CICS coupling facility data tables

A CICS® file is a representation of a dataset on DASD. If you specify that the file is to use data table services, CICS copies the contents of the dataset into in-memory shared storage when the file is opened and uses that copy whenever possible. For a Shared Data Table (CICS-maintained data table or user-maintained data table), the contents are copied into a z/OS® data space which is shared across a z/OS image. For a coupling facility data table, the contents are copied into a coupling facility structure, which is shared across a sysplex. For coupling facility data tables, it is also possible to populate the table at run time without previously loading it from a data set.

Coupling facility data tables (CFDTs) provide some of the performance benefits of shared data tables (SDTs) while also providing the flexibility to access the data table from anywhere in the sysplex. CFDTs are similar in many ways to a shared user-maintained data table (UMT). CFDTs and the CF structures they use are managed by a CFDT server.

The typical uses of CFDTs do not require the data to be long-lasting or permanent, although it is possible to define a CF data table to be recoverable from transaction and system failures. Updates to the data are not automatically hardened to the source data set, and the contents of the CFDT do not survive a failure of the CF or of the CF structure. It is the user's responsibility to copy updates to the data table back to the source data set on DASD if that is required. To protect the CFDT structure from a CF failure, you can use System-Managed Duplexing.

The performance of a CFDT (with its data in a CF structure), is not as good as with a shared data table (with its data in a a data space), but it is better than accessing the data from the data set, or by function shipping requests between CICS regions.

Note: No application changes are required if you want to change a user-maintained data table to make it a Coupling Facility Data Table. The mapping of the location of the data table (in an z/OS data space or in a CF structure) is controlled by CICS, and application programs do not need to be aware of the location of the data table. For application considerations specifically relating to CFDTs, see Coupling facility data tables.

For information about setting up and running a coupling facility data table server, see Setting up and running a coupling facility data table server. For information about FILE resources and CFDTs, see Coupling facility data tables.