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How a single expired key took Black Friday down, the maths of the herd, five defences (locking, coalescing, probabilistic expiry, background refresh, stale-while-revalidate) and how to choose, the edge cases that break them, and how Instagram and a trending-topics design keep the database safe.
Previously, on Day 2, you learned to get stale data out: TTLs, events, versioned keys, and a TTL safety net under your events.