Spring Cloud Gateway im Interview: Routing, Filter und Load Balancing

// Grundlegende Konfiguration von Spring Cloud Gateway
@SpringBootApplication
public class GatewayApplication {

    public static void main(String[] args) {
        // Startet den reaktiven Netty-Server (kein Tomcat)
        SpringApplication.run(GatewayApplication.class, args);
    }
}

# application.yml
# Minimale Gateway-Konfiguration
spring:
  cloud:
    gateway:
      routes:
        # Route zum User-Service
        - id: user-service
          uri: http://localhost:8081
          predicates:
            - Path=/api/users/**
        # Route zum Order-Service
        - id: order-service
          uri: http://localhost:8082
          predicates:
            - Path=/api/orders/**

// Programmatische Routenkonfiguration
@Configuration
public class RouteConfiguration {

    @Bean
    public RouteLocator customRouteLocator(RouteLocatorBuilder builder) {
        return builder.routes()
            // Route mit mehreren Predicates
            .route("product-service", r -> r
                // Predicate: URL-Pfad
                .path("/api/products/**")
                // Predicate: HTTP-Methode
                .and()
                .method(HttpMethod.GET, HttpMethod.POST)
                // Predicate: Header vorhanden
                .and()
                .header("X-Api-Version", "v2")
                // Filter: Pfad-Rewrite
                .filters(f -> f
                    .rewritePath("/api/products/(?<segment>.*)", "/products/${segment}")
                    // Filter: Header hinzufügen
                    .addRequestHeader("X-Gateway-Source", "spring-cloud-gateway")
                )
                // Ziel-URI
                .uri("http://localhost:8083"))
            .build();
    }
}

Eingehende Anfrage
    │
    ▼
┌─────────────────┐
│   Predicates    │ → Bedingungen prüfen (Pfad, Methode, Header...)
└────────┬────────┘
         │ Treffer gefunden
         ▼
┌─────────────────┐
│  Pre-Filters    │ → Anfrage vor dem Routing verändern
└────────┬────────┘
         │
         ▼
┌─────────────────┐
│   HTTP Proxy    │ → Weiterleitung an den Zielservice
└────────┬────────┘
         │
         ▼
┌─────────────────┐
│  Post-Filters   │ → Antwort vor der Auslieferung an den Client verändern
└────────┬────────┘
         │
         ▼
   Antwort an den Client

# application.yml
# Beispiele gängiger Predicates
spring:
  cloud:
    gateway:
      routes:
        # Pfad-Routing mit Variablenerfassung
        - id: user-details
          uri: http://user-service
          predicates:
            - Path=/users/{userId}

        # Routing nach HTTP-Methode
        - id: user-create
          uri: http://user-service
          predicates:
            - Path=/users
            - Method=POST

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

        # Routing nach Query-Parametern
        - id: search-api
          uri: http://search-service
          predicates:
            - Query=q

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

        # Zeitbasiertes Routing
        - id: maintenance-mode
          uri: http://maintenance-service
          predicates:
            - Between=2026-03-20T02:00:00Z,2026-03-20T04:00:00Z

// Erstellung eines eigenen Predicates
@Component
public class ApiKeyRoutePredicateFactory
    extends AbstractRoutePredicateFactory<ApiKeyRoutePredicateFactory.Config> {

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

    @Override
    public Predicate<ServerWebExchange> apply(Config config) {
        return exchange -> {
            // Prüft das Vorhandensein und die Gültigkeit des API-Keys
            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;
        }
    }
}

// Globaler Logging-Filter
@Component
@Slf4j
public class LoggingFilter implements GlobalFilter, Ordered {

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

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

        // Fügt die Request-ID den Headern hinzu
        ServerHttpRequest modifiedRequest = exchange.getRequest()
            .mutate()
            .header("X-Request-Id", requestId)
            .build();

        // Setzt die Kette fort und behandelt die Antwort
        return chain.filter(exchange.mutate().request(modifiedRequest).build())
            .then(Mono.fromRunnable(() -> {
                // POST-FILTER: nach der Antwort
                long duration = System.currentTimeMillis() - startTime;
                HttpStatusCode status = exchange.getResponse().getStatusCode();

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

    @Override
    public int getOrder() {
        // Negativer Wert = wird zuerst ausgeführt
        return -1;
    }
}

// JWT-Authentifizierungsfilter
@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);

        // Prüft das Vorhandensein des Tokens
        if (authHeader == null || !authHeader.startsWith("Bearer ")) {
            return handleUnauthorized(exchange, "Missing or invalid Authorization header");
        }

        String token = authHeader.substring(7);

        // Validiert das Token reaktiv
        return tokenValidator.validate(token)
            .flatMap(claims -> {
                // Reichert die Anfrage mit Benutzerinformationen an
                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));
    }
}

# application.yml
# Gängige eingebaute Filter
spring:
  cloud:
    gateway:
      routes:
        - id: order-service
          uri: lb://order-service
          predicates:
            - Path=/api/orders/**
          filters:
            # Pfad-Rewrite
            - RewritePath=/api/orders/(?<segment>.*), /orders/${segment}

            # Request-Header hinzufügen
            - AddRequestHeader=X-Gateway-Version, 1.0

            # Sensible Response-Header entfernen
            - RemoveResponseHeader=X-Powered-By
            - RemoveResponseHeader=Server

            # Pfad-Präfix
            - PrefixPath=/v2

            # Präfix entfernen
            - StripPrefix=1

            # Automatischer Retry bei Fehlern
            - 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}"

// Konfiguration des Rate Limiters pro Benutzer
@Configuration
public class RateLimiterConfiguration {

    @Bean
    public KeyResolver userKeyResolver() {
        // Begrenzung pro authentifiziertem Benutzer
        return exchange -> Mono.just(
            exchange.getRequest()
                .getHeaders()
                .getFirst("X-User-Id")
        ).defaultIfEmpty("anonymous");
    }

    @Bean
    public KeyResolver ipKeyResolver() {
        // Begrenzung pro IP-Adresse
        return exchange -> Mono.just(
            Objects.requireNonNull(exchange.getRequest()
                .getRemoteAddress())
                .getAddress()
                .getHostAddress()
        );
    }
}

// Filter zur Anpassung des Request-Bodys
@Component
@RequiredArgsConstructor
public class RequestBodyModificationFilter implements GlobalFilter, Ordered {

    private final ObjectMapper objectMapper;

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        // Nur POST/PUT-Anfragen mit JSON anpassen
        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 {
                    // Parst das JSON und passt es an
                    Map<String, Object> body = objectMapper.readValue(
                        bytes,
                        new TypeReference<Map<String, Object>>() {}
                    );

                    // Fügt Metadaten hinzu
                    body.put("processedAt", Instant.now().toString());
                    body.put("gatewayVersion", "1.0");

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

                    // Erstellt eine neue Anfrage mit angepasstem 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;
    }
}

// Konfiguration zur Anpassung der Antworten
@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) -> {
                        // Verpackt die Antwort in ein Standardformat
                        return Mono.just(String.format(
                            "{\"success\": true, \"data\": %s, \"timestamp\": \"%s\"}",
                            responseBody,
                            Instant.now()
                        ));
                    }
                ))
                .uri("lb://user-service"))
            .build();
    }
}

# application.yml
# Konfiguration des Load Balancing
spring:
  cloud:
    gateway:
      routes:
        - id: user-service
          # lb:// aktiviert das Load Balancing
          uri: lb://user-service
          predicates:
            - Path=/api/users/**

    # Konfiguration des Load Balancers
    loadbalancer:
      ribbon:
        enabled: false  # Verwendet Spring Cloud LoadBalancer (nicht Ribbon)

      # Konfiguration pro Service
      configurations: default

      # Health Check für das Load Balancing
      health-check:
        path:
          user-service: /actuator/health
        interval: 10s

# Service Discovery (Eureka oder anderes)
eureka:
  client:
    serviceUrl:
      defaultZone: http://localhost:8761/eureka/

// Eigene Konfiguration des Load Balancers
@Configuration
@LoadBalancerClients(defaultConfiguration = CustomLoadBalancerConfig.class)
public class LoadBalancerConfiguration {
}

// CustomLoadBalancerConfig.java
// Eigene Load-Balancing-Strategie
public class CustomLoadBalancerConfig {

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

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

    @Bean
    public ServiceInstanceListSupplier serviceInstanceListSupplier(
        ConfigurableApplicationContext context
    ) {
        // Fügt den Instanzen einen Health Check hinzu
        return ServiceInstanceListSupplier.builder()
            .withDiscoveryClient()
            .withHealthChecks()
            .withCaching()
            .build(context);
    }
}

// Gewichteter 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();
                }

                // Berechnet die Gewichte aus den Metadaten
                List<WeightedInstance> weighted = instances.stream()
                    .map(instance -> {
                        int weight = Integer.parseInt(
                            instance.getMetadata().getOrDefault("weight", "1")
                        );
                        return new WeightedInstance(instance, weight);
                    })
                    .toList();

                // Gewichtete Auswahl
                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) {}
}

# application.yml
# Konfiguration des Resilience4j Circuit Breakers
spring:
  cloud:
    gateway:
      routes:
        - id: order-service
          uri: lb://order-service
          predicates:
            - Path=/api/orders/**
          filters:
            # Circuit Breaker mit Fallback
            - name: CircuitBreaker
              args:
                name: orderServiceCB
                fallbackUri: forward:/fallback/orders

resilience4j:
  circuitbreaker:
    configs:
      default:
        # Anzahl bewerteter Anfragen
        slidingWindowSize: 10
        # Fehlergrenzwert für das Öffnen des Stromkreises
        failureRateThreshold: 50
        # Dauer im offenen Zustand vor dem nächsten Versuch
        waitDurationInOpenState: 30s
        # Erlaubte Anfragen im Half-Open-Zustand
        permittedNumberOfCallsInHalfOpenState: 3
        # Automatische Übergänge
        automaticTransitionFromOpenToHalfOpenEnabled: true

    instances:
      orderServiceCB:
        baseConfig: default
        failureRateThreshold: 60

  timelimiter:
    configs:
      default:
        timeoutDuration: 5s

    instances:
      orderServiceCB:
        timeoutDuration: 3s

// 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));
    }
}

// Überwachung der Circuit-Breaker-Ereignisse
@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-Metrik
        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());
    }
}

# application.yml
# Konfiguration von Retry mit Backoff
spring:
  cloud:
    gateway:
      routes:
        - id: payment-service
          uri: lb://payment-service
          predicates:
            - Path=/api/payments/**
          filters:
            - name: Retry
              args:
                # Anzahl der Versuche
                retries: 3
                # HTTP-Codes, die einen Retry auslösen
                statuses: BAD_GATEWAY,SERVICE_UNAVAILABLE,GATEWAY_TIMEOUT
                # Nur idempotente HTTP-Methoden
                methods: GET,PUT
                # Exceptions, die einen Retry auslösen
                exceptions:
                  - java.io.IOException
                  - java.net.ConnectException
                  - org.springframework.cloud.gateway.support.TimeoutException
                # Exponentieller Backoff
                backoff:
                  firstBackoff: 100ms
                  maxBackoff: 2000ms
                  factor: 2
                  basedOnPreviousValue: true

// Eigener Retry mit Geschäftslogik
@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) {
        // Wiederholt nur idempotente Methoden
        HttpMethod method = exchange.getRequest().getMethod();
        if (!Set.of(HttpMethod.GET, HttpMethod.PUT, HttpMethod.DELETE)
            .contains(method)) {
            return false;
        }

        // Prüft den Exception-Typ
        return throwable instanceof ConnectException ||
               throwable instanceof TimeoutException ||
               throwable instanceof ServiceUnavailableException;
    }

    private Duration calculateBackoff(int attempt) {
        // Exponentieller Backoff mit Jitter
        long baseBackoff = (long) (
            INITIAL_BACKOFF.toMillis() * Math.pow(BACKOFF_MULTIPLIER, attempt)
        );

        // Fügt zufälligen Jitter hinzu (±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;
    }
}

// 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) {
        // Parallele Aufrufe an verschiedene Services
        Mono<UserProfile> userMono = fetchUserProfile(userId);
        Mono<List<Order>> ordersMono = fetchRecentOrders(userId);
        Mono<NotificationCount> notificationsMono = fetchNotificationCount(userId);

        // Aggregation der Ergebnisse
        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());
    }
}

// DTO für die aggregierte Antwort
@Data
@Builder
@NoArgsConstructor
@AllArgsConstructor
public class DashboardResponse {
    private UserProfile user;
    private List<Order> recentOrders;
    private NotificationCount notificationCount;
    private Instant generatedAt;
    private String error;
}

# application.yml
# OAuth2-Resource-Server-Konfiguration
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

// Sicherheitskonfiguration des Gateways
@Configuration
@EnableWebFluxSecurity
public class SecurityConfig {

    @Bean
    public SecurityWebFilterChain securityWebFilterChain(ServerHttpSecurity http) {
        return http
            // CSRF für eine zustandslose API deaktivieren
            .csrf(ServerHttpSecurity.CsrfSpec::disable)

            // Konfiguration der Autorisierung
            .authorizeExchange(exchanges -> exchanges
                // Öffentliche Endpunkte
                .pathMatchers("/actuator/health", "/actuator/info").permitAll()
                .pathMatchers("/api/public/**").permitAll()
                .pathMatchers("/api/auth/**").permitAll()

                // Rollenspezifische Endpunkte
                .pathMatchers("/api/admin/**").hasRole("ADMIN")
                .pathMatchers(HttpMethod.DELETE, "/api/**").hasRole("ADMIN")

                // Alles andere erfordert Authentifizierung
                .anyExchange().authenticated()
            )

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

            .build();
    }

    @Bean
    public ReactiveJwtAuthenticationConverter jwtAuthenticationConverter() {
        JwtGrantedAuthoritiesConverter grantedAuthoritiesConverter =
            new JwtGrantedAuthoritiesConverter();
        // Extrahiert die Rollen aus dem Claim "roles"
        grantedAuthoritiesConverter.setAuthoritiesClaimName("roles");
        grantedAuthoritiesConverter.setAuthorityPrefix("ROLE_");

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

        return converter;
    }
}

// Weiterleitung des Tokens an die 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 -> {
                // Reicht das Token an die Downstream-Services weiter
                ServerHttpRequest request = exchange.getRequest()
                    .mutate()
                    .header(HttpHeaders.AUTHORIZATION, "Bearer " + jwt.getTokenValue())
                    // Fügt aus dem Token extrahierte Benutzerinformationen hinzu
                    .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() {
        // Nach der Authentifizierung, vor dem Routing
        return SecurityWebFiltersOrder.AUTHENTICATION.getOrder() + 1;
    }
}

# application.yml
# Konfiguration der Observability
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

// Filter für eigene Metriken
@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();

        // Liest den Zielservice aus der 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 für die Latenz
        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);

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

        // Logs für langsame Anfragen
        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;
    }
}

// Weitergabe des Tracing-Kontexts
@Component
@RequiredArgsConstructor
public class TracingFilter implements GlobalFilter, Ordered {

    private final Tracer tracer;

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        // Erstellt oder ruft den Tracing-Span ab
        Span span = tracer.nextSpan()
            .name("gateway-request")
            .tag("http.method", exchange.getRequest().getMethod().name())
            .tag("http.url", exchange.getRequest().getURI().toString())
            .start();

        // Schleust die Tracing-Header ein
        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;
    }
}

| Metrik                            | Beschreibung                     |
|-----------------------------------|----------------------------------|
| gateway.request.duration          | Latenz pro Route/Status          |
| gateway.requests.total            | Zähler der Anfragen              |
| resilience4j.circuitbreaker.state | Zustände des Circuit Breakers    |
| http.server.requests              | Standard-HTTP-Metriken           |
| spring.cloud.gateway.routes       | Aktive Routen                    |