Frequency Without Forgetting

Practical implementations add decay, so counters age downward and yesterday's popularity stops pinning entries in place

It resists a sequential scan only when every key carries an explicit TTL, because otherwise the scanned keys have no way to leave the cache

A key that was extremely hot during a one-time event can occupy space indefinitely, because its counter stays high after the traffic stops

A newly inserted key starts near zero and can be evicted almost immediately, even when it is about to become the hottest key

It evicts the entry whose last access is oldest, which is why it has to keep every entry in a recency-ordered list rather than a counter per key