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Build a quota manager: per-key token buckets with lazy refill, reservations, and refund on failure

Sliding-window limiters count requests; an API that bills by tokens needs to reserve a variable cost up front and refund it if the call fails. The trick is lazy refill: never run a timer, compute the current balance from elapsed time on each check.

Updated Aug 2026 · Grounded in real Forward Deployed Engineer interview loops and written to a senior-engineer editorial bar.

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