Glossary

Circuit Breaker

Circuit Breaker is a resilience pattern for distributed systems. It protects against cascading failures: if a downstream service goes down, it automatically "trips" and returns a fallback response instead of hanging.

Three states

Closed (normal operation) — requests flow through. Failure counter increments on errors
Open (breaker tripped) — requests are blocked; fallback is returned immediately. Set when failures exceed the threshold
Half-Open (probing) — after a timeout, one test request is let through. If it succeeds → Closed; if not → back to Open

Why

Without a circuit breaker: slow service A → threads in service B hang waiting → service B exhausts its thread pool → the entire cluster fails. The circuit breaker "disconnects" service A and lets the rest of the system continue operating.