Circuit Breaker: implementation and states of the algorithm.

Circuit Breaker is a pattern for increasing the resilience of distributed systems, preventing repeated calls to a failing service.

Main states:

1. Closed — all requests pass as usual.
2. Open — requests are blocked immediately to avoid overloading the non-functioning service.
3. Half-Open — after a timeout, a limited number of requests are attempted to check if the service has recovered.

Working principle:

• When the error threshold is exceeded (e.g., 5 consecutive failures), it switches to Open.
• After a specified timeout, it transitions to Half-Open and attempts requests.
• If requests succeed, it returns to Closed; otherwise, it switches back to Open.

Implementations in Java:

• Resilience4j library — modern, lightweight, supports Circuit Breaker, Rate Limiter, etc.
• Netflix Hystrix — popular but outdated.

Example of using Resilience4j CircuitBreaker:

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

Try<String> result = Try.ofSupplier(decoratedSupplier)
    .recover(throwable -> "Fallback response");

This approach helps avoid cascading failures and improves system stability.