Basics of UMDT - User Maintained Data table

In February 1989, CICS announced enhancements for the CICS/MVS product and CICS/MVS 2.1. Most notable was the introduction of CICS Data Tables support, either as CICS Maintained Data Tables (CMDT) or User Maintained Data Tables (UMDT). ‘Data tables’ is the concept of having data in main storage for faster access and improved terminal response time.
 
User Maintained Data Table (UMDT)
 
A User Maintained Data Table is a sort of fake VSAM file. It does not exist on DASD, it only lives in CICS memory. You browse, read and write to it like any other VSAM file but at the end of the day (when CICS goes down, or the file is closed) its content disappears. UMDT's provide a good "scratch file" facility. They can be accessed very fast, and do not suffer from disorganization. They are a good candidate for high volume read-only applications.
 
The application programming interface (API) gives the programmer the impression that VSAM data was being accessed, however with UMDT the data need not have come from a VSAM data set.
 
When a UMDT is first accessed, a "seed" VSAM file is read into memory - one record or many, depending on the application. The seed file's attributes (key position, maximum record size and key size) are used for the UMDT's attributes. One seed file can be used by many UMDT's as appropriate.
 
Updates to real VSAM data are done in main storage as well as the actual file but
Updates to UMDT are done in main storage only; with the user having responsibility to update the source data if that was necessary. No updates to the UMDT will be applied against the seed file.
 
UMDT Basics
 
·         All UMDTs have variable length records.
·         Maximum LRECL is 32K.
·         Maximum record count is 16,777,215.
·         A “seed” VSAM file must be identified. It can contain one record or many records. The values of key position, key lengths etc. are copied from the seed file.
·         That seed file is read into memory at OPEN time.
·         Open will occur at first reference (WRITE, READ, STARTBR functions as used in the application programme) following startup or file close.
·         All subsequent activity is against the data in memory.
·         Memory is cleared at CLOSE time.
·         The seed VSAM file remains unchanged.
 
To get a UMDT available to CICS, you must tell us:
 
·         You must have a UMDT defined.
·         The DDNAME for the file to be referred to in the program
·         The DSN of the seed file
 
What you have to do:
 
·         Define the seed file with the appropriate key length, key position and maximum record size.
·         Load the seed file with the appropriate initial data.
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