Primary and secondary circuit breakers with failover

A service reads from a primary backend and can fail over to a secondary backend (for example a leader database and a follower). Put a circuit breaker in front of each backend so that a backend that keeps failing is skipped instead of being called again and again.

Implement two classes.

CircuitBreaker(threshold: int, cooldown: float) guards one backend and has three states:

• CLOSED: calls are allowed. Each failure adds 1 to a consecutive-failure count, and a success resets the count to 0. When the count reaches threshold, the breaker becomes OPEN.
• OPEN: calls are refused without calling the backend. Once cooldown seconds have passed since it opened, the breaker becomes HALF_OPEN.
• HALF_OPEN: one trial call is allowed. If it succeeds, the breaker becomes CLOSED. If it fails, the breaker becomes OPEN again and the cooldown restarts.

DualCircuitBreaker(primary, secondary, threshold, cooldown), where each backend is a function request -> response that raises an exception on failure:

• call(request, now) -> (backend_name, response): try the primary if its breaker allows it. If the primary is refused or fails, try the secondary the same way. Record every real call's outcome on that backend's breaker. If neither backend returns a response, raise AllBackendsUnavailable. When both breakers are open, raise it without calling either backend.
• states() -> dict returns each breaker's current state.

Time is passed in as now (seconds) so the behaviour is deterministic.

Constraints: threshold >= 1 and cooldown > 0. Calls arrive one at a time with non-decreasing now.

Example 1 (threshold=2, cooldown=10). The primary is down and the secondary is up.

call("x", 0) -> ("secondary", ...)   primary: 1 failure, closed
call("y", 1) -> ("secondary", ...)   primary: 2 failures, now open
call("z", 2) -> ("secondary", ...)   primary is not called

Example 2. Continue from Example 1. The primary recovers at t = 5.

call("w", 8)  -> ("secondary", ...)  primary still open (opened at 1, cooldown 10)
call("v", 11) -> ("primary", ...)    trial call succeeds, primary closed again

Reconstructed from the reported question; details may differ from what interviewers ask.