def get(key): if key in cache: return cache[key] else: value = db.query("SELECT * FROM items WHERE id = ?", key) // slow cache[key] = value return value Because the cache was empty, all 10,000 threads saw a at the exact same moment. They all rushed to the database.
KQR> ROW CACHE CONTENTION CHECK GETS It printed: kqr row cache contention check gets
KQR had a job: cache frequently accessed rows so the main disk could rest. For years, this worked beautifully. Until . def get(key): if key in cache: return cache[key]
But they didn’t just rush to the database — they collided at the . You see, KQR’s cache was protected by a single, global synchronized block for writes. For years, this worked beautifully
From that day on, KQR’s monitoring dashboard had a new rule: If row cache contention check gets > 1000 per second — flip on single-flight mode. And the team learned a valuable lesson: sometimes, the most dangerous lock isn’t in your database — it’s in your cache’s eagerness to help .
— KQR had a little-known diagnostic command:
She hot-patched KQR’s logic to use :