LRU with TTL

Combining LRU eviction with time-based expiry:

TTL handling options:

Lazy — only check on access. Cheap; expired entries linger until evicted normally.

Active — background thread periodically scans for expired. Predictable memory; CPU cost.

Sorted by expiry — maintain a min-heap by expires_at. O(log n) for insertion; O(1) to peek the next-to-expire. Used by Redis-style sorted-set TTL.

Most caches do lazy + occasional sweeps. Memcached, Redis, Caffeine all blend.