GDG - A Basic Introduction

Introduction

• GDG stands for Generation Data Group. Here a group of files are related functionally or chronologically and can be accessed individually or as a group. These files share a unique name.
•  Every dataset in a GDG has a Generation number and version number. Eg.TESTGDG.DATA.G0001.V00
• Subsequent files are named by incrementing the Generation number(G0001-G9999).
• The current generation is referred a 0. The next generation as +1 an previous generation as -1
• Every time a new gen is created, it becomes the current gen and the current gen becomes the previous generation.
• All generations must be cataloged to keep track of the relative generation  numbers
• Maximum number of versions for a GDG is 255.
• Each generation can have same or different DCB parameters.

Why We Need IT:

1. Record keeping
2. Easy to relate data sets
3. Auto deletion of outdated datasets
4. No need to change the JCL every time.

How to define a GDG :

To use the GDG we have to define the GDG base / index.

//STEP10 EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN DD *
            DEFINE GDG(NAME(TESTGDG.DATA.GDG) -
            LIMIT(10) -
            NOEMPTY -
            SCRATCH)
/*

Where,

• GDG name can range from 1-44
• LIMIT can range from 1-255

Various Other Options: 

Empty Vs NoEmpty

Empty:-This option will delete all the older generations if the limit of generations is reached

Example: If a GDG is defined with a limit of 10 and 11th generation is created then all the old 10 generations are deleted and the 11th generation is created.

NoEmpty:- This option will delete the oldest generation if the limit of generations is reached .

Example: If a GDG is defined with a limit of 10 and 11th generation is created then the 1^st  generation is  deleted and the 11th generation is created.

Scratch Vs NoScratch

Scratch:-This option will delete the generation completely if the dataset is deleted. 

NoScratch-> This option will uncatalog the dataset if the dataset is deleted.

Creating a GDG version:

GDG version can be created in same JCL by giving +1 to the GDG base name.

STEP1   EXEC PGM=IEFBR14                                 

INFILE  DD  DSN= TESTGDG.DATA.GDG (+1),   

        DISP=(NEW,CATLG,DELETE),                         

        UNIT=SYSDA,                                      

        SPACE=(CYL,(200,200),RLSE),                      

       DCB=(LRECL=2700,BLKSIZE=0,RECFM=FB)                

                                                         

Altering a GDG:

We can also change the attributes of a GDG using the ALTER keyword.

//STEP1   EXEC PGM=IDCAMS                      

//SYSPRINT DD SYSOUT=*                         

//SYSIN    DD *                                

    ALTER TESTGDG.DATA.GDG  NOSCRATCH     

/*                                             

Deleting a GDG generation:

//STEP1   EXEC PGM=IEFBR14                    

//DL       DD  DSN=TESTGDG.DATA.GDG(+1),      

//         DISP=(OLD,DELETE,DELTE)            

/*      

Deleting a GDG base /Index:

Two parameters for deleting the base .

1.PURGE:    Object will be deleted regardless of the expiration date.   

//SYSIN    DD  *                  

   DELETE TESTGDG.DATA.GDG PURGE    

2.FORCE: Object will be delete even though it is not empty.   

//SYSIN    DD  *                  

   DELETE TESTGDG.DATA.GDG FORCE