Circuit Breaker Pattern

The Circuit Breaker Pattern is a resilience design pattern used in distributed systems and microservices to prevent cascading failures when a dependent service is unavailable or slow.

Why It’s Needed

In microservices, one failing service can overload others if retries keep happening. Without control, this may lead to system-wide outages.

How It Works

A circuit breaker acts like an electrical fuse:

1. Closed State

   • Requests flow normally.
   • Failures are monitored.

2. Open State

   • After a threshold of failures is reached, the breaker "opens."
   • Requests fail fast without calling the service.

3. Half-Open State

   • After a cooldown, a limited number of test requests are allowed.
   • If successful → back to Closed.
   • If failing → stay Open.

Benefits

• Fail Fast: Prevents wasting resources on failing services.
• Stability: Stops cascading failures across microservices.
• Recovery: Gradually restores requests when services recover.

Example (Java – Resilience4j)

CircuitBreaker circuitBreaker = CircuitBreaker.ofDefaults("serviceA");

Supplier<String> decoratedSupplier = 
  CircuitBreaker.decorateSupplier(circuitBreaker, () -> callRemoteService());

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

Real-World Usage

• Netflix Hystrix (legacy, now replaced by Resilience4j).
• Resilience4j for Java microservices.
• Spring Cloud supports circuit breakers in distributed systems.

Interview Tip

If asked in a system design interview: “How would you prevent cascading failures when a microservice is down?” → Mention the Circuit Breaker Pattern with fallback logic.