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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.

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. == Examples ==
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”. • "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. == See also ==
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