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JCL - Generation Data Group or GDG

  • Generation Data Groups (GDGs) are group of datasets related to each other by a common name.

  • The common name is referred as GDG base and each dataset associated with the base is called a GDG version.

  • A Maximum of 255 generations are allowed for a GDG base.
For example,

MYDATA.TEST.SAMPLE.GDG is the GDG base name.

The datasets are named as MYDATA.TEST.SAMPLE.GDG.G0001V00, MYDATA.TEST.SAMPLE.GDG.G0002V00 and so on.
  • G – Stands for Generation number – Value range 0000 – 9999
  • V – Stands for Version number – Value range 00 – 99

Use of GDG:
  1. We do not need to create a new JCL or change the name of the JCL every time in a weekly run or daily run or monthly run or yearly run of a JCL.

  2. You can set the limit of the related files(generations).

  3. We can easily keep track of all generation of data sets.

  4. We can delete or uncatalog the older generation.

  5. Any particular generation can be referred easily.
Current version of the GDG is referred as MYDATA.TEST.SAMPLE.GDG(0),
Previous versions are referred as (-1), (-2) and so on.
Next(new) version to be created in a program is refered as MYDATA.TEST.SAMPLE.GDG(+1).
How to create a GDG?

STEP1: Create a GDG base (using IDCAMS utility – Access method service utility)

//DEFGDG1   JOB  ...
//STEP1    EXEC PGM=IDCAMS
//SYSPRINT DD   SYSOUT=A
//SYSIN    DD   *
     DEFINE GDG -
           (NAME(userid.MYGDG.TEST) -
           EMPTY -
           NOSCRATCH -
           LIMIT(15))
/*

List of PARAMETERS used to create GDG:

NAME – Name of the GDG Base.

LIMIT – To limit the maximum number of generations.

EMPTY/NOEMPTY:

NOEMPTY – Uncatalog only the oldest generation in GDG when the limit is reached.
EMPTY – Uncatalog all the generations when a limit is reached.

SCRATCH/NOSCRATCH:

SCRATCH -Physically delete the dataset(generation) which is uncataloged.
NOSCRATCH – Don’t Physically delete the dataset(generation) which is uncataloged.

STEP2: Using IEFBR14 utility, we create the new generations.

//DEFGDS   JOB  ...
//STEP1    EXEC PGM=IEFBR14
//GDSDD1   DD   DSN=ICFUCAT1.GDG02(+1),DISP=(NEW,CATLG),
//         SPACE=(TRK,(5,2)),STORCLAS=GRPVOL1,DATACLAS=ALLOC01
//SYSPRINT DD   SYSOUT=A
//SYSIN    DD   *
/*

If there is no generation created for this gdg base, this job creates new generation - ICFUCAT1.GDG02.G0001V00. otherwise, it will create a new generation based on existing generation number.

After you have created several generations. if you display your GDG would look like this in ISPF 3.4:

ICFUCAT1.GDG02
ICFUCAT1.GDG02.G0001V00
ICFUCAT1.GDG02.G0002V00
ICFUCAT1.GDG02.G0003V00
Difference between EMPTY and NOEMPTY?

Some of you might be confused between EMPTY and NOEMPTY.

Let us take an example –

Suppose, you have a GDG Base, IBM.SALES.REPORT and you have created 3 generations like – 

IBM.SALES.REPORT.G0001V00
IBM.SALES.REPORT.G0002V00
IBM.SALES.REPORT.G0003V00

Now, when you try to create another generation and if you have coded ‘NOEMPTY‘ – It means that it will remove the the oldest generation from the catalog when the limit is reached which means that only IBM.SALES.REPORT.G0001V00 will be uncataloged. So, now the 3 generations which are present are -

IBM.SALES.REPORT.G0002V00
IBM.SALES.REPORT.G0003V00
IBM.SALES.REPORT.G0004V00

Now, Suppose, you have a GDG Base, IBM.SALES.STATUS and you have created 3 generations like IBM.SALES.STATUS.G0001V00, IBM.SALES.STATUS.G0002V00, and IBM.SALES.STATUS.G0003V00.

Now, what happens when you try to create another generation and If you have coded ‘EMPTY‘ – It means that it will remove all the older generations from the catalog which means that IBM.SALES.STATUS.G0001V00, IBM.SALES.STATUS.G0002V00, and IBM.SALES.STATUS.G0003V00 will be uncataloged and IBM.SALES.STATUS.G00004V00 will be created.
How to use GDG in a JCL?

In the following example, the latest version of MYDATA.IBM.SAMPLE.GDG is used as input to the program and a new version of MYDATA.IBM.SAMPLE.GDG is created as the output.

//CNDSAMP JOB CLASS=6,NOTIFY=&SYSUID
//*
//STP01   EXEC PGM=MYCOBB
//IN1     DD DSN=MYDATA.IBM.SAMPLE.GDG(0),DISP=SHR
//OUT1    DD DSN=MYDATA.IBM.SAMPLE.GDG(+1),DISP=(,CALTG,DELETE)
//        LRECL=100,RECFM=FB

Here, if the GDG had been referred by the actual name like MYDATA.IBM.SAMPLE.GDG.G0001V00, then it leads to changing the JCL every time before execution. Using (0) and (+1) makes it dynamically substitute the GDG version for execution.
How to delete a GDG?

If you need to delete your GDG, delete the individual data sets (G0001V00, G0002V00, etc.) and then run this IDCAMS job to delete the GDG.

//STEP010 EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN DD *
  DELETE (MYDATA.IBM.SAMPLE.GDG) GDG FORCE
/*
How to modify existing GDG parameters?

If you wish to change the parameter of the GDG Base. We use ALTER command,

For example, if you want to change the number of generations run this IDCAMS alter example where the number of generation is increased to 50. 

//STEP010 EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN DD *
  ALTER MYDATA.IBM.SAMPLE.GDG LIMIT(50)
/*


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