How circuit breakers in Resilience4j prevent cascading failures in microservices

A circuit breaker is a fault-tolerance pattern that monitors outgoing service calls and stops forwarding traffic to a failing dependency once its error rate crosses a set threshold. Instead of letting threads pile up on a dead service, the breaker returns a fast fallback response, giving the struggling dependency time to recover. The pattern operates as a three-state machine — CLOSED for normal operation, OPEN to block calls during an outage, and HALF-OPEN to probe whether the dependency has recovered. Resilience4j, a lightweight JVM fault-tolerance library, implements this via a simple annotation and YAML-based configuration in Spring Boot 3 applications. A key pitfall is that the @CircuitBreaker annotation relies on Spring AOP and silently does nothing if the spring-boot-starter-aop dependency is missing from the classpath.