Implement a Circuit Breaker for Backend Servers

Problem: Implement a Circuit Breaker for Backend Servers A load balancer distributes requests across multiple backend servers. Implement a circuit breaker used independently for each backend so that traffic is not continuously sent to unhealthy servers. The breaker has three states: CLOSED: requests are allowed normally. OPEN: requests are rejected and no traffic is sent to that backend. HALF_OPEN: after the cooldown period, a limited probe request is allowed; success recovers the backend, while failure opens the breaker again. Design and implement: logic for deciding whether a request may be sent to a backend; state updates after request success or failure; a consecutive-failure threshold; an OPEN cooldown period; probing and transitions in HALF_OPEN; a thread-safety or concurrency strategy. Explain time complexity, key state fields, and how this component integrates with backend selection in the load balancer. Example behavior: With a failure threshold of 3, three consecutive failures open a backend's breaker. The load balancer must not select that backend before the cooldown expires. After cooldown, one probe request is allowed. A successful probe returns the breaker to CLOSED; a failed probe returns it to OPEN.