A coupling facility is a mainframe processor (runs in an own LPAR, with dedicated physical CP, defined through Hardware Management Console (HMC)), with memory and special channels (CF Links), and a specialized operating system called Coupling Facility Control Code (CFCC).
When originally introduced, the CFCC executed in a separate 9674 mainframe unit that was essentially a processor without I/O facilities other than the CF links.
Later[a] 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.
Recovery support in the z/OS operating system allows structures to be rebuilt in the alternate CF in the event of a failure.
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 issuing requests to the CF.
The System z Architecture includes 18 special machine instructions and additional hardware features supporting CF operation.
Duplexing is often used as part of an installation's drive to remove single points of failure, with the aim of reducing the incidence and duration of application outages.