Lecture
A flag day, as used in system administration, is a change that requires a complete restart or conversion of a significant body of software or data. The change is large and expensive, and—in the event of failure—is also difficult and costly to undo.
The situation can arise when there are limits on the backward compatibility and forward compatibility between components of a system, which then requires that updates be performed almost simultaneously (during a «flag day cutover») in order for the system to function after the update. This contrasts with the method of gradual, phased upgrading, which avoids the interruption of service caused by en masse updates.
This system terminology arose from a major change to the ASCII definition of the Multics operating system, which was scheduled for the U.S. holiday Flag Day, June 14, 1966.
Another historic flag day was January 1, 1983, when the ARPANET switched from NCP to the TCP/IP protocol suite. This major change required shutting down and restarting all ARPANET nodes and interfaces across the entire network.
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