A flag day, as used in system administration, is a change which requires a complete restart or conversion of a sizable body of software or data. The change is large and expensive, and—in the event of failure—similarly difficult and expensive to reverse. The situation may arise if there are limitations on

backward compatibility Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especiall ...

and

forward compatibility Forward compatibility or upward compatibility is a design characteristic that allows a system to accept input intended for a later version of itself. The concept can be applied to entire systems, electrical interfaces, telecommunication signals, ...

among system components, which then requires that updates be performed almost simultaneously (during a "flag day cutover") for the system to function after the upgrade. This contrasts with the method of gradually phased-in upgrades, which avoids the disruption of service caused by ''

en masse Many words in the English vocabulary are of French origin, most coming from the Anglo-Norman spoken by the upper classes in England for several hundred years after the Norman Conquest, before the language settled into what became Modern Engli ...

'' upgrades. This systems terminology originates from a major change in the

Multics Multics ("Multiplexed Information and Computing Service") is an influential early time-sharing operating system based on the concept of a single-level memory.Dennis M. Ritchie, "The Evolution of the Unix Time-sharing System", Communications of ...

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...

's definition of

ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because ...

, which was scheduled for the United States holiday, Flag Day, on June 14, 1966. Another historical flag day was January 1, 1983, when the

ARPANET The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switched network with distributed control and one of the first networks to implement the TCP/IP protocol suite. Both technologies became the technical fou ...

changed from NCP to the

TCP/IP The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the set of communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suit ...

protocol suite. This major change required all ARPANET nodes and interfaces to be shut down and restarted across the entire network. Jon Postel
NCP/TCP Transition Plan, RFC 801
/ref>

References

Computer jargon {{compsci-stub

See also

References