Coupling facility data table structures and servers
Coupling facility data tables (CFDTs) are held in coupling facility list structures. Access to a CFDT is through a named CFDT server.
The CFDT server can be thought of as similar to a shared data tables file-owning region. For information about shared data tables support, see The data table sharing environment. You can use CFDT support to separate related groups of CFDTs by storing them in separate pools. For example, you can have one pool for production and another for test.
A CFDT pool is a coupling facility list structure, and access to it is provided by a coupling facility data table server. Within each MVS image, there must be one CFDT server for each CFDT pool accessed by CICS regions in the MVS image.
The names of the servers are formed by adding the server address space name to the pool name. For example, for FACILTY class RACF profiles, the server name is DFHCF.poolname.
Coupling facility list structures are defined in the coupling facility resource management (CFRM) policy. Structure names for CFDTs take the form DFHCFLS_poolname. The CFDT pool name is then specified in the startup JCL for the CFDT server.
Access using file control API
A CFDT is accessed from CICS through file control commands.
The file name specified on the command indicates the name of the table and pool in which it resides. The table name is either specified on the file definition or is the same as the file name, and the pool name is specified on the file definition. The table is uniquely identified by the pool name and table name, so that two tables with the same name can exist in different pools, and are entirely separate entities.
Automatic connection to CFDT pools
CICS connects to the CFDT server for a given pool the first time that a CFDT in that pool is referenced. CICS also reconnects automatically to the CFDT server when the server restarts after a failure.
CFDT servers are protected against misuse by CICS regions that call them, thus ensuring system integrity. Protection is provided so that the calling region cannot modify sensitive parameters to authorized functions.
Similarly, CICS is protected from any side effects if a CFDT server fails. If a CICS region issues a file control request to a CFDT server that has failed, the resulting MVS abend is trapped and returned to the application program as a SYSIDERR condition.
CFDT creation and deletion
CICS automatically creates a coupling facility data table (CFDT) when a first reference requires the CFDT to be opened. This CFDT is then used by the same region, or other CICS regions, that issue subsequent open requests for other files that name the same coupling facility data table.
CICS can optionally load the CFDT automatically from a source VSAM (KSDS) data set when it is first opened. Unlike user-maintained data tables, with CFDTs you can specify that there is no associated source data set, allowing you to create an empty CFDT.
Your application programs have access to a CFDT as soon as it is created, although there are some restrictions on the keys that can be accessed while data is being loaded.
When a CFDT is loaded, it becomes an independent entity, separate from the behavior of the CICS regions that access the table, or caused the table to be loaded. Even when all CICS regions have terminated, either normally or abnormally, a CFDT remains in the coupling facility until you take explicit action to delete the structure or the coupling facility.
MODIFY cfdt_server, DELETE TABLE=nameCFDT administration
CICS provides some utility functions that you can use to obtain summary information and periodic statistics from a coupling facility data table server about CFDTs defined in a pool. See Coupling facility data table statistics and Coupling Facility Data Table Pools report.
You can use this information when you administer coupling facility data table pools, and to evaluate capacity. See Coupling facility data table server parameters for details.