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.
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..).
What are the different types of VSAM datasets?
Entry sequenced datasets (ESDS), Key sequenced datasets (KSDS), Relative record dataset (RRDS) and Linear dataset (LDS).
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.
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.