A Korean flash sale at 9PM cost me $40 in a single month — not from KV reads, but from 800 concurrent Workers all racing to fetch the same product JSON from my origin in a 120ms window.
The naive KV pattern looks fine on paper: check KV, miss, fetch origin, write KV, return. At low concurrency it works. Under burst traffic, though, KV write latency was running ~80ms and my origin fetch took ~120ms. That gap was wide enough for roughly 400 Workers to independently conclude the cache was cold and hammer the origin simultaneously. It started returning 429s by 9:03PM.
The fix isn't faster KV writes — it's ensuring only one Worker ever starts the fetch. Everything else waits on the same in-flight promise. That's request coalescing.
The trick is that Workers are stateless across isolates — you can't share a Promise between them directly. But a Durable Object runs in a single-threaded JS environment, which makes a Map of in-flight promises inside a DO completely race-condition-free. The structure is straightforward:
if (!this.inflight.has(key)) {
