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Overview of VSAM SHAREOPTIONS

First, happy new year 2003. We are working in OS/390 v2r10, VSAM files and CICS TS 1.3. Also we have online and...

batch processing. Which is the best definition for the VSAM files? Some clusters have in the parameter shareoptions (2,3). Others have (4,3). We also have primary and secondary cluster VSAM.

A quick rundown of the VSAM SHAREOPTIONS for those not familiar with them: Cross-region, cross-system

Cross-region:

1 -- Any number of opens for just reading, but only for a single region if opened for writing: Complete integrity

2 -- Any number of opens for just reading and also one region for updating: VSAM enqueues on the control interval for update integrity, and users have to ENQ themselves on keys to provide read integrity

3 -- Fully shared by regions: Users have to look after integrity themselves

4 -- Fully shared by regions: Users have to look after integrity by using ENQ/DEQ and buffers are flushed after each direct access to a CI.

Cross-system:

3 -- Fully shared by regions: Users have to look after integrity themselves

4 -- Fully shared by regions: Users have to look after integrity themselves by using ENQs and RESERVEs and buffers are flushed after each direct access to a CI.

Consequently, in the CICS environment you only want CICS to be able to do the writing, and (maybe) permit other jobs to do reading at the same time. However, this reading will not necessarily get the most recently updated version of a given record. It depends on what else is going on in the CI that contains it.

If you need a guaranteed most accurate batch read access to a particular VSAM record, then you must use the RLS support within CICS and ensure that the batch job also uses RLS access.

So SHAREOPTIONS(2,3) is what you want on all VSAM files accessed by CICS. If you want to do batch update processing on a VSAM file that is used by CICS, you have to use CEMT to close it within CICS, so allowing a batch job to be the single permitted writer.

This was last published in January 2003

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