Flag day (computing)
Coordinated cutover for incompatible changes in software or protocols
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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]
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
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]
