Compatibility Library Facility (CLF) provides the DBMS Server with a platform independent interface to the operating system. It contains functions that perform services such as disk I/O, memory management, Semaphore manipulation, and context switching (when Ingres threading is configured). It also provides low level system communications functions.

For example, when a thread in the DBMS is idle, it normally shows up in iimonitor, IPM, or VDBA as having current facility CLF. This indicates that the thread is executing a CLF function to read information from the communications channel that connects the thread to the system process that instigated the database connection associated with this thread.