ims training

04/09/23 IMS DB Concepts 1

IMS DB CONCEPTS

IMS


Course Plan Introduction to IMS Getting Started Application Programming IMS Calls - Retrieve Processing IMS Calls - Loading and Inserting IMS Calls - Update Processing IMS Environments & Regions Recovery & Restart GSAM Processing


Introduction to IMS

Session 1


Tree Structure


IMS Terminology

Segments Fields Path Parent Child Level Twins Twin Chain

Record Database Key Sequence field Search Field


Segment Types & Segment Occurrences


IMS Hierarchic Sequence

Top to Bottom Left to Right Front to Back


Data Manipulation

Locate a specific Segment Move to next Segment occurrence Move to next Segment type Browse a the whole database or a specific

tree in the Hierarchic Sequence Update a segment Delete a segment Insert a new segment


Components and Environments

IMS is a Database/Data Communication (DB/DC) System IMS has two components:

• IMS DB (Database Manager)

• IMS DC (Transaction Manager)

DB and DC components can be implemented independently

IMS DB is required for both Online and Batch. IMS DB is referred sometimes as DL/I or Data Language/I DL/I can interface with CICS/VS and IMS DC. IMS DC can interface with DB2


IMS Software Development

IMS DC

IMSDatabase

OperatingSystem

DL/I

ApplicationProgram

Terminal

Reports

IMS ControlBlocks


IMS Advantages

Performance

Simplicity Maintenance Control Security

Transaction Manager


Getting Started

Session 2


Database Definition (DBD)

Database Definition Block (DBD) Control Block that describes the physical structure of a database.

Defines the Database to IMS

One DBD is used to define one Database

DBD specifies the segment types for a database and the fields in each segment.

Created by a series of control statements.

The DBDGEN process compiles the DBD and the database is defined to IMS. The whole process is normally done by the DBA

The DBD definition is used by Dl/I whenever the database is used.



Sample DBD1 DBD NAME=EDUCPDBD

2 SEGM NAME=COURSE, BYTES=36

3 FIELD NAME=(COURSE#,SEQ), BYTES=3, START=1

4 FIELD NAME=TITLE, BYTES=33, START=4

5 SEGM NAME=PREREQ, PARENT=COURSE, BYTES=3

6 FIELD NAME=(PREREQ#,SEQ), BYTES=3, START=1

7 SEGM NAME=OFFERING, PARENT=COURSE, BYTES=21

8 FIELD NAME=(OFF#,SEQ), BYTES=3, START=1

9 FIELD NAME=DATE, BYTES=6, START=4

One for each Segment Type

One for each Segment Type



Sample DBD (Contd..)10 FIELD NAME=LOCATION, BYTES=12, START=10

11 SEGM NAME=TEACHER, PARENT=OFFERING, BYTES=24

12 FIELD NAME=(EMP#,SEQ), BYTES=6, START=1

13 FIELD NAME=NAME, BYTES=18, START=7

14 SEGM NAME=STUDENT, PARENT=OFFERING, BYTES=24

15 FIELD NAME=(EMP#,SEQ), BYTES=6, START=1

16 FIELD NAME=NAME, BYTES=18, START=7

17 FIELD NAME=GRADE, BYTES=1, START=25


Program Specification Block (PSB)

Defines a program’s view of the database Specifies the group of segments and fields that one or

more applications can access. Also specifies the level of access for such segments/fields. A PSB consists of one or more control blocks called

Program Communications Block (PCB) Each PCB in a PSB defines one logical data structure. All PCB’s in a PSB collectively is known as an

Application Data Structure.



Sample PSB1 PCB DBDNAME = EDUCPDBD

2 SENSEG NAME = COURSE, PROCOPT = G

3 SENSEG NAME = OFFERING, PARENT = COURSE, PROCOPT = G

4 SENFLD NAME = OFF#, START = 1

5 SENFLD NAME = DATE, START = 4

6 SENSEG NAME = STUDENT, PARENT = OFFERING, PROCOPT = G



Sensitive Segments– Segments in the database that a particular application is

sensitive to

– Key sensitive segments are those segments an application can use to gain access to gain access to segments below it in the hierarchy. Cannot access data.

– Data sensitive segments are those an application can use to retrieve data

– Sensitive segments are specified using SENSEG macro statement



Sensitive Fields– Fields in a (Sensitive)segment that a particular

application is sensitive to

– Sensitive fields are specified by using the SENFLD macro statement.


Program Specification Block (PSB)The PROCOPT (Processing Options) entry Used for restricting the level of Segment access Specified as “PROCOPT=<Processing Options>”



G – Get

I – Insert

R - Replace

D - Delete

P - Position

A - All the above previleges

L - Load

GO - Read Only access

K - Can only access the key

Valid PROCOPT entries



Data independence: A new segment type can be added to the database without affecting existing users.

Data security: Users cannot access those segment types that they are not sensitive to.


Application Programming

Session 3


Features

Independent of Data Storage Method Common call structures Independent of Line Protocols Enhancements Maintain Data Integrity Programmer Aids Data Sharing Different Access Methods


IMS Application Program Components

I/O AREA

PCB MASK

SEGMENTS TO/FROM DATABASE

RETURN INFO FROM DL/I

3

4

PROGRAM ENTRY

CALLS TO DL./I

GETINSERTREPLACEDELETE

PROCESSING

TERMINATON

1

2

5

DL/I MODULES

PROGRAMLOGIC

DL/I

LINKAGESECTION

WORKINGSTORAGE


Program Structure

Program EntryPCB maskI/O areaDL/I callsTermination


Program Entry & Termination

Entry DLITCBL to be declared as entry point Example:

ENTRY ‘DLITCBL’ USING <PCB Name>

PCB resides outside the program (Linkage Section). IMS passes the address.

GOBACK to be used instead of STOP RUN.


Process

Program entry specifies PCB’s used by the program

Call statements are interpreted by Dl/I, the function performed and segments transferred between programs’ I/O area and database

Program terminates by returning control to DL/I


Format of an IMS CALL

COBOL:

CALL 'CBLTDLI' USING [PARMCOUNT,]

FUNCTION,

DBPCB,

I/O AREA,

[,SSA1,SSA2,SSA3...,SSAn].


Format of an IMS CALL

PL/1:

CALL PLITDLI (PARMCOUNT, FUNCTION, DBPCB, I/O AREA [,SSA1,SSA2,SSA3...,SSAn])

FUNCTION, DBPCB,I/O area are mandatory For the PLITDLI interface, all parameters except

PARMCOUNT are indirect pointers


CALL Functions Get Functions

GU - Retrieve a Segment occurrence randomly

GN - Retrieve next segment occurrence from the current position

GNP - Retrieve next segment occurrence under the current parent from the current position

Get Hold FunctionsGHU

GHN - Identical to GU, GN and GNP except that we use these calls for update intent.

GHNP


CALL Functions Update Functions

ISRT - Insert a new segment

DLET - Delete an existing segment along with its dependents

REPL - Replace an existing segment

Other FunctionsROLL - Roll Back and Exit

ROLB - Roll Back and Return

CHKP - Check Point

XRST - Check for Restart


PCB Mask

Defined in the LINKAGE SECTION Entry Statement to access PCB thru USING

clause Order of PCBs in the ENTRY statement

need not be the same as that in the LINKAGE SECTION


PCB Mask

Database Name Segment Level Status Code Processing Options Segment Name Feedback Area Key Length Feedback Area Number of Sensitive Segments Key Feedback Area


PCB Mask Example (COBOL)

01 PCBNAME.

02 DBD-NAME PIC X(8).

02 SEG-LEVEL PIC XX.

02 STATUS CODE PIC XX.

02 PROC-OPTIONS PIC XXXX.

02 RESERVE-DLI PIC S9(5) COMPUTATIONAL.

02 SEG-NAME-FB PIC X(8).

02 LENGTH-FB-KEY PIC S9(5) COMPUTATIONAL.

02 NUMB-SENS-SEGS PIC S 9(5) COMPUTATIONAL.

02 KEY-FB-AREA PIC X(n).


PCB Mask Example (PL/1)

DECLARE PCB_POINTER POINTER;

DECLARE 1 PCBNAME BASED (PCB_POINTER),

2 DBD_NAME CHAR(8),

2 SEG_LEVEL CHAR(2),

2 STATUS_CODE CHAR(2),

2 PROC_OPTIONS CHAR(4),

2 RESERVE_DLI FIXED BIN(31,0),

2 SEG_NAME_FB CHAR(8),

2 LENGTH_FB_KEY FIXED BIN(31,0),

2 NUMB_SENS_SEGS FIXED BIN(31,0),

2 KEY_FB_AREA CHAR(n);


Segment Search Arguments

Unqualified SSA Area of storage that contains data which identifies 9 bytes

in length first 8 bytes to contain segment name

8 chars 1

SEGMENT NAME b



Qualified SSA 8 character segment name 8 character field name 2 byte relational or logical operator variable field into which the search value is to be moved command code

8 chars 1 1 1 1 1 8 2 1 – 255 1

SEGMENTNAME

( FIELDNAME

OPR COMPARATIVEVALUE

)* CMD1

CMD2

CMD3



Command codes C Concatenated Key D Path Call F First Occurrence L Last Occurrence N Path Call Ignore P Set Parentage U Maintain Position at this level V Maintain Position at this & all superior levels - Null Command


SSA Definition (COBOL)

01 UNQUAL-SSA.

02 SEG-NAME PIC X(08) VALUE ’COURSEbb'.

02 FILLER PIC X(01) VALUE ' '.

01 QUAL-SSA-MAST.

02 SEG-NAME-M PIC X(08) VALUE 'ROOT '.

02 AST PIC X(01) VALUE '*'.

02 CMD1 PIC X(01) VALUE '-'.



02 BEGIN-PAREN-M PIC X(01) VALUE '('.

02 KEY-NAME-M PIC X(08) VALUE 'KEY '.

02 REL-OPER-M PIC X(02) VALUE ' ='.

02 KEY-VALUE-M PIC X(n) VALUE 'vv...v'.

02 END-PAREN-M PIC X(01) VALUE ')'.


IMS CALLS

Add Operation ISRTthe insert function

Retrieve Operation GU Get Unique function GN Get next function GNP Get next within parent function


Data Manipulation Functions

Update Operation GHUGet hold unique function GHNGet hold next function GHNP Get hold next within parent

function REPL Replace function DLET Delete function


IMS Calls: Retrieve Processing

Session 4


Retrieve Processing

Get Unique, or GUWe can use GU to retrieve a particular segment directly

Get Next, or GNWe can use GN to sequentially retrieve segments.

Get Next in Parent, or GNPWe can use GNP to retrieve dependents of a particular segment.


Random retrieval Get Unique (GU) Call

Usage– when there is a specific segment you want

– to establish your position in the database.

About– A GU with an unqualified SSA at the root level

attempts to satisfy the call by starting at the beginning of the database



– A GU with a qualified SSA can retrieve the segment described in the SSA regardless of that segment's location relative to current position.

– When a segment has non unique keys or no keys at all, a GU with qualified SSAs will return only the first occurrence of that segment

– When you issue a GU call that mixes qualified and unqualified SSAs, DL/I retrieves the first occurrence of the segment type that satisfies the call at each level



– When there are missing levels, DL/I assumes an unqualified SSA for those levels

Status Codes– Spaces: The call was successful

– GE: Segment not found (Invalid key)



Example 1: GU call with No SSAs

CALL ‘CBLTDLI’ USING GU PCB-MASK IO-AREA

This call retrieves the first ROOT segment in the database.



Example 2: Fully Qualified SSAs in all levels

CALL ‘CBLTDLI’ USING GU PCB-MASK

IO-AREA COURSE-SSA

OFFERING-SSA

where COURSE-SSA - COURSE (COURSE# =M23)bb bb



Example 2 (Contd..)

OFFERING-SSA - OFFERING(OFF# =M19)

This call retrieves the OFFERING segment M19 for the course M23.

bbbbb



Example 3: Unqualified SSAs in all levels

CALL ‘CBLTDLI’ USING GU PCB-MASK IO-AREA COURSE-SSA OFFERING-SSA

where COURSE-SSA - COURSEbb



Example 3: Unqualified SSAs in all levels

and OFFERING-SSA - OFFERING

This retrieves the first offering segment (OFF# = 1) under the Course M23.

b



Example 4: GU call with both Unqualified and Qualified SSAs

CALL ‘CBLTDLI’ USING GU PCB-MASK IO-AREA COURSE-SSA OFFERING-SSA STUDENT-SSA



Example 4 (Contd..)

where COURSE-SSA - COURSE (COURSE# =M23) OFFERING-SSA - OFFERING

STUDENT-SSA - STUDENT (EMP# =36162)

This retrieves the STUDENT segment with EMP# 36162, under first offering segment (OFF# = 1) under the Course M23.

b bbbbb

bb bbb



Example 5: GU call with Missing Levels

CALL ‘CBLTDLI’ USING GU PCB-MASK IO-AREA STUDENT-SSA

where

STUDENT-SSA - STUDENT (EMP# =36162)bbbbbb



Example 5 (Contd..)

This assumes the COURSE-SSA and the OFFERING-SSA as unqualified and retrieves the STUDENT segment with EMP# 36162, under first offering segment (OFF# = 1) under the first Course segment (M23).


Sequential retrieval Get Next (GN) Call

Usage– when you want the next segment in the hierarchy

– when you want to sequentially browse the database in the hierarchic order

– often issued after a GU call

About– DL/I uses the current position set by the previous call

as the search starting point



– If no position is established in the hierarchy, GN retrieves the first segment in the database.

– The segment retrieved is determined by a combination of the current sequential position in the hierarchy and the SSAs included in the call.

– A GN call with an unqualified SSA retrieves the next occurrence of that segment type by going forward from the current position.



– A GN call with qualified SSAs retrieves the next occurrence of the specified segment type that satisfies the SSAs.

– When a segment has non unique keys or no keys at all, a GN with qualified SSAs will return the next occurrence of that segment

– When you issue a GN call that mixes qualified and unqualified SSAs, DL/I retrieves the next occurrence of the segment type that satisfies the call at each level



– When there are missing levels, DL/I assumes an unqualified SSA for those levels

– The above GN calls, when repeated, retrieve the next segment satisfying the call till no more segment found (‘GE’ status code) is reached

– Successive GN calls with ‘NO’ SSAs will result in the navigation in the database in the hierarchic sequence till the End of database is reached. The next GN call will retrieve the first Root segment in the database.



Status Codes– Blank - The call was successful

– GE - Segment not found

– GB - End of Database reached

– GA - DL/I moved to a higher level

– GK - DL/I moved to the next segment type



Example 1: GN call with No SSAs

CALL ‘CBLTDLI’ USING GN PCB-MASK IO-AREA

Repeated use of this call will get all the segments of the database in the hierarchical sequence

After the last segment is retrieved, IMS returns ‘GB’ return code



Example 1 (Contd..)

Issuing one more GN call will get the first occurrence of the ROOT segment

The SEGMENT field of the PCB mask will tell us the segment type last retrieved



Example 2: GN with Unqualified SSAs

CALL ‘CBLTDLI’ USING GU PCB-MASK IO-AREA COURSE-SSA

OFFERING-SSAwhere

COURSE-SSA - COURSEOFFERING-SSA - OFFERING

bbb



Example 2 (Contd..) Repeated use of this call retrieves all the occurrences

of the OFFERING segment, no matter what at they are dependent on.

After the last segment is retrieved, IMS returns ‘GE’ return code



Example 3: Fully Qualified SSAs

CALL ‘CBLTDLI’ USING GN PCB-MASK IO-AREA COURSE-SSA OFFERING-SSA

where COURSE-SSA - COURSE (COURSE# =M23) OFFERING-SSA - OFFERING(OFF# =1)

bb bbbbbbb



Example 3 (Contd..)

Repeated use of this call retrieves all the OFFERING segments with key 1, under all the COURSE segments with key M23. After the last segment is retrieved, IMS returns ‘GE’ return code



Example 4: Both Unqualified and Qualified SSAs

CALL ‘CBLTDLI’ USING GU PCB-MASK IO-AREA COURSE-SSA OFFERING-SSA STUDENT-SSA



Example 4 (Contd..)

where COURSE-SSA - COURSE (COURSE# =M23) OFFERING-SSA - OFFERING STUDENT-SSA: STUDENT (EMP# =36162)

Repeated use of this call retrieves the STUDENT segments with EMP# 36162, under the all the offering segments present under all the COURSE segments with key value M23.

bb bbb

b bbbbb



Example 5: GN call with Missing Levels

CALL ‘CBLTDLI’ USING GN PCB-MASK IO-AREA STUDENT-SSA

where STUDENT-SSA - STUDENT (EMP# =36162)bbbbbb



Example 5: GN call with Missing Levels

Repeated use of this call retrieves all the STUDENT segments with EMP# 36162, no matter what they are dependent on.

After the last occurrence, ‘GE’ return code is returned


Sequential retrieval Get next within Parent (GNP)

Usage– when you want the next segment in the hierarchy

under an established parent

– when you want to sequentially browse the database in the hierarchic order, under given parent

– often issued after a GU or GN call



About– DL/I uses the current position and parentage set by

the previous GU/GN call as the starting point.

– You can set parentage in two ways

• By issuing a successful GU or GN call

• By using the P command code with a GU, GN, or GNP call



– GNP behaves exactly like the GN call but the in a GNP call, the search for the next segment occurs only under the established parent (Refer to the explanation for the GN call)

– If a GNP call is issued without establishing prior parentage, IMS returns a ‘GP’ status code.



Status Codes– Blank - The call was successful

– GE - Segment not found

– GA - DL/I moved to a higher level

– GK - DL/I moved to the next segment type

– GP - Parentage not established


IMS Calls: Loading and Inserting

Session 5


Insert Call

Usage– Add new segments to an existing database

– Load records to a new database

– IMS knows whether the program is going to load a database from scratch or insert segments on to an existing database from the processing options in the PCB.


Insert Call Valid PROCOPT Values

– L - Load Mode

– LS - Load Mode ascending sequence (also allows insert calls)

– A - Get, Insert, Delete, Replace (not valid if loading from scratch)

– AS - Same as A, but in ascending sequence (not valid if loading from scratch)

– I - Insert mode

– IS - Insert mode ascending sequence


Loading a database A database is loaded from scratch usually using a load

program. The input records must be presented in hirearchical

sequence. The Load Program consists of a single ISRT call, which is

used to insert all segment types. Segment images are read from the input dataset and the

program determines the type segment that is to loaded The Unqualified SSA is formatted with the proper segment

name and the IOAREA populated. It is not correct to qualify the SSA for the segment to be

inserted.


Loading a database

Insert Call for LOAD

CALL ‘CBLTDLI’ USING ISRT COURSE-PCB-MASK IO-AREA LOAD-SSA

where LOAD-SSA is 01 LOAD-SSA. 03 SEGNAME PIC X(08). 03 FILLERPIC X(01) VALUE SPACES.


Loading a Database

Status CodesLB - Segment already exists

LC - Key Values out of sequence

LD - No parent for segment being loaded

LE - Segment types out of sequence


Insert Call

Example 1: Insert with unqualified SSA

CALL ‘CBLTDLI’ USING ISRT COURSE-PCB-MASK STUDENT-IO-AREA COURSE-SSA

STUDENT-SSA

where COURSE-SSA - COURSE (COURSE# =M23)

OFFERING-SSA - OFFERING bb bb

b


Insert Call

Example 2: (with qualified SSA)CALL ‘CBLTDLI’ USING DLI-ISRT

COURSE-PCB-MASK STUDENT-IO-AREA COURSE-SSA OFFERING-SSA

where COURSE-SSA - COURSE (COURSE# =M23)

OFFERING-SSA - OFFERING(OFF# =4)bb bb

bbbbb


Insert Call

Status Codes– GE : In a multiple SSA, at least one of the

segments is not present

– II: Duplicate Segment

– IX: Insert Rule Violation


Insert Call

Command Codes:– F & L

• Meaningful only for non-unique keys & segments without keys

– D

• For inserting more than one segment along the path


IMS Calls: Update Processing

Session 6


Get Hold Calls Usage

– The Get-Hold calls cause IMS to save information about the segment location, so that the segments can be later deleted (with a DLET) call or replaced (REPL) call.


Get Hold Calls Get Hold Functions

– "Get Hold Unique," or GHU, is the hold form for a GU call

– "Get Hold Next," or GHN, is the hold form for a GN call

– "Get Hold Next in Parent," or GHNP, is the hold form for a GNP call.

– No other call is allowed that reference the same PCB between the time the program issues a GH call and the REPL or DLET call.


Replace Call - REPL

Usage– Used to replace the original contents of a segment– Should always be preceded by a GHU, GHN or a GHNP call.– If the preceding Get Hold is a path call

• replace operates on the entire path• use N command with unqualified SSA if you don't want to

replace some of the segments in the path


Replace Call - REPL

Example

CALL ‘CBLTDLI’ USING GHU COURSE-PCB-MASK OFFERING-IO-AREA COURSE-SSA OFFERING-SSA


bbb

bb


Replace Call - REPL

Example (Contd..)

After retrieving the OFFERING Segment, with the GH call, its image in the I/O area is manipulated to effect the desired changes (except key field) and the REPL call is issued.

CALL ‘CBLTDLI’ USING REPL COURSE-PCB-MASK

OFFERING-IO-AREA


Replace Call - REPL

Status Codes– Blank - No error– AJ - Qualified SSA is used– DJ - No preceding Get Hold call– DA - Segment’s Key field is altered


Replace Call - REPL

Restrictions– Length of the segment can not be changed

– Key fields can not be changed

– Qualified SSA not allowed


Delete Call - DLET

Usage – Used to delete a segment

– DLET call must be preceded by one of the three Get Hold calls


Delete Call - DLET

Example

CALL ‘CBLTDLI’ USING GHU COURSE-PCB-MASK OFFERING-IO-AREA

COURSE-SSA OFFERING-SSA


Delete Call - DLET

Example (Contd..)


CALL ‘CBLTDLI’ USING DLET COURSE-PCB-MASK OFDERING-IO-AREA

bb bb


Delete Call - DLET

Restrictions– DLET call must be preceded by one of the three

Get Hold calls

– All of its physical dependents are also deleted from the database regardless of whether or not your program is sensitive to all of these segments

– Only one SSA is allowed in the DLET call

– No command codes apply to the DLET call


Delete Call - DLET

Status Codes– DJ - No previous Get Hold Call

– DA - Key field modified

– DX - Delete Rule Violation


IMS Environments and Regions

Session 7


IMS Application Processing Environments DB/DC Environment

– Message Processing Programs: MPPs– Batch Message Processing Programs: BMPs

DBCTL Environment Batch Environment


IMS Regions IMS control region

– Handles all the communication between remote terminals and IMS.

– Also controls the use of the various MP regions in the system

MP regions – Region where MP programs are executed

BMP regions. – Region where BMP programs are executed


Types of Programs

Batch programs Online message processing programs

(MPPs) Batch message processing programs

(BMPs)


Batch Programs

Also called DL/I Programs Typically produce a large amount of output Are not executed by another program or user Produce output that is not needed right away The databases and the online regions must be

down for Batch DL/I processing.


Batch Programs Users cannot use screens and other MP programs

that access these databases that are taken offline. DL/I programs are typically run from late

evening till midnight or till early morning to carry out daily (routine) processing, mass updates and/or to create daily reports.

Database datasets have to be defined in the batch JCL.


Online Message Processing Programs (MPP) Used for Screen Processing Steps to process a transaction

– Retrieve a message from IMS.

– Process the message and access the database as necessary.

– Respond to the message.

– Repeat the process until there are no more messages; terminate


Online Message Processing Programs (MPP) For each MPP, the following is to be

defined– The transaction's priority

– The number of messages for a particular transaction code that the MPP can process in a single scheduling

– The amount of time (in seconds) in which the MPP is allowed to process a single transactio

MPP’s cannot access MVS datasets.


Batch Message Processing Programs (BMP) Used when it is necessary to receive

substantial information from remote terminals and each message requires substantial processing and a response back to the terminal is generally not required.

BMPs can however send messages to other an MPP or another (transaction-oriented) BMP.


Batch Message Processing Programs (BMP) Also used for batch programs online,

without retrieving or sending messages from and to the terminals.

The databases need not be shut down and the online region can be up, so that users can use MP programs (screens).

Like the MPPs, The BMPs have to be defined to the IMS system.


Batch Message Processing Programs (BMP) BMPs can access MVS Datasets. There are two kinds of BMPs:

A transaction-oriented BMP accesses message queues for its input and output. It can also process input from MVS files, and it can create MVS files.

A batch-oriented BMP does not access the message queue for input; it is simply a batch program that runs online. It can send its output to an MVS output device.


Batch Message Processing Programs (BMP) Like Batch programs, BMP’s do have JCLs

defined to specify the MVS datasets to be used, PSB, Recovery options IMS log datasets etc.

The BMP’s can be scheduled to run every hour or every couple of hours, depending on the need.


IMS Recovery & Restart

Session 8


Recovery and Restart– Needed when a DL/I or BMP program abends.

– Helps in maintaining the integrity of the databases

– Once database is recovered to the last stable state, program(s) can be conveniently restarted.

– When a job which normally runs for 5 hrs crashes after 4 hrs, recovery and restart procedures help in restarting the job from the point it abended, so that it is not run from scratch.


Recovery and Restart DL/I abends

– A DL/I program does not abend the way a non-DL/I program abends, because DL/I programs are executed under the control if DL/I rather than the operating system

– DL/I intervenes to contain the damage to the databases by invoking an abnormal termination routine.

– DL/I however, does not correct the damage.


Recovery and Restart Abnormal Termination Routine

– Make sure that the database data sets are properly closed.

– Catalog entries are properly updated.

– The DL/I log used is closed

– Cancels the job

– If requested, puts out a storage dump, which can be manually analyzed to find what caused abend.


Recovery and Restart Logging

– When logging is used, DL/I records all the changes a program makes to its databases in a special file called IMS LOG file.

– The log is updated prior to the database update (Write-Ahead Logging.

– Can reside in Tape or Disk

– Records the before and after image of a segment, when changes are made to it.


Forward Recovery– Change data in the DL/I logs accumulated for a

period of time is applied to a image copy of the database as it existed before the changes began.

– Used only when a database is physically damaged in some way and may be a device error makes it inaccessible

– An old image copy of database (in stable state) and a log dataset containing the accumulated changes must be available.


Forward Recovery– The Database Image Copy Utility is used for

making image copies of the database

– The Database Change Accumulation Utility is used for consolidating the change logs.

– The Dataset recovery utility is used for recovering the database to a stable state


Backward Recovery– Also called Database Backout

– Used for reversing all the changes to the database till the last Stable State (Checkpoint), in case of an abend.

– The IMS log records are read backwards and their effects are reversed in the database.

– When the backout is complete, the databases are in the same state they were before the failure.


Backward Recovery– Database backout Utility can be used for the

purposes of performing a Backout.

– Backward Recovery is appropriate when program ends in a controlled fashion- that is, an abnormal termination routine is called by the application program, that records and reports the problem


Backward Recovery Note:

– An application program can also chose to invoke dymanic backout, by issuing a ROLLBACK call (ROLB).

– Generally speaking, ROLB calls are avoided in pure DL/I programs and Batch-Oriented BMPs. However, they will normally be used in MPPs and Transaction-oriented BMPs.


I/O PCB Mask

An IO-PCB is a special PCB used to request system related requests like:– Checkpoint

– Restart

– Receiving messages from the message queue into an application program

– Sending messages from an application program to the same logical terminal or application program


I/O PCB Mask

Logical Terminal Name (8 bytes) Reserved for IMS (2 bytes) Status Code (2 bytes) Current Date (4 bytes) Current Time ( 4 bytes) Input Message Sequence Number (4 bytes) Message Output Descriptor Name (8 bytes)


I/O PCB Mask

User Identification (8 bytes) Group name (8 bytes)


Checkpoint Synonymous to Sync point, commit point or

point of integrity. Database changes are committed during Checkpoint.

Point of execution in a the program where the database changes are considered complete and accurate.

Database changes before the most recent checkpoint are not reversed by backward recovery.


Checkpoint Database changes logged after the most recent

checkpoint are not applied to an image copy of the database during forward recovery.

Establishing Checkpoint

– A Checkpoint is established using a CHKP call

– A checkpoint record is written on to the IMS log file, which tells the DL/I recovery utilities that recovery should stop here.


Checkpoint Frequency Generally, Checkpoints are taken at regular

intervals (example every 10 minutes or 15 minutes) or after a specified number of transactions (say for every 1000 transactions)

The Checkpoint frequency may vary depending on the complexity and average execution time of the application and on the volume of data updated.


Basic Checkpoint The format of a Basic Checkpoint call is

CALL ‘CBLTDLI’ USING CHKP IO-PCB-MASK

CHECKPOINT-ID The IO-PCB format is as follows:

01 IO-PCB-MASK 05 FILLER PIC X(10). 05 IO-PCB-STATUS-CODE PIC X(02).

The IO-PCB has to be the first PCB in the ENTRY statement in the Procedure Division.


Basic Checkpoint Checkpoint-id Field:

– Contains Eight-bytes – Takes the place of a segment I/O area for an IMS

call. – Program places a value that identifies the

checkpoint record on the IMS log file The checkpoint-id can then be used to restart the

program


Basic Checkpoint Disadvantages

– Program has to provide a facility to accept the checkpoint-id, then decide what to do with it.

– May have to read transaction files to skip transactions that were posted before the last checkpoint.

– This means the resetting of working storage fields to the values they had when the checkpoint was taken.


Symbolic Checkpoint Similar to Basic Checkpoint but:

– Works with Extended Restart (XRST) call to restart the program which was terminated abnormally.

– Can save as many as 7 data areas in the program, which are restored when the program is restarted.

– Note: • In each Data area, the first item is the name of a full-word

field [PIC S9(5) COMP], that contains the name of length of the data area to be saved. The second is the name of the data area itself.


Symbolic Checkpoint The format of a Symbolic Checkpoint call

is CALL ‘CBLTDLI’ USING CHKP IO-PCB-MASK

CHECKPOINT-ID [DATA-AREA-1] [DATA-AREA-2] [DATA-AREA-3]

….. ….. ….. [DATA-AREA-7]


Extended Restart - XRST Call Causes restart to be performed. Must be used in a program if Symbolic Checkpoint

is used Issued only once - at the beginning of the program. If the program is not restarted, it indicates that

subsequent CHKP calls will be Symbolic Checkpoints rather than basic checkpoints.

Establishes the maximum number of areas within the program that can be Checkpointed on subsequent CHKP calls.


XRST Call The format of an XRST call is

CALL ‘CBLTDLI’ USING XRST IO-PCB-MASK

RESTART-IO-AREA [DATA-AREA-1]

[DATA-AREA-2] [DATA-AREA-3]

….. ….. ….. [DATA-AREA-7]


Ways of Starting a Program Normal start

– The IO-AREA pointed to in the XRST call must contain blanks. If IMS does not change the area after issuing the XRST call, then the program is being started normally.

Restart based on the I/O area provided by the XRST call– The first 8 bytes of the I/O area will be populated

with the Checkpoint ID and the remaining 4 bytes will be blank


Ways of Starting a Program Restart based on the PARM field of the

EXEC statement on the program’s JCL.


What happens during XRST IMS reads the system log defined in the //IMSLOGR

DD statement and searches for the checkpoint records with the Checkpoint ID supplied.

When this ID is found, IMS restores the areas that we identified in the Symbolic CHKP Call.

For restarting a BMP, instead of supplying the whole Checkpoint ID, we can just specify ‘LAST’ in the I/O area pointed to by the XRST call and leaving the remaining 8 bytes blank. This instructs IMS to use the last checkpoint issued by the BMP.


Repositioning after XRST The XRST call attempts to reposition all the databases

to the position held when the checkpoint was taken. This is done by including each PCB and PCB key

feedback area in the checkpoint record. Issuing XRST causes the key feedback area from the

PCB in the checkpoint record to be moved to the corresponding PCB in the PSB being restarted.

Then IMS issues a GU call qualified with the concatenated key for each PCB that held a position when the checkpoint was taken.


Restrictions After issuing a Checkpoint call, IMS loses the positions

held in all the databases. It is our responsibility to reposition our databases in the application program, after a CHKP call.

Do not take a checkpoint call after a DLET. If we do so, the next time we try to reposition `issue a XRST call to restart, IMS will issue a GU call with the relevant concatenated keys to find the segment. Since it is already deleted, IMS will return a ‘GE status code.

XRST cannot reposition to the right segment if a segment or any of its parents have a non-unique key.


GSAM Processing Generalized Sequential Access Method Allows an application program to access either a

standard dataset or a VSAM ESDS through IMS calls. GSAM datasets allow Symbolic Checkpoint and Restart. GSAM is non-hierarchical. GSAM does not have any

segments, segment type, key fields, search fields. The unit of data that is retrieved is merely a record. GSAM datasets are commonly used when the business

requires that the programs are re-startable. GSAM allows files with record formats F, FB, V,VB

and U.


GSAM Processing GSAM supports only GU, GN and ISRT, OPEN,

CLSE calls. GSAM files can be easily restarted Only sequential processing is allowed. Records can be added only at the end of the database It is not possible to delete records


GSAM DBD & PCBDBD

DBD NAME=GSAMDBD,ACCESS=(GSAM,VSAM)DATASET DD1=INPUT,DD2=OUTPUT,RECFM=F,RECORD=80DBDGENFINISHEND

PSB

PCB TYPE=GSAM,PROCOPT=G,DBDNAME=GSAMDBDPCB TYPE=GSAM,PROCOPT=L,DBDNAME=GSAMDBDPSBGEN LANG=COCOL,PSBNAME=<Name of the PSB>


GSAM IMS Callsa) CALL ‘CBLTDLI’ USING GU

GSAM-PCB IO-AREA

b) CALL ‘CBLTDLI’ USING GN GSAM-PCB

IO-AREA

c) CALL ‘CBLTDLI’ USING ISRT GSAM-PCB

IO-AREA


GSAM IMS Calls Note:

– GSAM automatically opens the database upon the first GU or GN call.

– GSAM automatically closes the database upon reaching the end of the database or when the program terminates

– The OPEN and CLSE call are used to explicitly open and close the database, in cases where the same GSAM file is created and read in a single job step.


Coding GSAM DD namesTo use Input GSAM files//INPUT DD DSN=FILE1,DISP=SHR

To use Output GSAM files

//OUTPUT DD DSN=FILE2,DISP=(,CATLG,CATLG), DCB=(LRECL=80,REFCM=FB,BLKSIZE=8000)


Checkpoint and Restart To Checkpoint GSAM databases, we must use Symbolic

CHKP and XRST calls. When we use XRST, IMS automatically repositions

GSAM for processing. Do not use Temporary datasets with Symbolic CHKP and

XRST SYSOUT used at restart time can give duplicate output

data After taking a checkpoint, IMS cancels all the positions

held normal IMS databases, but in GSAM, positions are maintained.

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