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Flag day (computing)

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A flag day (or flag day cutover) 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.[1][2]

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.[3]

Origin

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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 (14 June 1966), and the name was applied by analogy to other coordinated, incompatible changeovers.[1][2] This etymology appears in the Jargon File as early as 1981.[4]

Examples

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  • On 1 January 1983, the ARPANET conducted a coordinated transition from NCP to the TCP/IP protocol suite,[5] a milestone sometimes described as a network “flag day.”[citation needed] The irreversible transition took place on a fixed date; that is, it was conducted in "flag day" style.[6]
  • "DNS Flag Day 2019" was a coordinated change beginning on 1 February 2019, in which DNS software and service providers removed certain workarounds related to EDNS handling, potentially affecting name resolution for domains hosted on non-compliant authoritative servers.[7][8] Several "DNS Flag Days," coordinating the retirement of other obsolescent features, have been held since 2019.

See also

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References

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  1. 1 2 Guy Steele (1983). "Flag day". The Hacker's Dictionary: A Guide to the Computer Underground (1st ed.). New York: Harper & Row. p. 56. ISBN 0-06-091082-8.
  2. 1 2 Eric S. Raymond (1996). "Flag day". The New Hacker's Dictionary. MIT Press. p. 192. ISBN 978-0-262-68092-9.
  3. ↑ Partridge, Craig (November 1994). "Technical Criteria for Choosing IP The Next Generation (IPng)". RFC Editor. Retrieved 25 December 2025.
  4. ↑ Bud Spurgeon (Spring 1981). Stewart Brand (ed.). "Computer Slang". Coevolution Quarterly (29). Whole Earth Catalog: 30–31.
  5. ↑ Jon Postel (November 1981). "RFC 801: NCP/TCP Transition Plan". Retrieved 2025-12-25.
  6. ↑ Barry M. Leiner; Vint Cerf; David D. Clark; Robert E. Kahn; Leonard Kleinrock; Daniel C. Lynch; Jon Postel; Lawrence G. Roberts; Stephen Wolff (October 2009). "A Brief History of the Internet". ACM SIGCOMM Computer Communication Review. 39 (5): 22–31. doi:10.1145/1629607.1629613. One of the more interesting challenges was the transition of the ARPANET host protocol from NCP to TCP/IP as of January 1, 1983. This was a "flag-day" style transition, requiring all hosts to convert simultaneously or be left having to communicate via rather ad-hoc mechanisms.
  7. ↑ "Windows Server Domain Name System (DNS) Flag Day Compliance". Microsoft Support. Retrieved 25 December 2025.
  8. ↑ "Cyber Alert: DNS Flag Day". Center for Internet Security (MS-ISAC). 30 January 2019. Retrieved 25 December 2025.
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