Flag day (computing)


A flag day in computing and system administration is a planned change that requires many systems to be upgraded or converted in a coordinated way because the old and new versions are not mutually compatible. Such changes are typically costly to carry out and, if problems arise, difficult to roll back.
Flag days can occur when constraints on backward compatibility or forward compatibility prevent a gradual migration, requiring updates to be performed nearly simultaneously for the overall system to function. In contrast, phased deployments aim to preserve service continuity by allowing old and new versions to coexist during the transition.

Origin

The term is commonly traced to the Multics project: a system-wide change to the system's handling of the ASCII character set was scheduled for the U.S. holiday Flag Day, and the name was applied by analogy to other coordinated, incompatible changeovers.

Examples