What is the Circuit Breaker pattern and when should you use it?

Circuit Breaker prevents cascading failures by stopping calls to failing services. States: Closed (normal operation, tracking failures), Open (fails fast, doesn't call service), Half-Open (periodically tests if service recovered). Configuration: failure threshold, timeout, recovery time. Benefits: fail fast, prevent resource exhaustion, allow recovery time. Libraries: Hystrix, Resilience4j, Polly. Use for: external API calls, database connections, any unreliable dependency. Combine with: timeouts, retries with backoff, fallbacks.