# Spring Cloud Gateway Interview: Routing, Filters and Load Balancing

> Master Spring Cloud Gateway for technical interviews: 12 questions covering routing, filters, load balancing and API Gateway patterns with code examples.

- Published: 2026-03-20
- Updated: 2026-03-31
- Author: SharpSkill
- Tags: spring cloud gateway, microservices, api gateway, routing, technical interview
- Reading time: 15 min

---

Spring Cloud Gateway is the reference solution for implementing an API Gateway in a Spring microservices architecture. Technical interviews evaluate the ability to configure routing, create custom filters, and manage load balancing effectively.

> **Interview preparation tip**
>
> Recruiters test understanding of Gateway patterns: centralized authentication, rate limiting, and circuit breaker. Being able to explain why Spring Cloud Gateway over alternatives makes the difference.

## Architecture and Spring Cloud Gateway Fundamentals

### Question 1: What is Spring Cloud Gateway and why use it?

Spring Cloud Gateway is a reactive API Gateway built on Spring WebFlux and Project Reactor. It serves as the single entry point for all requests to microservices, providing routing, filtering, and load balancing capabilities.

```java
// GatewayApplication.java
// Basic Spring Cloud Gateway setup
@SpringBootApplication
public class GatewayApplication {

    public static void main(String[] args) {
        // Starts reactive Netty server (not Tomcat)
        SpringApplication.run(GatewayApplication.class, args);
    }
}
```

```yaml
# application.yml
# Minimal gateway configuration
spring:
  cloud:
    gateway:
      routes:
        # Route to user service
        - id: user-service
          uri: http://localhost:8081
          predicates:
            - Path=/api/users/**
        # Route to order service
        - id: order-service
          uri: http://localhost:8082
          predicates:
            - Path=/api/orders/**
```

The main advantages of Spring Cloud Gateway include: non-blocking architecture for high performance, native integration with the Spring Cloud ecosystem, and support for modern reactive patterns.

### Question 2: Explain Route, Predicate, and Filter concepts

Three fundamental concepts structure Spring Cloud Gateway: Routes define destinations, Predicates determine when to apply a route, and Filters modify requests/responses.

```java
// RouteConfiguration.java
// Programmatic route configuration
@Configuration
public class RouteConfiguration {

    @Bean
    public RouteLocator customRouteLocator(RouteLocatorBuilder builder) {
        return builder.routes()
            // Route with multiple predicates
            .route("product-service", r -> r
                // Predicate: URL path
                .path("/api/products/**")
                // Predicate: HTTP method
                .and()
                .method(HttpMethod.GET, HttpMethod.POST)
                // Predicate: header present
                .and()
                .header("X-Api-Version", "v2")
                // Filter: path rewrite
                .filters(f -> f
                    .rewritePath("/api/products/(?<segment>.*)", "/products/${segment}")
                    // Filter: add header
                    .addRequestHeader("X-Gateway-Source", "spring-cloud-gateway")
                )
                // Destination URI
                .uri("http://localhost:8083"))
            .build();
    }
}
```

The processing flow follows this sequence:

```text
Incoming request
    │
    ▼
┌─────────────────┐
│   Predicates    │ → Evaluate conditions (path, method, header...)
└────────┬────────┘
         │ Match found
         ▼
┌─────────────────┐
│  Pre-Filters    │ → Modify request before routing
└────────┬────────┘
         │
         ▼
┌─────────────────┐
│   HTTP Proxy    │ → Forward to target service
└────────┬────────┘
         │
         ▼
┌─────────────────┐
│  Post-Filters   │ → Modify response before returning to client
└────────┬────────┘
         │
         ▼
    Client response
```

### Question 3: What are the most commonly used predicates?

Spring Cloud Gateway provides many built-in predicates for varied routing conditions. Combining multiple predicates enables sophisticated routing rules.

```yaml
# application.yml
# Common predicate examples
spring:
  cloud:
    gateway:
      routes:
        # Path routing with variable capture
        - id: user-details
          uri: http://user-service
          predicates:
            - Path=/users/{userId}

        # HTTP method routing
        - id: user-create
          uri: http://user-service
          predicates:
            - Path=/users
            - Method=POST

        # Header-based routing
        - id: mobile-api
          uri: http://mobile-service
          predicates:
            - Header=X-Client-Type, mobile

        # Query parameter routing
        - id: search-api
          uri: http://search-service
          predicates:
            - Query=q

        # Host-based routing
        - id: admin-portal
          uri: http://admin-service
          predicates:
            - Host=admin.example.com

        # Time-based routing
        - id: maintenance-mode
          uri: http://maintenance-service
          predicates:
            - Between=2026-03-20T02:00:00Z,2026-03-20T04:00:00Z
```

```java
// CustomPredicateFactory.java
// Creating a custom predicate
@Component
public class ApiKeyRoutePredicateFactory
    extends AbstractRoutePredicateFactory<ApiKeyRoutePredicateFactory.Config> {

    public ApiKeyRoutePredicateFactory() {
        super(Config.class);
    }

    @Override
    public Predicate<ServerWebExchange> apply(Config config) {
        return exchange -> {
            // Check API key presence and validity
            String apiKey = exchange.getRequest()
                .getHeaders()
                .getFirst("X-Api-Key");

            return apiKey != null && config.getValidKeys().contains(apiKey);
        };
    }

    @Validated
    public static class Config {
        private List<String> validKeys = new ArrayList<>();

        public List<String> getValidKeys() {
            return validKeys;
        }

        public void setValidKeys(List<String> validKeys) {
            this.validKeys = validKeys;
        }
    }
}
```

> **Predicate order**
>
> Predicate order does not affect evaluation, but route order matters. Routes are evaluated sequentially, and the first match is used.

## Filters and Request Transformation

### Question 4: How do pre and post-processing filters work?

GatewayFilter filters execute in an ordered chain. "Pre" filters modify the request before routing, "post" filters modify the response after receiving from the target service.

```java
// LoggingFilter.java
// Global logging filter
@Component
@Slf4j
public class LoggingFilter implements GlobalFilter, Ordered {

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        // PRE-FILTER: before routing
        String requestId = UUID.randomUUID().toString();
        long startTime = System.currentTimeMillis();

        log.info("Request {} started: {} {}",
            requestId,
            exchange.getRequest().getMethod(),
            exchange.getRequest().getPath());

        // Add request ID to headers
        ServerHttpRequest modifiedRequest = exchange.getRequest()
            .mutate()
            .header("X-Request-Id", requestId)
            .build();

        // Continue the chain and handle response
        return chain.filter(exchange.mutate().request(modifiedRequest).build())
            .then(Mono.fromRunnable(() -> {
                // POST-FILTER: after response
                long duration = System.currentTimeMillis() - startTime;
                HttpStatusCode status = exchange.getResponse().getStatusCode();

                log.info("Request {} completed: status={}, duration={}ms",
                    requestId, status, duration);
            }));
    }

    @Override
    public int getOrder() {
        // Negative order = executed first
        return -1;
    }
}
```

```java
// AuthenticationFilter.java
// JWT authentication filter
@Component
@RequiredArgsConstructor
public class AuthenticationFilter implements GatewayFilter {

    private final JwtTokenValidator tokenValidator;

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        String authHeader = exchange.getRequest()
            .getHeaders()
            .getFirst(HttpHeaders.AUTHORIZATION);

        // Check token presence
        if (authHeader == null || !authHeader.startsWith("Bearer ")) {
            return handleUnauthorized(exchange, "Missing or invalid Authorization header");
        }

        String token = authHeader.substring(7);

        // Validate token reactively
        return tokenValidator.validate(token)
            .flatMap(claims -> {
                // Enrich request with user information
                ServerHttpRequest enrichedRequest = exchange.getRequest()
                    .mutate()
                    .header("X-User-Id", claims.getSubject())
                    .header("X-User-Roles", String.join(",", claims.getRoles()))
                    .build();

                return chain.filter(exchange.mutate().request(enrichedRequest).build());
            })
            .onErrorResume(e -> handleUnauthorized(exchange, e.getMessage()));
    }

    private Mono<Void> handleUnauthorized(ServerWebExchange exchange, String message) {
        exchange.getResponse().setStatusCode(HttpStatus.UNAUTHORIZED);
        exchange.getResponse().getHeaders().setContentType(MediaType.APPLICATION_JSON);

        String body = String.format("{\"error\": \"%s\"}", message);
        DataBuffer buffer = exchange.getResponse()
            .bufferFactory()
            .wrap(body.getBytes(StandardCharsets.UTF_8));

        return exchange.getResponse().writeWith(Mono.just(buffer));
    }
}
```

### Question 5: What are the most useful built-in filters?

Spring Cloud Gateway provides many built-in filters covering common use cases: URL rewriting, header modification, retry, and circuit breaker.

```yaml
# application.yml
# Common built-in filters
spring:
  cloud:
    gateway:
      routes:
        - id: order-service
          uri: lb://order-service
          predicates:
            - Path=/api/orders/**
          filters:
            # Path rewriting
            - RewritePath=/api/orders/(?<segment>.*), /orders/${segment}

            # Add request headers
            - AddRequestHeader=X-Gateway-Version, 1.0

            # Remove sensitive response headers
            - RemoveResponseHeader=X-Powered-By
            - RemoveResponseHeader=Server

            # Path prefix
            - PrefixPath=/v2

            # Strip prefix
            - StripPrefix=1

            # Automatic retry on errors
            - name: Retry
              args:
                retries: 3
                statuses: BAD_GATEWAY,SERVICE_UNAVAILABLE
                methods: GET
                backoff:
                  firstBackoff: 100ms
                  maxBackoff: 500ms
                  factor: 2

            # Rate limiting
            - name: RequestRateLimiter
              args:
                redis-rate-limiter.replenishRate: 10
                redis-rate-limiter.burstCapacity: 20
                key-resolver: "#{@userKeyResolver}"
```

```java
// RateLimiterConfiguration.java
// Per-user rate limiter configuration
@Configuration
public class RateLimiterConfiguration {

    @Bean
    public KeyResolver userKeyResolver() {
        // Limit by authenticated user
        return exchange -> Mono.just(
            exchange.getRequest()
                .getHeaders()
                .getFirst("X-User-Id")
        ).defaultIfEmpty("anonymous");
    }

    @Bean
    public KeyResolver ipKeyResolver() {
        // Limit by IP address
        return exchange -> Mono.just(
            Objects.requireNonNull(exchange.getRequest()
                .getRemoteAddress())
                .getAddress()
                .getHostAddress()
        );
    }
}
```

### Question 6: How to implement a body modification filter?

Modifying the request or response body requires a specific approach with ModifyRequestBodyGatewayFilterFactory or ModifyResponseBodyGatewayFilterFactory.

```java
// RequestBodyModificationFilter.java
// Request body modification filter
@Component
@RequiredArgsConstructor
public class RequestBodyModificationFilter implements GlobalFilter, Ordered {

    private final ObjectMapper objectMapper;

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        // Only modify POST/PUT requests with JSON
        if (!isJsonRequest(exchange)) {
            return chain.filter(exchange);
        }

        return DataBufferUtils.join(exchange.getRequest().getBody())
            .flatMap(dataBuffer -> {
                byte[] bytes = new byte[dataBuffer.readableByteCount()];
                dataBuffer.read(bytes);
                DataBufferUtils.release(dataBuffer);

                try {
                    // Parse and modify JSON
                    Map<String, Object> body = objectMapper.readValue(
                        bytes,
                        new TypeReference<Map<String, Object>>() {}
                    );

                    // Add metadata
                    body.put("processedAt", Instant.now().toString());
                    body.put("gatewayVersion", "1.0");

                    byte[] modifiedBytes = objectMapper.writeValueAsBytes(body);

                    // Create new request with modified body
                    ServerHttpRequest modifiedRequest = new ServerHttpRequestDecorator(
                        exchange.getRequest()
                    ) {
                        @Override
                        public Flux<DataBuffer> getBody() {
                            return Flux.just(
                                exchange.getResponse()
                                    .bufferFactory()
                                    .wrap(modifiedBytes)
                            );
                        }

                        @Override
                        public HttpHeaders getHeaders() {
                            HttpHeaders headers = new HttpHeaders();
                            headers.putAll(super.getHeaders());
                            headers.setContentLength(modifiedBytes.length);
                            return headers;
                        }
                    };

                    return chain.filter(exchange.mutate().request(modifiedRequest).build());

                } catch (IOException e) {
                    return Mono.error(new RuntimeException("Failed to parse request body", e));
                }
            });
    }

    private boolean isJsonRequest(ServerWebExchange exchange) {
        MediaType contentType = exchange.getRequest().getHeaders().getContentType();
        return contentType != null &&
               contentType.isCompatibleWith(MediaType.APPLICATION_JSON);
    }

    @Override
    public int getOrder() {
        return Ordered.HIGHEST_PRECEDENCE;
    }
}
```

```java
// ResponseBodyModificationConfig.java
// Configuration for modifying responses
@Configuration
public class ResponseBodyModificationConfig {

    @Bean
    public RouteLocator responseModifyingRoutes(
        RouteLocatorBuilder builder,
        ModifyResponseBodyGatewayFilterFactory modifyResponseBodyFilter
    ) {
        return builder.routes()
            .route("modify-response", r -> r
                .path("/api/users/**")
                .filters(f -> f.modifyResponseBody(
                    String.class,
                    String.class,
                    MediaType.APPLICATION_JSON_VALUE,
                    (exchange, responseBody) -> {
                        // Wrap response in standard format
                        return Mono.just(String.format(
                            "{\"success\": true, \"data\": %s, \"timestamp\": \"%s\"}",
                            responseBody,
                            Instant.now()
                        ));
                    }
                ))
                .uri("lb://user-service"))
            .build();
    }
}
```

## Load Balancing and Resilience

### Question 7: How to configure load balancing with Spring Cloud LoadBalancer?

Spring Cloud Gateway integrates with Spring Cloud LoadBalancer to distribute traffic between service instances. The `lb://` URI scheme activates automatic load balancing.

```yaml
# application.yml
# Load balancing configuration
spring:
  cloud:
    gateway:
      routes:
        - id: user-service
          # lb:// activates load balancing
          uri: lb://user-service
          predicates:
            - Path=/api/users/**

    # Load balancer configuration
    loadbalancer:
      ribbon:
        enabled: false  # Use Spring Cloud LoadBalancer (not Ribbon)

      # Per-service configuration
      configurations: default

      # Health check for load balancing
      health-check:
        path:
          user-service: /actuator/health
        interval: 10s

# Service discovery (Eureka or other)
eureka:
  client:
    serviceUrl:
      defaultZone: http://localhost:8761/eureka/
```

```java
// LoadBalancerConfiguration.java
// Custom load balancer configuration
@Configuration
@LoadBalancerClients(defaultConfiguration = CustomLoadBalancerConfig.class)
public class LoadBalancerConfiguration {
}

// CustomLoadBalancerConfig.java
// Custom load balancing strategy
public class CustomLoadBalancerConfig {

    @Bean
    public ReactorLoadBalancer<ServiceInstance> loadBalancer(
        Environment environment,
        LoadBalancerClientFactory clientFactory
    ) {
        String serviceId = environment.getProperty(
            LoadBalancerClientFactory.PROPERTY_NAME
        );

        // Use Round Robin by default
        return new RoundRobinLoadBalancer(
            clientFactory.getLazyProvider(serviceId, ServiceInstanceListSupplier.class),
            serviceId
        );
    }

    @Bean
    public ServiceInstanceListSupplier serviceInstanceListSupplier(
        ConfigurableApplicationContext context
    ) {
        // Add health check to instances
        return ServiceInstanceListSupplier.builder()
            .withDiscoveryClient()
            .withHealthChecks()
            .withCaching()
            .build(context);
    }
}
```

```java
// WeightedLoadBalancer.java
// Weighted load balancer
@Component
@RequiredArgsConstructor
public class WeightedLoadBalancer implements ReactorServiceInstanceLoadBalancer {

    private final String serviceId;
    private final ObjectProvider<ServiceInstanceListSupplier> supplierProvider;
    private final Random random = new Random();

    @Override
    public Mono<Response<ServiceInstance>> choose(Request request) {
        return supplierProvider.getIfAvailable()
            .get()
            .next()
            .map(instances -> {
                if (instances.isEmpty()) {
                    return new EmptyResponse();
                }

                // Calculate weights based on metadata
                List<WeightedInstance> weighted = instances.stream()
                    .map(instance -> {
                        int weight = Integer.parseInt(
                            instance.getMetadata().getOrDefault("weight", "1")
                        );
                        return new WeightedInstance(instance, weight);
                    })
                    .toList();

                // Weighted selection
                int totalWeight = weighted.stream()
                    .mapToInt(WeightedInstance::weight)
                    .sum();

                int randomWeight = random.nextInt(totalWeight);
                int currentWeight = 0;

                for (WeightedInstance wi : weighted) {
                    currentWeight += wi.weight();
                    if (randomWeight < currentWeight) {
                        return new DefaultResponse(wi.instance());
                    }
                }

                return new DefaultResponse(weighted.get(0).instance());
            });
    }

    private record WeightedInstance(ServiceInstance instance, int weight) {}
}
```

### Question 8: How to implement circuit breaker in the gateway?

The circuit breaker protects against cascading failures. Spring Cloud Gateway integrates with Resilience4j for advanced failure management.

```yaml
# application.yml
# Resilience4j Circuit Breaker configuration
spring:
  cloud:
    gateway:
      routes:
        - id: order-service
          uri: lb://order-service
          predicates:
            - Path=/api/orders/**
          filters:
            # Circuit breaker with fallback
            - name: CircuitBreaker
              args:
                name: orderServiceCB
                fallbackUri: forward:/fallback/orders

resilience4j:
  circuitbreaker:
    configs:
      default:
        # Number of requests evaluated
        slidingWindowSize: 10
        # Failure threshold to open circuit
        failureRateThreshold: 50
        # Duration circuit stays open before retry
        waitDurationInOpenState: 30s
        # Requests allowed in half-open
        permittedNumberOfCallsInHalfOpenState: 3
        # Automatic transitions
        automaticTransitionFromOpenToHalfOpenEnabled: true

    instances:
      orderServiceCB:
        baseConfig: default
        failureRateThreshold: 60

  timelimiter:
    configs:
      default:
        timeoutDuration: 5s

    instances:
      orderServiceCB:
        timeoutDuration: 3s
```

```java
// FallbackController.java
// Fallback controller
@RestController
@RequestMapping("/fallback")
@Slf4j
public class FallbackController {

    @GetMapping("/orders")
    public Mono<ResponseEntity<Map<String, Object>>> ordersFallback(
        ServerWebExchange exchange
    ) {
        log.warn("Circuit breaker activated for orders service");

        Map<String, Object> response = Map.of(
            "success", false,
            "error", "Service temporarily unavailable",
            "code", "SERVICE_UNAVAILABLE",
            "retryAfter", 30
        );

        return Mono.just(ResponseEntity
            .status(HttpStatus.SERVICE_UNAVAILABLE)
            .body(response));
    }

    @PostMapping("/orders")
    public Mono<ResponseEntity<Map<String, Object>>> ordersPostFallback() {
        Map<String, Object> response = Map.of(
            "success", false,
            "error", "Order creation temporarily unavailable",
            "code", "SERVICE_UNAVAILABLE",
            "message", "Please try again later"
        );

        return Mono.just(ResponseEntity
            .status(HttpStatus.SERVICE_UNAVAILABLE)
            .body(response));
    }
}
```

```java
// CircuitBreakerEventListener.java
// Circuit breaker event monitoring
@Component
@Slf4j
@RequiredArgsConstructor
public class CircuitBreakerEventListener {

    private final MeterRegistry meterRegistry;

    @EventListener
    public void onCircuitBreakerStateTransition(
        CircuitBreakerOnStateTransitionEvent event
    ) {
        CircuitBreaker.StateTransition transition = event.getStateTransition();

        log.info("Circuit breaker {} state changed: {} -> {}",
            event.getCircuitBreakerName(),
            transition.getFromState(),
            transition.getToState());

        // Micrometer metric
        meterRegistry.counter(
            "circuit_breaker.state_transition",
            "name", event.getCircuitBreakerName(),
            "from", transition.getFromState().name(),
            "to", transition.getToState().name()
        ).increment();
    }

    @EventListener
    public void onCircuitBreakerFailure(CircuitBreakerOnErrorEvent event) {
        log.error("Circuit breaker {} error: {}",
            event.getCircuitBreakerName(),
            event.getThrowable().getMessage());
    }
}
```

> **Timeout and circuit breaker**
>
> Configure consistent timeout between circuit breaker and HTTP client. A timeout that is too long blocks threads, while one that is too short triggers false positives.

### Question 9: How to implement retry with exponential backoff?

Smart retry with exponential backoff avoids overwhelming a struggling service while maximizing success chances.

```yaml
# application.yml
# Retry with backoff configuration
spring:
  cloud:
    gateway:
      routes:
        - id: payment-service
          uri: lb://payment-service
          predicates:
            - Path=/api/payments/**
          filters:
            - name: Retry
              args:
                # Number of attempts
                retries: 3
                # HTTP codes triggering retry
                statuses: BAD_GATEWAY,SERVICE_UNAVAILABLE,GATEWAY_TIMEOUT
                # Idempotent HTTP methods only
                methods: GET,PUT
                # Exceptions triggering retry
                exceptions:
                  - java.io.IOException
                  - java.net.ConnectException
                  - org.springframework.cloud.gateway.support.TimeoutException
                # Exponential backoff
                backoff:
                  firstBackoff: 100ms
                  maxBackoff: 2000ms
                  factor: 2
                  basedOnPreviousValue: true
```

```java
// CustomRetryFilter.java
// Custom retry with business logic
@Component
@Slf4j
public class CustomRetryFilter implements GatewayFilter, Ordered {

    private static final int MAX_RETRIES = 3;
    private static final Duration INITIAL_BACKOFF = Duration.ofMillis(100);
    private static final double BACKOFF_MULTIPLIER = 2.0;
    private static final double JITTER_FACTOR = 0.1;

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        return Mono.defer(() -> attemptRequest(exchange, chain, 0));
    }

    private Mono<Void> attemptRequest(
        ServerWebExchange exchange,
        GatewayFilterChain chain,
        int attempt
    ) {
        return chain.filter(exchange)
            .onErrorResume(throwable -> {
                if (attempt >= MAX_RETRIES || !isRetryable(throwable, exchange)) {
                    return Mono.error(throwable);
                }

                Duration backoff = calculateBackoff(attempt);
                log.warn("Retry attempt {} after {}ms for {} {}",
                    attempt + 1,
                    backoff.toMillis(),
                    exchange.getRequest().getMethod(),
                    exchange.getRequest().getPath());

                return Mono.delay(backoff)
                    .then(Mono.defer(() ->
                        attemptRequest(exchange, chain, attempt + 1)
                    ));
            });
    }

    private boolean isRetryable(Throwable throwable, ServerWebExchange exchange) {
        // Only retry idempotent methods
        HttpMethod method = exchange.getRequest().getMethod();
        if (!Set.of(HttpMethod.GET, HttpMethod.PUT, HttpMethod.DELETE)
            .contains(method)) {
            return false;
        }

        // Check exception type
        return throwable instanceof ConnectException ||
               throwable instanceof TimeoutException ||
               throwable instanceof ServiceUnavailableException;
    }

    private Duration calculateBackoff(int attempt) {
        // Exponential backoff with jitter
        long baseBackoff = (long) (
            INITIAL_BACKOFF.toMillis() * Math.pow(BACKOFF_MULTIPLIER, attempt)
        );

        // Add random jitter (±10%)
        double jitter = 1.0 + (Math.random() - 0.5) * 2 * JITTER_FACTOR;
        long finalBackoff = (long) (baseBackoff * jitter);

        return Duration.ofMillis(finalBackoff);
    }

    @Override
    public int getOrder() {
        return Ordered.LOWEST_PRECEDENCE - 1;
    }
}
```

## Advanced Patterns and Best Practices

### Question 10: How to implement request aggregation?

Aggregation combines multiple microservice calls into a single response for the client, reducing latency and frontend complexity.

```java
// AggregationController.java
// Multi-service aggregation
@RestController
@RequestMapping("/api/aggregate")
@RequiredArgsConstructor
@Slf4j
public class AggregationController {

    private final WebClient.Builder webClientBuilder;
    private final CircuitBreakerRegistry circuitBreakerRegistry;

    @GetMapping("/user-dashboard/{userId}")
    public Mono<DashboardResponse> getUserDashboard(@PathVariable Long userId) {
        // Parallel calls to different services
        Mono<UserProfile> userMono = fetchUserProfile(userId);
        Mono<List<Order>> ordersMono = fetchRecentOrders(userId);
        Mono<NotificationCount> notificationsMono = fetchNotificationCount(userId);

        // Result aggregation
        return Mono.zip(userMono, ordersMono, notificationsMono)
            .map(tuple -> DashboardResponse.builder()
                .user(tuple.getT1())
                .recentOrders(tuple.getT2())
                .notificationCount(tuple.getT3())
                .generatedAt(Instant.now())
                .build())
            .timeout(Duration.ofSeconds(5))
            .onErrorResume(this::handleAggregationError);
    }

    private Mono<UserProfile> fetchUserProfile(Long userId) {
        return webClientBuilder.build()
            .get()
            .uri("lb://user-service/users/{id}", userId)
            .retrieve()
            .bodyToMono(UserProfile.class)
            .transform(CircuitBreakerOperator.of(
                circuitBreakerRegistry.circuitBreaker("user-service")
            ))
            .onErrorReturn(new UserProfile(userId, "Unknown", null));
    }

    private Mono<List<Order>> fetchRecentOrders(Long userId) {
        return webClientBuilder.build()
            .get()
            .uri("lb://order-service/orders?userId={id}&limit=5", userId)
            .retrieve()
            .bodyToFlux(Order.class)
            .collectList()
            .transform(CircuitBreakerOperator.of(
                circuitBreakerRegistry.circuitBreaker("order-service")
            ))
            .onErrorReturn(Collections.emptyList());
    }

    private Mono<NotificationCount> fetchNotificationCount(Long userId) {
        return webClientBuilder.build()
            .get()
            .uri("lb://notification-service/notifications/count/{id}", userId)
            .retrieve()
            .bodyToMono(NotificationCount.class)
            .transform(CircuitBreakerOperator.of(
                circuitBreakerRegistry.circuitBreaker("notification-service")
            ))
            .onErrorReturn(new NotificationCount(0, 0));
    }

    private Mono<DashboardResponse> handleAggregationError(Throwable error) {
        log.error("Dashboard aggregation failed: {}", error.getMessage());
        return Mono.just(DashboardResponse.builder()
            .error("Partial data available")
            .generatedAt(Instant.now())
            .build());
    }
}
```

```java
// DashboardResponse.java
// Aggregated response DTO
@Data
@Builder
@NoArgsConstructor
@AllArgsConstructor
public class DashboardResponse {
    private UserProfile user;
    private List<Order> recentOrders;
    private NotificationCount notificationCount;
    private Instant generatedAt;
    private String error;
}
```

### Question 11: How to secure the gateway with OAuth2?

OAuth2 integration centralizes authentication at the gateway level, avoiding logic duplication in each microservice.

```yaml
# application.yml
# OAuth2 Resource Server configuration
spring:
  security:
    oauth2:
      resourceserver:
        jwt:
          issuer-uri: https://auth.example.com/realms/myrealm
          jwk-set-uri: https://auth.example.com/realms/myrealm/protocol/openid-connect/certs
```

```java
// SecurityConfig.java
// Gateway security configuration
@Configuration
@EnableWebFluxSecurity
public class SecurityConfig {

    @Bean
    public SecurityWebFilterChain securityWebFilterChain(ServerHttpSecurity http) {
        return http
            // Disable CSRF for stateless API
            .csrf(ServerHttpSecurity.CsrfSpec::disable)

            // Authorization configuration
            .authorizeExchange(exchanges -> exchanges
                // Public endpoints
                .pathMatchers("/actuator/health", "/actuator/info").permitAll()
                .pathMatchers("/api/public/**").permitAll()
                .pathMatchers("/api/auth/**").permitAll()

                // Role-specific endpoints
                .pathMatchers("/api/admin/**").hasRole("ADMIN")
                .pathMatchers(HttpMethod.DELETE, "/api/**").hasRole("ADMIN")

                // Everything else requires authentication
                .anyExchange().authenticated()
            )

            // OAuth2 Resource Server with JWT
            .oauth2ResourceServer(oauth2 -> oauth2
                .jwt(jwt -> jwt
                    .jwtAuthenticationConverter(jwtAuthenticationConverter())
                )
            )

            .build();
    }

    @Bean
    public ReactiveJwtAuthenticationConverter jwtAuthenticationConverter() {
        JwtGrantedAuthoritiesConverter grantedAuthoritiesConverter =
            new JwtGrantedAuthoritiesConverter();
        // Extract roles from "roles" claim
        grantedAuthoritiesConverter.setAuthoritiesClaimName("roles");
        grantedAuthoritiesConverter.setAuthorityPrefix("ROLE_");

        ReactiveJwtAuthenticationConverter converter =
            new ReactiveJwtAuthenticationConverter();
        converter.setJwtGrantedAuthoritiesConverter(
            new ReactiveJwtGrantedAuthoritiesConverterAdapter(grantedAuthoritiesConverter)
        );

        return converter;
    }
}
```

```java
// TokenRelayFilter.java
// Token relay to downstream services
@Component
public class TokenRelayFilter implements GlobalFilter, Ordered {

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        return ReactiveSecurityContextHolder.getContext()
            .map(SecurityContext::getAuthentication)
            .filter(auth -> auth instanceof JwtAuthenticationToken)
            .cast(JwtAuthenticationToken.class)
            .map(JwtAuthenticationToken::getToken)
            .map(jwt -> {
                // Relay token to downstream services
                ServerHttpRequest request = exchange.getRequest()
                    .mutate()
                    .header(HttpHeaders.AUTHORIZATION, "Bearer " + jwt.getTokenValue())
                    // Add user information extracted from token
                    .header("X-User-Id", jwt.getSubject())
                    .header("X-User-Email", jwt.getClaimAsString("email"))
                    .build();

                return exchange.mutate().request(request).build();
            })
            .defaultIfEmpty(exchange)
            .flatMap(chain::filter);
    }

    @Override
    public int getOrder() {
        // After authentication, before routing
        return SecurityWebFiltersOrder.AUTHENTICATION.getOrder() + 1;
    }
}
```

### Question 12: What are the best practices for monitoring and observability?

Gateway monitoring is crucial for identifying performance and availability issues in a microservices architecture.

```yaml
# application.yml
# Observability configuration
management:
  endpoints:
    web:
      exposure:
        include: health,info,metrics,prometheus,gateway
  metrics:
    tags:
      application: api-gateway
    distribution:
      percentiles-histogram:
        http.server.requests: true
      percentiles:
        http.server.requests: 0.5, 0.95, 0.99
  tracing:
    sampling:
      probability: 1.0

spring:
  cloud:
    gateway:
      metrics:
        enabled: true
        tags:
          path:
            enabled: true
```

```java
// MetricsFilter.java
// Custom metrics filter
@Component
@RequiredArgsConstructor
@Slf4j
public class MetricsFilter implements GlobalFilter, Ordered {

    private final MeterRegistry meterRegistry;

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        long startTime = System.nanoTime();
        String path = exchange.getRequest().getPath().value();
        String method = exchange.getRequest().getMethod().name();

        // Extract target service from route
        Route route = exchange.getAttribute(ServerWebExchangeUtils.GATEWAY_ROUTE_ATTR);
        String routeId = route != null ? route.getId() : "unknown";

        return chain.filter(exchange)
            .doOnSuccess(v -> recordMetrics(exchange, startTime, routeId, "success"))
            .doOnError(e -> recordMetrics(exchange, startTime, routeId, "error"));
    }

    private void recordMetrics(
        ServerWebExchange exchange,
        long startTime,
        String routeId,
        String outcome
    ) {
        long duration = System.nanoTime() - startTime;
        HttpStatusCode status = exchange.getResponse().getStatusCode();
        String statusCode = status != null ? String.valueOf(status.value()) : "0";

        // Timer for latency
        Timer.builder("gateway.request.duration")
            .tag("route", routeId)
            .tag("method", exchange.getRequest().getMethod().name())
            .tag("status", statusCode)
            .tag("outcome", outcome)
            .register(meterRegistry)
            .record(duration, TimeUnit.NANOSECONDS);

        // Request counter
        meterRegistry.counter(
            "gateway.requests.total",
            "route", routeId,
            "status", statusCode
        ).increment();

        // Log slow requests
        if (duration > TimeUnit.SECONDS.toNanos(1)) {
            log.warn("Slow request: {} {} - {}ms (route: {})",
                exchange.getRequest().getMethod(),
                exchange.getRequest().getPath(),
                TimeUnit.NANOSECONDS.toMillis(duration),
                routeId);
        }
    }

    @Override
    public int getOrder() {
        return Ordered.HIGHEST_PRECEDENCE;
    }
}
```

```java
// TracingFilter.java
// Tracing context propagation
@Component
@RequiredArgsConstructor
public class TracingFilter implements GlobalFilter, Ordered {

    private final Tracer tracer;

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        // Create or retrieve tracing span
        Span span = tracer.nextSpan()
            .name("gateway-request")
            .tag("http.method", exchange.getRequest().getMethod().name())
            .tag("http.url", exchange.getRequest().getURI().toString())
            .start();

        // Inject tracing headers
        ServerHttpRequest request = exchange.getRequest()
            .mutate()
            .header("X-Trace-Id", span.context().traceId())
            .header("X-Span-Id", span.context().spanId())
            .build();

        return chain.filter(exchange.mutate().request(request).build())
            .doOnSuccess(v -> {
                HttpStatusCode status = exchange.getResponse().getStatusCode();
                span.tag("http.status_code",
                    status != null ? String.valueOf(status.value()) : "0");
                span.end();
            })
            .doOnError(e -> {
                span.tag("error", e.getMessage());
                span.end();
            });
    }

    @Override
    public int getOrder() {
        return Ordered.HIGHEST_PRECEDENCE + 1;
    }
}
```

Essential metrics table:

```text
| Metric                            | Description                      |
|-----------------------------------|----------------------------------|
| gateway.request.duration          | Latency per route/status         |
| gateway.requests.total            | Request counter                  |
| resilience4j.circuitbreaker.state | Circuit breaker states           |
| http.server.requests              | Standard HTTP metrics            |
| spring.cloud.gateway.routes       | Active routes                    |
```

> **Recommended dashboards**
>
> Use Grafana with Spring Cloud Gateway and Resilience4j dashboards to visualize metrics. Configure alerts on P99 latency and error rate.

## Conclusion

Spring Cloud Gateway is an essential component of modern microservices architectures. Key points to remember for interviews:

**Architecture and concepts:**
- ✅ Routes, Predicates, and Filters form the basic model
- ✅ Reactive architecture with WebFlux and Netty
- ✅ Native integration with Spring Cloud ecosystem

**Essential features:**
- ✅ Dynamic routing based on multiple criteria
- ✅ Pre/post filters for request/response transformation
- ✅ Load balancing with Spring Cloud LoadBalancer

**Resilience and security:**
- ✅ Circuit breaker with Resilience4j and fallback
- ✅ Retry with exponential backoff and jitter
- ✅ Centralized OAuth2/JWT authentication

**Observability:**
- ✅ Micrometer metrics with per-route tags
- ✅ Distributed tracing with context propagation
- ✅ Health checks and actuator endpoints

Mastering Spring Cloud Gateway demonstrates deep understanding of microservices patterns and scalability concerns. These skills are essential for designing robust and high-performance API Gateways.

---

Source: SharpSkill (https://sharpskill.dev), tech interview preparation for your real stack.
HTML version of this page: https://sharpskill.dev/en/blog/spring-boot/spring-cloud-gateway-interview-routing-filters