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VSAM Interview Questions and Answers

1
How do you convert flat files to VSAM files?

Flat files are converted into VSAM files by following process:

  • Identify the primary key, when the whole record can be primary key.
  • Alternate key can also be used, if needed
  • File is to be sorted by primary key or the whole record
  • Utilize the SORT statement for dropping duplicates.
  • A JCL command ( IDCAMS – Define Cluster) need to be submitted by using the result of sorting mechanism(mentioned above) as input
  • Use the command IDCAMS REPRO for loading the flat file into the VSAM file.
2
What is IDCAMS ?

IDCAMS is the Access Method Services program. You run the IDCAMS program and supply AMS commands thru SYSIN. (examples of AMS commands are DELETE, DEFINE, REPRO etc..).

3
What are the different types of VSAM datasets?

Entry sequenced datasets (ESDS), Key sequenced datasets (KSDS), Relative record dataset (RRDS) and Linear dataset (LDS).

4
What is meant by dirty read in VSAM?
  • High degree of concurrent access to VSAM files is achieved by dirty read - It is done by avoiding the complications associated with CI and CA splits - The dirty read protocol is summarized as:
  • The VSAM file mandatorily be defined with cross-region SHARE POINTS 4.
  • The file allocation must be with DISP=SHR - ENQ must be followed for all operations of the files, for data set - Data set name is specified for ENQ name - Any string can be used for the ENQ gname - The user’s buffer must be refreshed with GET request, after obtaining a lock - No lock is held during a wait - DEQ function should be called at the end of all the file operations of enqueue.
5
What happens when you open an empty VSAM file in a COBOL program for input?

A VSAM file that has never contained a record is treated as unavailable. Attempting to open for input will fail. An empty file can be opened for output only. When you open for output, COBOL will write a dummy record to the file & then delete it out.

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