Circuit breaker state machine.

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:

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

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().

In JavaScript and TypeScript the options are one object, new CircuitBreaker(name, clock, { failureThreshold, failureWindow, recoveryTimeout, successThreshold, failureExceptions }) (the same defaults, failureExceptions: [Error]), call(fn, ...args) passes positional arguments only, and state is a getter that returns one of the CircuitState strings ("closed", "open", "half_open").