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Banner: race condition where a websocket fill arrives before the REST order response

Crypto Trading Systems Bug Report

Order state machine crashes with unknown order: the fill arrived before the REST response

Ryan Pham · Sep 16, 2026 · 6 min read

Our order manager occasionally raised KeyError: unknown order when a fill event arrived on the websocket before the REST place-order call returned. It keyed orders by exchange order id, which it only learned from the REST response. Keying by client order id and buffering unknown events fixed it.

Banner: a single-lane road with one stalled truck holding up every other request behind it

Programming Languages Bug Report

FastAPI freezes all requests: a blocking call inside async def stalls the event loop

Daniel Tran · Jul 22, 2025 · 9 min read

One slow partner API call made our whole FastAPI service stop answering, health checks included, until Kubernetes restarted the pods. A synchronous requests call and a time.sleep inside async def were blocking the event loop.