How does a Circuit Breaker help reduce failures when accessing an external service?

Circuit Breaker is a pattern that helps protect a system from cascading failures when calling external services. It works like an automatic switch: if an external service starts responding with errors frequently or does not respond at all, the Circuit Breaker "opens" the circuit and temporarily blocks further requests to that service.

This reduces the load on the external service and allows the system to recover more quickly. After some time, the Circuit Breaker transitions to a "half-open" state, allowing a limited number of requests to check if the service has recovered. If the requests succeed, the circuit closes, and normal operation resumes.

An example in Java using the Resilience4j library:

CircuitBreaker circuitBreaker = CircuitBreaker.ofDefaults("myService");
Supplier<String> decoratedSupplier = CircuitBreaker
    .decorateSupplier(circuitBreaker, () -> externalService.call());

try {
    String result = decoratedSupplier.get();
} catch (CallNotPermittedException ex) {
    // Circuit Breaker is open, requests are not sent
}

Thus, the Circuit Breaker helps reduce the number of failures by preventing excessive requests to a non-working service and ensuring more stable system operation.