How does the Circuit Breaker pattern work and what are its three states?

The Circuit Breaker pattern prevents cascading failures by detecting when a downstream service is unavailable and fast-failing subsequent calls instead of letting them queue up and exhaust threads. It is named after the electrical circuit breaker that trips when current exceeds a safe threshold.

The pattern has three states:

• Closed (normal) — calls pass through to the downstream service. The breaker counts failures. When the failure rate (or failure count) exceeds a configured threshold within a time window, the breaker trips to Open.
• Open — all calls are immediately rejected with an error (or the Fallback is invoked) without contacting the downstream service. A timer starts. This gives the failing service time to recover without being bombarded with traffic.
• Half-Open — after the timer expires, the breaker allows a limited number of probe requests through. If they succeed, the breaker transitions back to Closed. If they fail, it returns to Open.

// Resilience4j Circuit Breaker (Java)
CircuitBreakerConfig config = CircuitBreakerConfig.custom()
    .failureRateThreshold(50)          // open if >50% fail
    .waitDurationInOpenState(Duration.ofSeconds(30))
    .permittedNumberOfCallsInHalfOpenState(5)
    .slidingWindowSize(10)
    .build();

CircuitBreaker cb = CircuitBreakerRegistry.of(config)
                        .circuitBreaker("inventoryService");

Supplier<Stock> decorated = CircuitBreaker
    .decorateSupplier(cb, () -> inventoryClient.getStock(sku));

Try<Stock> result = Try.ofSupplier(decorated)
    .recover(CallNotPermittedException.class, e -> defaultStock());

The circuit breaker is most effective when combined with a Fallback (Q31) — when the circuit is Open, the fallback returns a cached or degraded response so the caller can still serve the request in a degraded mode rather than propagating a hard error to the user.

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