Exploit CICS Data Tables!

If you, like almost any other user of IBM System/390 and zSeries processors, use CICS, you should use CICS Data Tables whenever it is possible.

With CICS Transaction Server for VSE and for zOS (or MVS) the data table support provides very efficient processing of data in memory with no references to VSAM buffers, cache storage in the processor or I/O subsystem, or disk for read activity, whether it is direct or sequential access. Updates need to be written to disk for integrity, of course.

If you are using an older version of CICS (e.g. CICS/VSE) on VSE/ESA 1.3 or later, data table support benefits are limited to full key read activity -- other activity will still need to access the file on disk by way of VSAM.

You may find it hard to believe, but measurements made by the Swami in conjunction with IBM's Dr. Wolfgang Kraemer showed that even if a file is completely contained in VSAM buffers, so that no real I/O activity is being done, significant CPU time savings can be had through the use of CICS Data Tables. This time saved permits a higher level of real throughput for the whole system.

With CICS Transaction Server, you can also share a single copy of a data table in memory with other CICS subsystems! This saves storage and reduces CPU load further.

Note that VSE/ESA 2.5's implementation of VSAM provides hashed buffer searches. This reduces the CPU cost of searching large LSR buffer pools, but CICS Data Tables are still more effective when they are applicable.

NOTE! The initial CICS Data Tables Guide manuals were written in such a way that you might conclude that if a file has update, browse, or generic-key retrievals, it cannot be used in a CICS Data Table. This is incorrect -- the true message is that CICS Data Table benefits only apply for full-key direct reads. Thus, if a file has many full-key reads and some other activity, we can still have significant savings on the full-key read activity.

CICS Transaction Server's Data Table support now includes browse and generic key retrievals, so additional savings can be realized by the latest and greatest levels of VSE, VSAM and CICS.