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

Thank YouThank You