Coupling Facility

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 (runs in an own LPAR, with dedicated physical CP, defined thru HMC), with memory and special channels (CF Links), and a specialised operating system called Coupling Facility Control Code (CFCC). It has no I/O devices, other than the CF links. The information in the CF resides entirely in memory as CFCC is not a virtual memory operating system. A CF typically has a large memory – of the order of several gigabytes. In principle any IBM mainframe can serve as a coupling facility. The CF runs no application software.

When originally introduced, the CFCC executed in a separate mainframe unit that was essentially a processor without I/O facilities other than the CF links. Later IBM enabled the use of an Internal Coupling Facility where the CFCC runs in a logical partition (LPAR) defined in standard processor complex and communicates over internal links within that processor complex hardware. Links to another processor unit are over copper cables. More than one CF is typically configured in a Sysplex cluster for reliability and availability. Recovery support in the z/OS operating system allows structures to be rebuilt in the alternate CF in the event of a failure.