JCL Generation Data Group Interview Questions and Answers

11/14/2009 No Comment

JCL Interview Questions on Generation Data Group (GDG). 

Explain what do you mean by Generation Data Group (GDG)?
Answer : A Generation Data Group in JCL is a group of chronologically/functionally related datasets, with each having a father-son relationship. GDGs are processed periodically, often by adding a new generation, retaining previous generations, and sometimes discarding the oldest generation. Every dataset within a group is assigned a unique Generation Number.

How is GDG base created in the system?
Answer : A GDG base is created in the system catalog and keeps track of the generation numbers used for datasets in the group. Then the IDCAMS utility is used to define the GDG base for MVS/XA, and MVS/ESA. However in older systems, the IEHPROGM utility be used.

Explain what is model dataset label (Model DSCB)?
Answer : A model dataset label is a pattern for the dataset label created for any dataset named as part of the GDG group. The system needs an existing dataset to serve as a model to supply the DCB parameters for the generation data group one wish to create. The model dataset label must be cataloged. The model DSCB name is placed on the DCB parameter on the DD statement that creates the generation data group.

What is the advantage in using generation data groups?

Answer : The advantage in using the generation data groups is that all datasets have the name, and the system keeps track of adding and deleting successive generations. The JCL does not need to be changed between runs.

How are GDG concatenated?
Answer : Generation data groups are concatenated by specifying each dataset name and the generation number of all generations of the generation data group . To retrieve all generations of a generation data group, omit the generation number. The DD statements refers to all generations
The result is the same as if all individual datasets were concatenated. If generations are not on the same volume, this will not work.

Can you tell how is a previous GDG coded?
Answer : Previous GDGs are coded as (-1) after the dataset name. An example would be DSN=JAN.DATA(-1).

Can you tell how is the current GDG coded?

Answer : Current GDGs are coded as (0), (+0), or (-0) after the dataset name as follows: DSN=JAN.DATA(0). The +(0) and (-0) have the same effect as (0) Normally, it is coded as (0).

Can you tell how is a new GDG coded?

Answer : A new GG is coded as (+1) after the dataset name as follows:
DSN=JAN.DATA(+1). This will cause all generations to be pushed down one level at the end of the job.

How are different generations specified?

Answer : Different generations are specified by providing the dataset name and generation number for each GDG desired.

What is the status of the GDG when an abend occurs?

Answer : The GDG is in a bad state because it may consist of partial information. If used in this state, it is possible to get incorrect or improper results. It is recommended to reset the current generation by deleting the bad generation before executing the job, which will re-create a new generation.
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