Circuit Breakers

A circuit breaker is a per-backend state machine that prevents cascade failures when a backend is misbehaving:

   CLOSED ----failure_rate exceeds threshold----> OPEN
     ^                                              |
     |                                          (timeout)
     |                                              v
     +---success_rate exceeds threshold------ HALF_OPEN

States:

• CLOSED: normal operation; requests flow through; track failures
• OPEN: short-circuit immediately; return error without contacting backend
• HALF_OPEN: allow a small number of "probe" requests; based on success rate, transition to CLOSED or back to OPEN

Why this matters: when a backend is overloaded, sending more requests makes it worse (queues fill, threads exhaust). The circuit breaker says "stop trying for 30 seconds, give it room to recover".

Implementation:

This pattern came from Netflix's Hystrix. Now standard in service meshes (Istio, Linkerd) and most language frameworks (resilience4j, Polly).