Overview of shared data tables support in a sysplex

A shared data table can exploit shared data tables support only within a single MVS™ image.4 However, you can extend the use of shared data tables to a sysplex environment for an application that requires only read access to a shared user-maintained data table, or for one that does not require that changes are seen immediately.

You can replicate a user-maintained data table across the sysplex, with one data table per MVS. You must have one Shared Data Tables (SDT) server region in each MVS image, each owning a user-maintained data table that can be accessed using SDT sharing by any of the other CICS® regions within that MVS. These other regions require remote file definitions that refer to the user-maintained data table in their server region. Each user-maintained data table (UMT) must have the same source data set, and this data set must be readable by all of the SDT server regions. If the access is read-only, with the data never being updated, this will in effect provide a shared user-maintained data table in a sysplex.

If, as is probably more likely, the underlying data changes from time to time, but it is not necessary to reflect such changes immediately in the UMTs, you may periodically perform some processing to refresh the contents of the UMTs so that they are updated to match the underlying data without the need to close and then reload the UMTs. Changes are applied to the source data set, rather than to the user-maintained data table, using CICS applications that refer to the data set by a non-data table file definition, or using batch programs. An example application program (see Figure 10) illustrates how the UMTs can be refreshed to reflect the current contents of the source data set. The program would run on each MVS image and update the UMT in that image. Such a program could be run at regular times during the day, or at user request. It would be most efficient to run it in the SDT server regions, to avoid function shipping updates to the UMT.

If it is critical that the CICS regions in all the MVS images in the sysplex are synchronized in their view of the data, the transactions that read the data must be stopped while the refresh programs run, and restarted only after the programs have completed on all MVS systems.

This technique is appropriate only for user-maintained data tables because:

Figure 10 shows an example COBOL application program that demonstrates how you can refresh the replicated UMTs.

Related concepts
Using shared data tables support in a sysplex
How to refresh replicated user-maintained data tables
Example program for refreshing a user-maintained data table
Source code for the example program to refresh a replicated user-maintained data table

4.
But note that a shared data table can be shared using function shipping across MVS images.

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