System design interview question
"Design a circuit breaker. Service A calls service B, B starts failing, and you want A to stop calling it for a while rather than piling up timeouts. Give me the state machine, the thresholds, and tell me what goes wrong if you get them wrong."
A focused resilience-pattern question, small enough that there is nowhere to hide. The signal is in the details: what counts as a failure, how you avoid tripping on low traffic, how the half-open probe avoids re-breaking a recovering service, and — the part most candidates miss — what the caller should actually do when the circuit is open.
On Practick another engineer interviews you live for 60 minutes on a question from this bank. They have the full interviewer guide beside the call: the answers to your clarifying questions, the numbers to hand out, a ladder of follow-ups and a scoring rubric. You get the model solution once the session settles. It is free: interview someone for an hour and you have earned your own.
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