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Generation Data Groups (GDGs)

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Introduction

Generation Data Groups (GDGs) are a method used in JCL to manage related datasets that are created and used sequentially. GDGs allow for the automatic handling of dataset versions, making it easier to manage data over time.

Key Concepts

What is a GDG?

  • GDG Base: The GDG base is a catalog entry that defines the GDG and its attributes.
  • GDG Generation: Each dataset within a GDG is called a generation and is identified by a relative generation number (e.g., G0001V00).

Benefits of Using GDGs

  • Automatic Versioning: GDGs automatically handle the creation of new dataset versions.
  • Simplified Dataset Management: GDGs simplify the process of managing datasets that are used in a sequence.
  • Retention and Deletion: GDGs can be configured to retain a specific number of generations, automatically deleting older ones.

Creating a GDG

Defining a GDG Base

To define a GDG base, you use the IDCAMS utility. Here is an example:

//DEFINEGDG EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN    DD *
  DEFINE GDG (NAME(MY.GDG.BASE) LIMIT(5) SCRATCH)
/*
  • NAME: Specifies the name of the GDG base.
  • LIMIT: Specifies the maximum number of generations to retain.
  • SCRATCH: Indicates that the oldest generation should be deleted when the limit is exceeded.

Creating a GDG Generation

To create a new generation within a GDG, you use the (+1) notation in the DD statement:

//NEWGEN   EXEC PGM=MYPROG
//MYDATA   DD DSN=MY.GDG.BASE(+1),
//             DISP=(NEW,CATLG,DELETE),
//             SPACE=(CYL,(5,5)),
//             DCB=(RECFM=FB,LRECL=80,BLKSIZE=800)
  • DSN: Specifies the GDG base name followed by (+1) to create a new generation.
  • DISP: Specifies the dataset disposition.
  • SPACE: Defines the space allocation for the dataset.
  • DCB: Defines the dataset control block attributes.

Accessing GDG Generations

Accessing the Latest Generation

To access the latest generation, use the (0) notation:

//READGEN  EXEC PGM=MYPROG
//MYDATA   DD DSN=MY.GDG.BASE(0),
//             DISP=SHR

Accessing Previous Generations

To access previous generations, use negative relative numbers:

//READOLD  EXEC PGM=MYPROG
//MYDATA   DD DSN=MY.GDG.BASE(-1),
//             DISP=SHR

Practical Example

Example: Creating and Accessing GDG Generations

  1. Define the GDG Base:
//DEFINEGDG EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN    DD *
  DEFINE GDG (NAME(MY.GDG.BASE) LIMIT(3) SCRATCH)
/*
  1. Create a New Generation:
//NEWGEN   EXEC PGM=MYPROG
//MYDATA   DD DSN=MY.GDG.BASE(+1),
//             DISP=(NEW,CATLG,DELETE),
//             SPACE=(CYL,(5,5)),
//             DCB=(RECFM=FB,LRECL=80,BLKSIZE=800)
  1. Access the Latest Generation:
//READGEN  EXEC PGM=MYPROG
//MYDATA   DD DSN=MY.GDG.BASE(0),
//             DISP=SHR
  1. Access the Previous Generation:
//READOLD  EXEC PGM=MYPROG
//MYDATA   DD DSN=MY.GDG.BASE(-1),
//             DISP=SHR

Exercises

Exercise 1: Define a GDG Base

Define a GDG base named TEST.GDG.BASE with a limit of 4 generations and set it to delete the oldest generation when the limit is exceeded.

Solution:

//DEFINEGDG EXEC PGM=IDCAMS
//SYSPRINT DD SYSOUT=*
//SYSIN    DD *
  DEFINE GDG (NAME(TEST.GDG.BASE) LIMIT(4) SCRATCH)
/*

Exercise 2: Create a New Generation

Write a JCL job to create a new generation in the TEST.GDG.BASE GDG.

Solution:

//NEWGEN   EXEC PGM=MYPROG
//MYDATA   DD DSN=TEST.GDG.BASE(+1),
//             DISP=(NEW,CATLG,DELETE),
//             SPACE=(CYL,(5,5)),
//             DCB=(RECFM=FB,LRECL=80,BLKSIZE=800)

Exercise 3: Access the Latest Generation

Write a JCL job to read the latest generation from the TEST.GDG.BASE GDG.

Solution:

//READGEN  EXEC PGM=MYPROG
//MYDATA   DD DSN=TEST.GDG.BASE(0),
//             DISP=SHR

Common Mistakes and Tips

  • Incorrect GDG Base Name: Ensure the GDG base name is correctly specified in both the definition and usage.
  • Exceeding GDG Limit: Be aware of the GDG limit to avoid unexpected deletion of older generations.
  • DISP Parameter: Use the correct DISP parameter to avoid accidental deletion or cataloging issues.

Conclusion

Generation Data Groups (GDGs) are a powerful feature in JCL for managing sequential datasets. By understanding how to define, create, and access GDG generations, you can efficiently handle data versioning and retention in your JCL jobs. Practice the exercises provided to reinforce your understanding and become proficient in using GDGs.

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