At 9:00:00 AM, a surge of traffic hit. Every user, in every time zone, suddenly demanded the same piece of data: the flash sale metadata for item ID #42.

def get(key): if key in cache: return cache[key] else: // Only one thread goes to DB; others wait for its result return cache.load_or_wait(key) Within 30 seconds, the contention ratio dropped from 1.00 to 0.001.

KQR had a job: cache frequently accessed rows so the main disk could rest. For years, this worked beautifully. Until .

— KQR’s row cache for item:42 expired. 9:00:02 — 10,000 concurrent GET requests arrived simultaneously.