In z/OS, it is possible to catalog successive updates or generations of related data, which are called generation data groups (GDGs).
Each data set within a GDG is called a generation or generation data set (GDS). A generation data group (GDG) is a collection of historically related non-VSAM data sets that are arranged in chronological order. That is, each data set is historically related to the others in the group.
Within a GDG, the generations can have like or unlike DCB attributes and data set organizations. If the attributes and organizations of all generations in a group are identical, the generations can be retrieved together as a single data set.
Advantages to grouping related data sets include:
Generation data sets have sequentially ordered absolute and relative names that represent their age. The operating system's catalog management routines use the absolute generation name. Older data sets have smaller absolute numbers. The relative name is a signed integer used to refer to the latest (0), the next to the latest (-1), and so forth, generation.
For example, the data set name LAB.PAYROLL(0) refers to the most recent data set of the group; LAB.PAYROLL(-1) refers to the second most recent data set; and so forth. The relative number can also be used to catalog a new generation (+1). A generation data group (GDG) base is allocated in a catalog before the generation data sets are cataloged. Each GDG is represented by a GDG base entry.
For new non-system-managed data sets, if you do not specify a volume and the data set is not opened, the system does not catalog the data set. New system-managed data sets are always cataloged when allocated, with the volume assigned from a storage group.
In order for GDGs to work, the GDGs have to be created before data sets that are to be included in them can be made a part of them. In order to create a GDG, the following must be specified to the operating system:
When a GDG is created, it will not have any data set belonging to it. However, each data set that is added to it must of the same type. A model containing parameter information, which includes all the data sets added to a GDG, must be specified to the system. Once a model for a GDG has been established, the system must be informed each time a data set is to be added to it. The system must be able to specify the generation number of each data set within a GDG. The system must be informed if a data set within a GDG is to be deleted. The system must be informed if a data set within a GDG is to be deleted, even if its retention period has not expired. The system must be informed if only the index of a GDG is to be deleted. The system must be informed if the entire GDG is to be deleted; this includes the index and all related data sets.
Before a GDG is created an index, which defines its name along with other, features must be created and cataloged. The IDCAMS utility is used to*create this index. The DEFINE GDG statement is used to convey the information to the IDCAMS utility relating to the index. The following sub-parameters are used to convey information about the index to the IDCAMS utility:
No more than 255 data sets can exist within one GDG. All rules that apply to coding data sets apply equally to data sets within GDGs. The only difference is that a generation number must be coded within brackets for the data set name in the DSN parameter. GDGs must be cataloged. GDGs must reside on tape or direct access device. The DSN and UNIT parameters must be coded for all new generation data sets. The DISP parameter must be set to GATLG for all new generation data sets.
Features of GDGs
GDGs are characterized by the following:
Advantages of GDGs
The record keeping (as to what generation number should be assigned to new data sets, and which data set goes if the limit is reached), is the responsibility of the operating system, not the application programmer. Further more, GDGs provide a convenient method of relating data sets together, and automatically discarding those data sets, which are outdated.
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