Circuit Breaker Pattern (Netflix Hystrix / Resilience4j).

In Simple Words: Prevent cascading failures when a downstream microservice is down. Instead of exhausting worker threads on blocking timeouts, the Circuit Breaker trips to OPEN state and fails fast.

• CLOSED: Requests flow normally. Failures increment failure counter.
• OPEN: Error threshold exceeded. Requests fail immediately with fallback without network hop.
• HALF-OPEN: After sleep window, allows trial requests. If successful, resets to CLOSED.

stateDiagram-v2
    [*] --> Closed
    Closed --> Open: Failure Rate > Threshold (e.g. 50%)
    Open --> HalfOpen: Sleep Window Elapsed (e.g. 30s)
    HalfOpen --> Closed: Trial Requests Succeed
    HalfOpen --> Open: Trial Request Fails