAM database works through it sequentially from beginning to end.The application programs cannot replace or delete segments without copying the entire database.
– HISAM (Hierarchic Indexed Sequential Access Method): In HISAM the data is stored with hierarchic sequential organization. An index is also maintained to allow random access to any database record.
– SHSAM( Simple Hierarchical Sequential Access Method):Similar to HSAM but used to support databases that consist only of root segments.
– SHISAM(Simple Hierarchic Indexed Sequential Access Method ) : Similar to HISAM and used in cases in which the database consist of only root segments.
– SHSAM & SHISAM are used primarily for converting standard files to DL/I data bases.
Hierarchic Direct Organization Access Method
• HDAM ( Hierarchic Direct Access Method ):
– HDAM stores root segment occurrences based on a randomizing routine.
– Occurrences of dependent segments are related to root and one another by a system of pointers the HD Organization is based upon.
– HDAM databases are not appropriate for sequential processing.
• HIDAM (Hierarchic Indexed Direct Access Method) :
– Segment data in HIDAM is stored in the same way like that in HDAM.– In HIDAM, unlike HDAM root segment is located through an index.– Root segments can be retrieved in sequence.
Additional IMS Access Methods• GSAM( Generalized Sequential Access Methods):
– GSAM lets application files to be treat OS sequential files as databases. – Data is processed on a record to record to basis but through DL/I calls.– Processing of database is sequential , ISRT add data only at the end of database & REPL
and DLET calls are not supported. – They are typically used during conversion from a system that uses standard files to one
that uses data bases. – Since files are considered by IMS to be databases, IMS recovery facilities can be used.
• Fast Path data bases:
– Fast Path data bases provide fast processing of simple data structures. – Two types of Fast Path databases : MSDB(Main Storage Data Base ) & DEDB( Data Entry
Data Base ).• MSDB(Main Storage Data Base) :
– It is used to store an application’s most intensively used data and resides in virtual storage.– It provides fast access to data and it contains only a small amount of data.– These are root-segment-only data bases.
Additional IMS Access Methods (contd..)
• DEDB( Data Entry Data Base ) :
– DEBD is stored in disk and has a hierarchical structure
– They are organized in typical DL/I fashion, as direct dependent segment types.
– DEBD’s use a complicated storage scheme that involves separating the data base into as many as 240 areas and this allows very large data bases.
Module 11Advanced DL/I features
Variable Length SegmentsDBD for GSAMsPCB for GSAMs
Variable Length Segments• When a field length that is stored in a segment type varies, for example Description or Explanatory text, then we
define those fields as variable length fields• The segment with such a field defined in it is called Variable Length Segment • For description and explanatory fields, if we define them long enough to accommodate the longest possible text, then
a lot of space is wasted in cases where it contains shorter strings.• The SEGM macro in DBD is defined as
SEGM NAME=INVENSEG,PARENT=0,POINTER=TR,BYTES=m,n
• m=maximum length of the segment + 2 bytes• n=minimum length of the segment + 2 bytes
• The extra two bytes is used to store the length field of the occurrence of the variable length segment• In Application Program :
– The length field has to be included in the I-O Area for the segment. Length PIC S 9(4)
– The I-O area should be large enough to accommodate the Maximum variable length segment + Length field
– Before an ISRT / REPLACE / DELETE call is issued we have to move the actual length to the length field in the I/O area
Variable Length Segments (contd.)• Variable Length Segments are appropriate when segment occurrence length vary but once
created and made stabilized.
• Disadvantage:
– If the occurrence of the segment type grows in length then Variable length segment will drop performance
– When segment type occurrences grow in size then it split's into 2 parts which are not stored in the same physical record, so we require two I/O operations to fetch the segment therefore the performance drops
DBD for GSAMs• During DBD generation for a GSAM database we should specify one dataset group• The DD name of the input dataset that is used when the application retrieves data from the
database• The DD name of the output dataset used when loading the database.
• The DBD for a GSAM is shown belowDBD NAME=CARDS,ACCESS=(GSAM,BSAM)DATASET D1=ICARDS,DD2=OCARDS,RECFM=F,RECORD=80DBDGENFINISHEND
• In GSAM DBD's you can't specify
– SEGM and FIELD statements– The use of logical or index relationships between segments
• IMS adds 2 bytes to the record length value specified in the DBD in order to accommodate the ZZ field.
DBD for GSAMs (contd.)• Whenever the database is GSAM/BSAM and the records are variable (V or VB),
IMS adds 2 bytes. • The record size of the GSAM database is 2 bytes greater than the longest segment
that is passed to IMS by the application program.• A database if defined as GSAM has the advantage of the usage of CHECKPOINT
and RESTART • Disadvantage of GSAM database : Only inserts can be done to the DB which is
defined as GSAM, no delete operation can be performed on GSAM Database.
PCB for GSAMs• The PCB for a GSAM database is coded as shown below
PCB TYPE=GSAM,DBDNAME=REPORT,PROCOPT=LS• The GSAM PCB statement must follow the PCB statements with TYPE=TP or DB if any exist in
the PSB generation, the rule is:
– TP PCBs First– DB PCBs Second – GSAM PCBs Last
• A sample PSB is shown belowPCB TYPE=TP,NAME=OUTPUT1PCB TYPE=DB,DBDNAME=PARTMSTR,PROCOPT=A,KEYLEN=100SENSEG NAME=PARTMAST,PARENT=0,PROCOPT=ASENSEG NAME=CPWS,PARENT=PARTMAST,PROCOPT=A PCB TYPE=GSAM,DBDNAME=REPORT,PROCOPT=LS PSBGEN LANG=COBOL,PSBNAME=APPLPGM3END
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