Circuit breaker
Resilience pattern — when a downstream service is failing, stop calling it for a while instead of hammering a wounded system.
Three states: closed (normal traffic), open (failure rate over threshold → short-circuit, fail fast), half-open (let a few probes through to see if the service is back). Popularised by Netflix Hystrix (now in maintenance mode); today's common choices are Polly (.NET), Resilience4j (JVM), failsafe-go, or service mesh policies in Istio/Linkerd.
Protects against cascade failures and thundering herd when the dependency comes back. Always pair with timeouts, retries (with jitter) and bulkheads so you don't exhaust your thread pool.