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Code lab · World 02 · Failure-First Design

Circuit breaker state machine.

CLOSED, OPEN and HALF-OPEN with a sliding failure window, a recovery timer on a fake clock, and business errors that don't trip it.

Medium~35 min3 example + 10 hidden tests+90 XPJump to the code ↓
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PractisesW02 · D31.2 The Three StatesW02 · D32.2 Basic Implementation

When the bank API is down, every call that waits 3 seconds for a timeout holds a thread, and the payment service dies with it. A circuit breaker notices the failures and fails fast instead (Day 3, chapter 1). Build the lesson's breaker from section 2.2:

From To When
CLOSED OPEN failures inside the window reach failure_threshold
OPEN HALF_OPEN recovery_timeout seconds have passed since the last failure
HALF_OPEN CLOSED success_threshold trial calls succeed
HALF_OPEN OPEN a trial call fails

Implement CircuitBreaker(name, clock, failure_threshold=5, failure_window=60, recovery_timeout=30, success_threshold=1, failure_exceptions=(Exception,)):

Member Does
call(func, *args, **kwargs) runs func and returns its result, re-raising its exceptions unchanged. While OPEN it raises CircuitOpenError without calling func
state (property) the current CircuitState, after any time-based transition

The state machine, exactly as the lesson implements it:

  • CLOSED: record the time of each failure. Drop failures older than failure_window seconds. When the failures still in the window reach failure_threshold, open. A success in CLOSED does not clear the window: it is a time window, not a consecutive-failure counter.
  • OPEN: once recovery_timeout seconds have passed since the last failure, move to HALF_OPEN (checked lazily, on call or state). Rejected calls don't restart the timer.
  • HALF_OPEN: let calls through as trials. success_threshold successes close the circuit and clear the failures. One failure re-opens it and restarts the timer.
  • Only exceptions that are instances of failure_exceptions count as failures. Anything else (say, a declined card) propagates to the caller and is neither a failure nor a success: the service answered correctly (Day 3, "What counts as a failure?").
  • Read time only through clock.now().
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