Explain circuit breaker states and the specific failure it prevents.

It prevents cascading failure: when a downstream is slow/down, callers pile up threads waiting on timeouts until the caller itself exhausts its pool and falls over, taking its own callers with it.

CLOSED passes calls through while counting failures over a sliding window; OPEN fast-fails without a network call once a failure-rate threshold trips; after a cooldown it goes HALF-OPEN and lets a few probes through, closing on success or reopening on failure. The breaker must be paired with sane timeouts (the original cause), bulkheads, and a fallback for graceful degradation — a breaker with infinite timeouts still hangs.