Sample IDMS data sharing definitions

Products

• IDMS

Issue/Introduction

This article provides sample IDMS data sharing definitions.

Environment

Release: All supported releases.

Cause

Data sharing is the ability for multiple IDMS CVs in a sysplex (in the same or different LPARs) to simultaneously share update access to the same database area(s).

Resolution

Note: the CF definitions described below are defined in the IBM Coupling Facility, not in IDMS. Most likely, System Programmers will need to be consulted for making those definitions.

All of the sizing information in these samples are examples only and in practice will be site-specific. Consult the Data Sharing documentation page for more specific guidelines.

1. The catalog definitions of the areas being data shared must have the same physical attributes in the involved CVs, because they describe the same physical data. See Sharing Update Access to Data.
    
2. For an area to be able to use data sharing, all of its files must use a shared cache. Define a shared cache in the CF to be used for this purpose. Example:
   
   STRUCTURE NAME(IDMSEMPDSGCACHE1) SIZE(24M) PREFLIST(COUPLET1)
   
   For sizing purposes, a shared cache can be thought of as a database buffer, with the difference that it can be used by multiple files with varying page sizes, so its size needs to be specified as an amount of storage, not a number of pages. As with database buffers, the larger it is, the better it will perform. 
   
   
3. Decide on a data sharing group name. In this example, EMPDSG will be used.
   
   
4. Define a list structure and a lock structure in the CF. The structure names must follow the naming standard of CAIDMSxxxxxxLI and CAIDMSxxxxxxLK respectively, where xxxxxx is the data sharing group name decided in point 3 above.
   
   Examples:

   STRUCTURE NAME(CAIDMSEMPDSGLI) SIZE(24M) PREFLIST(COUPLET1)
STRUCTURE NAME(CAIDMSEMPDSGLK) SIZE(24M) PREFLIST(COUPLET1)

5. Update the DMCL of all of the involved CVs to identify the shared cache structures to be used and whether or not a segment or area is to be shared. In this example, an entire segment is being shared. If any of the files of an area that is to be shared do not have a shared cache defined to them, the shared cache defined in the DATA SHARING DEFAULT SHARED CACHE clause will be used for that file. This technique is used in the following example.
   
   ALTER
DMCL CVnnDMCL
    DATA SHARING DEFAULT SHARED CACHE IDMSEMPDSGCACHE1
        LOCK ENTRIES 131072 MEMBERS 4
        ON CONNECTIVITY LOSS NOABEND
    INCLUDE SEGMENT EMPDEMO
        ON STARTUP SET STATUS TO UPDATE
        ON WARMSTART MAINTAIN CURRENT STATUS
        DATA SHARING YES
        DEFAULT SHARED CACHE NULL
    ;

6. Update the SYSIDMS file of all of the involved CVs, specifying the data sharing group name (which must be the same) and a unique member name within the group for each CV.
   
   //SYSIDMS  DD *
DSGROUP=EMPDSG
DCNAME=SYSTEMnn

Additional Information

Further details on these topics and everything else regarding IDMS's sysplex exploitation can be found at IDMS in a Sysplex Environment.