distributed systems / LEVEL 4
THEORY / RETRIEVAL
Closed admits ordinary work and counts consecutive failures. Reaching the threshold opens the circuit so calls fail fast instead of amplifying an unhealthy dependency. After the cooldown, HalfOpen admits one controlled probe: success closes the circuit and failure starts a new open interval. A probe stampede defeats the recovery boundary.
Requests already in flight when a circuit opens may finish much later. Without a generation token, an old success can close a newer Open cycle or an old failure can reopen a recovered circuit. A move-only permit carries the admission generation, and record consumes it so stale completions become explicit no-ops.
A breaker estimates dependency health from recent outcomes. Timeouts bound one attempt, retry budgets bound amplification, rate limiters control admission and bulkheads isolate capacity. Production systems compose these controls and define which failures count; the exercise isolates the state-machine contract so every transition is deterministic.
CHECKPOINT
Select every applicable option. Credit requires an exact set match.
02 / DEBUGGING
Repair the circuit breaker without changing its public API. Closed must open at the exact failure threshold; Open must admit exactly one probe at the exact cooldown boundary; failed probes reopen from their completion time; and stale permits must be ignored before they can advance logical time or mutate a newer generation.
Runner is waiting for a submission.