Resilience4j

Introduction

Resilience4j brings circuit breakers, retries, rate limiters and bulkheads to Spring Boot — preventing cascading failures when downstream services are slow or down.

Informative example

Circuit breaker + retry on a service call:

@Service
public class PaymentClient {
    @CircuitBreaker(name = "payment", fallbackMethod = "fallback")
    @Retry(name = "payment")
    @TimeLimiter(name = "payment")
    public CompletableFuture<Receipt> charge(Order order) {
        return CompletableFuture.supplyAsync(() -> rest.post("/charge", order));
    }

    private CompletableFuture<Receipt> fallback(Order o, Throwable t) {
        return CompletableFuture.completedFuture(Receipt.pending(o.id()));
    }
}

# application.yml
resilience4j.circuitbreaker.instances.payment.slidingWindowSize=10
resilience4j.circuitbreaker.instances.payment.failureRateThreshold=50

Best practices

• Always provide a fallback — open circuit without fallback = errors.
• Tune thresholds from production metrics, not guesses.
• Combine circuit breaker + retry + timeout — not just one.

Purpose of this lesson

Master Resilience4j so you can apply it confidently in production Java code, technical interviews, and code reviews.

Step-by-step explanation

1. Understand the core idea behind Resilience4j.
2. Walk through the runnable example and tweak it in the playground.
3. Apply the pattern in a small Spring Boot or CLI exercise of your own.
4. Re-read the common mistakes and interview Q&A to lock the concept in.

Interactive workflow diagram

Debugging tips

• Circuit stuck open? Check failureRateThreshold and waitDurationInOpenState.
• Fallback not called? Method signature must match: (args..., Throwable).

Optimization strategies

• Profile before optimizing — JFR (Java Flight Recorder) and async-profiler reveal real hotspots.
• Prefer immutable data and stream pipelines over hand-rolled loops when readability matters.
• Reach for the right JDK collection (ArrayList vs LinkedList vs ArrayDeque) before writing custom data structures.

Enterprise example

Teams at Netflix, Uber and Goldman Sachs apply Resilience4j daily — usually wrapped behind Spring Boot services with observability hooks (Micrometer + OpenTelemetry).

Interview questions & answers

Summary

In this lesson you learned Resilience4j — the concept, syntax, a runnable example, and the production pitfalls to avoid. Apply it in the playground before moving on.