spring-boot-resilience4j

# Spring Boot Resilience4j Patterns

## When to Use

To implement resilience patterns in Spring Boot applications, use this skill when:
- Preventing cascading failures from external service unavailability with circuit breaker pattern
- Retrying transient failures with exponential backoff
- Rate limiting to protect services from overload or downstream service capacity constraints
- Isolating resources with bulkhead pattern to prevent thread pool exhaustion
- Adding timeout controls to async operations with time limiter
- Combining multiple patterns for comprehensive fault tolerance

Resilience4j is a lightweight, composable library for adding fault tolerance without requiring external infrastructure. It provides annotation-based patterns that integrate seamlessly with Spring Boot's AOP and Actuator.

## Instructions

### 1. Setup and Dependencies

Add Resilience4j dependencies to your project. For Maven, add to pom.xml:

<dependency>
    <groupId>io.github.resilience4j</groupId>
    <artifactId>resilience4j-spring-boot3</artifactId>
    <version>2.2.0</version> // Use latest stable version
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-aop</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-actuator</artifactId>
</dependency>

For Gradle, add to build.gradle:

implementation "io.github.resilience4j:resilience4j-spring-boot3:2.2.0"
implementation "org.springframework.boot:spring-boot-starter-aop"
implementation "org.springframework.boot:spring-boot-starter-actuator"

Enable AOP annotation processing with @EnableAspectJAutoProxy (auto-configured by Spring Boot).

### 2. Circuit Breaker Pattern

Apply @CircuitBreaker annotation to methods calling external services:

@Service
public class PaymentService {
    private final RestTemplate restTemplate;

    public PaymentService(RestTemplate restTemplate) {
        this.restTemplate = restTemplate;
    }

    @CircuitBreaker(name = "paymentService", fallbackMethod = "paymentFallback")
    public PaymentResponse processPayment(PaymentRequest request) {
        return restTemplate.postForObject("http://payment-api/process",
            request, PaymentResponse.class);
    }

    private PaymentResponse paymentFallback(PaymentRequest request, Exception ex) {
        return PaymentResponse.builder()
            .status("PENDING")
            .message("Service temporarily unavailable")
            .build();
    }
}

Configure in application.yml:

resilience4j:
  circuitbreaker:
    configs:
      default:
        registerHealthIndicator: true
        slidingWindowSize: 10
        minimumNumberOfCalls: 5
        failureRateThreshold: 50
        waitDurationInOpenState: 10s
    instances:
      paymentService:
        baseConfig: default

See @references/configuration-reference.md for complete circuit breaker configuration options.

### 3. Retry Pattern

Apply @Retry annotation for transient failure recovery:

@Service
public class ProductService {
    private final RestTemplate restTemplate;

    public ProductService(RestTemplate restTemplate) {
        this.restTemplate = restTemplate;
    }

    @Retry(name = "productService", fallbackMethod = "getProductFallback")
    public Product getProduct(Long productId) {
        return restTemplate.getForObject(
            "http://product-api/products/" + productId,
            Product.class);
    }

    private Product getProductFallback(Long productId, Exception ex) {
        return Product.builder()
            .id(productId)
            .name("Unavailable")
            .available(false)
            .build();
    }
}

Configure retry in application.yml:

resilience4j:
  retry:
    configs:
      default:
        maxAttempts: 3
        waitDuration: 500ms
        enableExponentialBackoff: true
        exponentialBackoffMultiplier: 2
    instances:
      productService:
        baseConfig: default
        maxAttempts: 5

See @references/configuration-reference.md for retry exception configuration.

### 4. Rate Limiter Pattern

Apply @RateLimiter to control request rates:

@Service
public class NotificationService {
    private final EmailClient emailClient;

    public NotificationService(EmailClient emailClient) {
        this.emailClient = emailClient;
    }

    @RateLimiter(name = "notificationService",
        fallbackMethod = "rateLimitFallback")
    public void sendEmail(EmailRequest request) {
        emailClient.send(request);
    }

    private void rateLimitFallback(EmailRequest request, Exception ex) {
        throw new RateLimitExceededException(
            "Too many requests. Please try again later.");
    }
}

Configure in application.yml:

resilience4j:
  ratelimiter:
    configs:
      default:
        registerHealthIndicator: true
        limitForPeriod: 10
        limitRefreshPeriod: 1s
        timeoutDuration: 500ms
    instances:
      notificationService:
        baseConfig: default
        limitForPeriod: 5

### 5. Bulkhead Pattern

Apply @Bulkhead to isolate resources. Use type = SEMAPHORE for synchronous methods:

@Service
public class ReportService {
    private final ReportGenerator reportGenerator;

    public ReportService(ReportGenerator reportGenerator) {
        this.reportGenerator = reportGenerator;
    }

    @Bulkhead(name = "reportService", type = Bulkhead.Type.SEMAPHORE)
    public Report generateReport(ReportRequest request) {
        return reportGenerator.generate(request);
    }
}

Use type = THREADPOOL for async/CompletableFuture methods:

@Service
public class AnalyticsService {
    @Bulkhead(name = "analyticsService", type = Bulkhead.Type.THREADPOOL)
    public CompletableFuture<AnalyticsResult> runAnalytics(
            AnalyticsRequest request) {
        return CompletableFuture.supplyAsync(() ->
            analyticsEngine.analyze(request));
    }
}

Configure in application.yml:

resilience4j:
  bulkhead:
    configs:
      default:
        maxConcurrentCalls: 10
        maxWaitDuration: 100ms
    instances:
      reportService:
        baseConfig: default
        maxConcurrentCalls: 5

  thread-pool-bulkhead:
    instances:
      analyticsService:
        maxThreadPoolSize: 8

### 6. Time Limiter Pattern

Apply @TimeLimiter to async methods to enforce timeout boundaries:

@Service
public class SearchService {
    @TimeLimiter(name = "searchService", fallbackMethod = "searchFallback")
    public CompletableFuture<SearchResults> search(SearchQuery query) {
        return CompletableFuture.supplyAsync(() ->
            searchEngine.executeSearch(query));
    }

    private CompletableFuture<SearchResults> searchFallback(
            SearchQuery query, Exception ex) {
        return CompletableFuture.completedFuture(
            SearchResults.empty("Search timed out"));
    }
}

Configure in application.yml:

resilience4j:
  timelimiter:
    configs:
      default:
        timeoutDuration: 2s
        cancelRunningFuture: true
    instances:
      searchService:
        baseConfig: default
        timeoutDuration: 3s

### 7. Combining Multiple Patterns

Stack multiple patterns on a single method for comprehensive fault tolerance:

@Service
public class OrderService {
    @CircuitBreaker(name = "orderService")
    @Retry(name = "orderService")
    @RateLimiter(name = "orderService")
    @Bulkhead(name = "orderService")
    public Order createOrder(OrderRequest request) {
        return orderClient.createOrder(request);
    }
}

Execution order: Retry → CircuitBreaker → RateLimiter → Bulkhead → Method

All patterns should reference the same named configuration instance for consistency.

### 8. Exception Handling and Monitoring

Create a global exception handler using @RestControllerAdvice:

@RestControllerAdvice
public class ResilienceExceptionHandler {

    @ExceptionHandler(CallNotPermittedException.class)
    @ResponseStatus(HttpStatus.SERVICE_UNAVAILABLE)
    public ErrorResponse handleCircuitOpen(CallNotPermittedException ex) {
        return new ErrorResponse("SERVICE_UNAVAILABLE",
            "Service currently unavailable");
    }

    @ExceptionHandler(RequestNotPermitted.class)
    @ResponseStatus(HttpStatus.TOO_MANY_REQUESTS)
    public ErrorResponse handleRateLimited(RequestNotPermitted ex) {
        return new ErrorResponse("TOO_MANY_REQUESTS",
            "Rate limit exceeded");
    }

    @ExceptionHandler(BulkheadFullException.class)
    @ResponseStatus(HttpStatus.SERVICE_UNAVAILABLE)
    public ErrorResponse handleBulkheadFull(BulkheadFullException ex) {
        return new ErrorResponse("CAPACITY_EXCEEDED",
            "Service at capacity");
    }
}

Enable Actuator endpoints for monitoring resilience patterns in application.yml:

management:
  endpoints:
    web:
      exposure:
        include: health,metrics,circuitbreakers,retries,ratelimiters
  endpoint:
    health:
      show-details: always
  health:
    circuitbreakers:
      enabled: true
    ratelimiters:
      enabled: true

Access monitoring endpoints:
- GET /actuator/health - Overall health including resilience patterns
- GET /actuator/circuitbreakers - Circuit breaker states
- GET /actuator/metrics - Custom resilience metrics

## Best Practices

- **Always provide fallback methods**: Ensure graceful degradation with meaningful responses rather than exceptions
- **Use exponential backoff for retries**: Prevent overwhelming recovering services with aggressive backoff (exponentialBackoffMultiplier: 2)
- **Choose appropriate failure thresholds**: Set failureRateThreshold between 50-70% depending on acceptable error rates
- **Use constructor injection exclusively**: Never use field injection for Resilience4j dependencies
- **Enable health indicators**: Set registerHealthIndicator: true for all patterns to integrate with Spring Boot health
- **Separate failure vs. client errors**: Retry only transient errors (network timeouts, 5xx); skip 4xx and business exceptions
- **Size bulkheads based on load**: Calculate thread pool and semaphore sizes from expected concurrent load and latency
- **Monitor and adjust**: Continuously review metrics and adjust timeouts/thresholds based on production behavior
- **Document fallback behavior**: Make fallback logic clear and predictable to users and maintainers

## Common Mistakes

Refer to references/testing-patterns.md for:
- Testing circuit breaker state transitions
- Simulating transient failures with WireMock
- Validating fallback method signatures
- Avoiding common misconfiguration errors

Refer to references/configuration-reference.md for:
- Complete property reference for all patterns
- Configuration validation rules
- Exception handling configuration

## References and Examples

- [Complete property reference and configuration patterns](references/configuration-reference.md)
- [Unit and integration testing strategies](references/testing-patterns.md)
- [Real-world e-commerce service example using all patterns](references/examples.md)
- [Resilience4j Documentation](https://resilience4j.readme.io/)
- [Spring Boot Actuator Skill](/skills/spring-boot-actuator/SKILL.md) - Monitoring resilience patterns with Actuator