Circuit Breaker

State Machine

HALF-OPEN: success → CLOSED  |  failure → OPEN

✓ CLOSED — Requests passing through

Request History (latest → right)

🟢 accepted🔴 failed⬛ fail-fast (open)

📖 State Explanation

🎯 When to Use

Any synchronous call to an external service that can degrade — databases, microservices, third-party APIs. Prevents cascading failures.

⚖️ Trade-offs

False positives cause unnecessary outages; slow detection if window too large; need fallback behavior when open (cached data, degraded response).

💡 Interview Tip

"Netflix's Hystrix popularised circuit breakers. In OPEN state, return cached data or a degraded response — don't just error to the user."