One verdict change. Fifty cache misses a second.

transcript

A scam number. 50 lookups a second. At 10:22 its verdict changes. So you delete the cache entry. Wrong move.

Delete it, and every lookup misses. All at once. Fifty a second, straight to the store. A stampede.

The busiest numbers aren't even scams. A courier's: 2,000 lookups a second. So keep 8 copies, on 8 nodes. Read one at random. Now each node sees 250 a second. Easy.

But changes still hurt. Delete, and every reader misses. So overwrite instead. Version 43 over 42, in every copy. No reader ever misses. A late 42? Ignored.

But how do you know a number is hot? The spam pipeline already counts calls per number, per minute. Above a threshold, it writes the copies. Before the herd arrives. Heat and spam are the same measurement.

Hot key changing? Don't delete it. Spread it across copies. Spot the heat on the write path. Overwrite every copy with a newer version.

Full breakdown in the lecture.