Dogpile / cache stampede protection
When a hot key expires, only one client should recompute. Everyone else waits or uses stale.
Patterns:
1. Mutex / lock around recompute (single-flight)
2. Stale-while-revalidate (serve old, refresh in background)
3. Probabilistic early expiration (jitter who refreshes first)
Related: thundering herd on the cache miss path
Scenario: Product page cache key expires under Black Friday load. Without protection, every edge node hits the DB for the same SKU. With a lock or single-flight: one recomputes, others block briefly or keep serving stale. Same idea as stampede protection in Redis/memcached clients.