Circuit breaker

A circuit breaker tracks failure rate to a dependency. Closed state allows calls; open state fails fast without loading the sick service; half-open probes occasional calls to detect recovery. This preserves caller resources and prevents cascading failure.

Trade-off: fast failure and protection versus stale open state blocking legitimate traffic after recovery.

On interviews: describe state transitions and thresholds; combine breaker with fallback and bulkhead.

Common pitfalls: breaker on every call without shared state; thresholds too sensitive; no half-open probes.

Checklist:

• Configure failure rate and sliding window.
• Fail fast with clear errors when open.
• Probe recovery in half-open state.
• Expose breaker metrics on dashboards.