Dragon is a snoopy cache coherence protocol from Xerox PARC that uses update-based invalidation instead of invalidation: writes broadcast new values on the bus and sharers update their copies in place rather than discarding them. This favors workloads where shared lines are read frequently after writes.

Dragon became niche because invalidate-based protocols like MESI are more efficient on write-heavy workloads and lines that are rarely re-read by other cores.

Update-based approach keeps copies valid without reload.

Invalidate (MESI):  write -> sharers drop copy -> reload on next read
Update (Dragon):    write -> sharers update locally -> immediate valid