How does a Resilience4j Circuit Breaker work together with thread pools?

A circuit breaker tracks the outcomes of recent calls using a sliding window, either count-based or time-based, and moves through three states based on that data. In the CLOSED state, calls pass through normally. If the failure rate crosses a configured threshold, the circuit trips to OPEN, at which point it fast-fails every call immediately without sending any traffic downstream at all. After a configured wait period, it moves to HALF_OPEN, where it allows a small number of test calls through to see if the downstream service has recovered, returning to CLOSED if they succeed or back to OPEN if they don't. Combining this with the bulkhead pattern, using a separate bounded thread pool per downstream dependency, means the circuit breaker wraps each call: while OPEN, it throws a CallNotPermittedException immediately without ever consuming a thread from the pool, and while HALF_OPEN, it allows only a limited amount of traffic through. It's also common to pair a circuit breaker with a Retry policy, configured not to retry while the circuit is open, and with a TimeLimiter that times out slow calls before the circuit even has a chance to trip. The overall goal is to prevent cascading failures, so that one slow or failing downstream dependency doesn't exhaust the thread pool that upstream calls also depend on.