IBM Coupling Facility
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In IBM mainframe computers, a Coupling Facility or CF is a piece of computer hardware which allows multiple processors to access the same data.
A Parallel Sysplex relies on one or more Coupling Facilities (CFs). A coupling facility is a mainframe processor, with memory and special channels (CF Links), and an operating system primitive called Coupling Facility Control Code (CFCC). It has no I/O devices, other than the CF links. The information in the CF resides in memory and a CF typically has a large memory. In principle, any IBM mainframe can serve as a coupling facility. The CF runs no application software.
A CF is used for three purposes:
1. Locking information that is shared among all attached systems
2. Cache information (such as for a data base) that is shared among all attached systems
3. Data list information that is shared among all attached systems
Supportet by a CF, a Sysplex cluster scales very well up to several hundreds of CPUs running transaction and data base applications. Using the CF links, data can be directly exchanged between the CF memory and the memory of the attached systems, using a direct memory access like mechanism, without interterrupting a running program. Systems in a Sysplex cluster store CF information in local memory in an area called a bit vector. This enables them to locally query critical state information of other systems in the Sysplex without the need for interrogation. The System z Architecture includes 18 special machine instructions and additional hardware features supporting CF operation.
CFs are unique to S/390 and System z mainframes. They are key to Parallel Sysplex technology.
