Flag day (computing)

This article is about the system administration term. For the holiday, see Flag Day.

Not to be confused with Feature toggle.

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]

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. ^[2]

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 (14 June 1966), and the name was applied by analogy to other coordinated, incompatible changeovers. ^{[1] [3]}

Examples

• On 1 January 1983, the ARPANET conducted a coordinated transition from NCP to the TCP/IP protocol suite, a milestone sometimes described as a network “flag day”. ^{[4] [5]}
• 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. ^{[6] [7]}

See also

• Backward compatibility
• Forward compatibility
• Protocol ossification

References

1. Raymond, Eric S. (1996). The New Hacker's Dictionary. MIT Press. p. 192. ISBN   978-0-262-68092-9.
2. Partridge, Craig (November 1994). "Technical Criteria for Choosing IP The Next Generation (IPng)". RFC Editor. Retrieved 25 December 2025.
3. "Multics Glossary". MIT. Retrieved 25 December 2025.
4. Postel, Jon (November 1981). "NCP/TCP Transition Plan". RFC Editor. Retrieved 25 December 2025.
5. Leiner, Barry M.; Cerf, Vinton G.; Clark, David D.; Kahn, Robert E.; Kleinrock, Leonard; Lynch, Daniel C.; Postel, Jon; Roberts, Lawrence G.; Wolff, Stephen (October 2009). "A Brief History of the Internet". ACM SIGCOMM Computer Communication Review. 39 (5): 22–31. doi:10.1145/1629607.1629613.
6. "Windows Server Domain Name System (DNS) Flag Day Compliance". Microsoft Support. Retrieved 25 December 2025.
7. "Cyber Alert: DNS Flag Day". Center for Internet Security (MS-ISAC). 30 January 2019. Retrieved 25 December 2025.

External links

• DNS Flag Day 2019