Written from production.
One production failure per post, dissected in 10–15 minutes.
Idempotency keys that survive concurrent retries
A client times out, retries — and the customer is charged twice
▶Cache stampede: one expired key, one dead database
A hot key expires — a thousand requests rebuild it against the database at once
▶The five-second ALTER TABLE that took the site down
"Instant" DDL queued behind one long transaction — and every query queued behind it
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