Chapter 4: API Gateway with Spring Cloud Gateway

🎯 Chapter Objectives

• Understand the role of an API Gateway
• Configure Spring Cloud Gateway
• Define routes and predicates
• Implement filters (pre/post)
• Configure rate limiting
• Integrate with Eureka for load balancing

---

1. Why an API Gateway?

1.1 Without Gateway

Problems:

• Client must know all services
• CORS to configure on each service
• Authentication duplicated everywhere
• No centralized rate limiting
• Dispersed logging
• SSL to manage everywhere

1.2 With Gateway

Advantages:

• Single entry point
• Centralized authentication
• Rate limiting
• Logging/Monitoring
• SSL termination
• Load balancing
• Request transformation

---

2. Spring Cloud Gateway vs Zuul

Criterion	Spring Cloud Gateway	Zuul 1.x
Architecture	Reactive (WebFlux)	Blocking (Servlet)
Performance	Excellent	Good
Programming	Functional	Imperative
Maintenance	Active	Deprecated
Spring Boot 3	✅ Supported	❌ Not supported

Recommendation: Use Spring Cloud Gateway for new projects.

---

3. Key Concepts

3.1 Route

A route is the basic building block: it defines where to send a request.

spring:
  cloud:
    gateway:
      routes:
        - id: user-service-route
          uri: http://localhost:8081
          predicates:
            - Path=/api/users/**

3.2 Predicate

A predicate is a condition to match a request.

predicates:
  - Path=/api/users/**        # By path
  - Method=GET,POST           # By HTTP method
  - Header=X-Request-Id, \d+  # By header
  - Query=name                # By query param
  - Host=**.example.com       # By hostname
  - After=2024-01-01T00:00:00 # By date

3.3 Filter

A filter modifies the request or the response.

filters:
  - AddRequestHeader=X-Gateway, true
  - AddResponseHeader=X-Response-Time, 123
  - RewritePath=/api/(?<segment>.*), /$\{segment}
  - CircuitBreaker=name=myCircuitBreaker

3.4 Processing Flow

---

4. Gateway Configuration

4.1 Project Structure

api-gateway/
├── pom.xml
└── src/main/
    ├── java/com/example/gateway/
    │   ├── ApiGatewayApplication.java
    │   ├── config/
    │   │   ├── GatewayConfig.java
    │   │   └── SecurityConfig.java
    │   └── filter/
    │       └── LoggingFilter.java
    └── resources/
        └── application.yml

4.2 Maven Dependencies

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 
         https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.2.0</version>
    </parent>
    
    <groupId>com.example</groupId>
    <artifactId>api-gateway</artifactId>
    <version>1.0.0</version>
    
    <properties>
        <java.version>17</java.version>
        <spring-cloud.version>2023.0.0</spring-cloud.version>
    </properties>
    
    <dependencies>
        <!-- Spring Cloud Gateway -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-gateway</artifactId>
        </dependency>
        
        <!-- Eureka Client -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
        </dependency>
        
        <!-- Circuit Breaker -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-circuitbreaker-reactor-resilience4j</artifactId>
        </dependency>
        
        <!-- Rate Limiter (Redis) -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-data-redis-reactive</artifactId>
        </dependency>
        
        <!-- Actuator -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-actuator</artifactId>
        </dependency>
    </dependencies>
    
    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.cloud</groupId>
                <artifactId>spring-cloud-dependencies</artifactId>
                <version>${spring-cloud.version}</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>
    
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

4.3 Main Application

package com.example.gateway;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class ApiGatewayApplication {

    public static void main(String[] args) {
        SpringApplication.run(ApiGatewayApplication.class, args);
    }
}

---

5. YAML Route Configuration

5.1 Basic Routes

server:
  port: 8080

spring:
  application:
    name: api-gateway
  cloud:
    gateway:
      routes:
        # Route to User Service
        - id: user-service
          uri: http://localhost:8081
          predicates:
            - Path=/api/users/**
        
        # Route to Product Service
        - id: product-service
          uri: http://localhost:8082
          predicates:
            - Path=/api/products/**
        
        # Route to Order Service
        - id: order-service
          uri: http://localhost:8083
          predicates:
            - Path=/api/orders/**

# Eureka
eureka:
  client:
    service-url:
      defaultZone: http://localhost:8761/eureka/

5.2 Routes with Eureka (Load Balancing)

spring:
  cloud:
    gateway:
      routes:
        - id: user-service
          # lb:// = Load Balanced via Eureka
          uri: lb://user-service
          predicates:
            - Path=/api/users/**
        
        - id: product-service
          uri: lb://product-service
          predicates:
            - Path=/api/products/**
        
        - id: order-service
          uri: lb://order-service
          predicates:
            - Path=/api/orders/**
      
      # Automatic service discovery
      discovery:
        locator:
          enabled: true
          lower-case-service-id: true

5.3 Routes with Filters

spring:
  cloud:
    gateway:
      routes:
        - id: user-service
          uri: lb://user-service
          predicates:
            - Path=/api/users/**
          filters:
            # Add headers
            - AddRequestHeader=X-Gateway-Source, api-gateway
            - AddResponseHeader=X-Response-Time, ${timestamp}
            
            # Rewrite path
            - RewritePath=/api/users/(?<segment>.*), /users/$\{segment}
            
            # Strip prefix
            - StripPrefix=1
            
            # Circuit Breaker
            - name: CircuitBreaker
              args:
                name: userServiceCircuitBreaker
                fallbackUri: forward:/fallback/users

---

6. Detailed Predicates

6.1 Path Predicate

predicates:
  # Exact path
  - Path=/api/users
  
  # With single wildcard (one segment)
  - Path=/api/users/*
  
  # With multiple wildcards (multiple segments)
  - Path=/api/users/**
  
  # Multiple paths
  - Path=/api/users/**,/api/accounts/**

6.2 Method Predicate

predicates:
  - Path=/api/users/**
  - Method=GET,POST,PUT,DELETE

6.3 Header Predicate

predicates:
  - Path=/api/users/**
  # Header present with regex value
  - Header=X-Api-Key, ^[a-zA-Z0-9]{32}$
  # Header present (without checking value)
  - Header=Authorization

6.4 Query Predicate

predicates:
  - Path=/api/search
  # Query param present
  - Query=q
  # Query param with regex value
  - Query=category, ^(electronics|books|clothing)$

6.5 Host Predicate

predicates:
  - Host=**.example.com
  - Host=api.example.com,admin.example.com

6.6 DateTime Predicates

predicates:
  # Active after this date
  - After=2024-01-01T00:00:00+00:00[UTC]
  
  # Active before this date
  - Before=2025-01-01T00:00:00+00:00[UTC]
  
  # Active between two dates
  - Between=2024-01-01T00:00:00+00:00[UTC], 2024-12-31T23:59:59+00:00[UTC]

6.7 Weight Predicate (A/B Testing)

routes:
  - id: user-service-v1
    uri: lb://user-service-v1
    predicates:
      - Path=/api/users/**
      - Weight=group1, 8  # 80% of traffic
  
  - id: user-service-v2
    uri: lb://user-service-v2
    predicates:
      - Path=/api/users/**
      - Weight=group1, 2  # 20% of traffic

---

7. Detailed Filters

7.1 AddRequestHeader / AddResponseHeader

filters:
  - AddRequestHeader=X-Request-Source, gateway
  - AddResponseHeader=X-Gateway-Version, 1.0

7.2 RemoveRequestHeader / RemoveResponseHeader

filters:
  - RemoveRequestHeader=Cookie
  - RemoveResponseHeader=X-Powered-By

7.3 RewritePath

filters:
  # /api/users/123 → /users/123
  - RewritePath=/api/(?<segment>.*), /$\{segment}
  
  # /old-api/users → /api/v2/users
  - RewritePath=/old-api/(?<segment>.*), /api/v2/$\{segment}

7.4 StripPrefix / PrefixPath

filters:
  # /api/v1/users → /users (removes 2 segments)
  - StripPrefix=2
  
  # /users → /api/users (adds prefix)
  - PrefixPath=/api

7.5 SetStatus

filters:
  # Force a return code
  - SetStatus=401

7.6 Retry

filters:
  - name: Retry
    args:
      retries: 3
      statuses: BAD_GATEWAY,SERVICE_UNAVAILABLE
      methods: GET
      backoff:
        firstBackoff: 100ms
        maxBackoff: 500ms
        factor: 2

7.7 RequestRateLimiter

filters:
  - name: RequestRateLimiter
    args:
      redis-rate-limiter.replenishRate: 10
      redis-rate-limiter.burstCapacity: 20
      redis-rate-limiter.requestedTokens: 1
      key-resolver: "#{@userKeyResolver}"

@Bean
public KeyResolver userKeyResolver() {
    return exchange -> Mono.just(
        exchange.getRequest()
            .getHeaders()
            .getFirst("X-User-Id") != null 
            ? exchange.getRequest().getHeaders().getFirst("X-User-Id")
            : exchange.getRequest().getRemoteAddress().getAddress().getHostAddress()
    );
}

---

8. Java Configuration (Programmatic)

8.1 RouteLocator Bean

package com.example.gateway.config;

import org.springframework.cloud.gateway.route.RouteLocator;
import org.springframework.cloud.gateway.route.builder.RouteLocatorBuilder;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class GatewayConfig {
    
    @Bean
    public RouteLocator customRouteLocator(RouteLocatorBuilder builder) {
        return builder.routes()
            // Route User Service
            .route("user-service", r -> r
                .path("/api/users/**")
                .filters(f -> f
                    .stripPrefix(1)
                    .addRequestHeader("X-Gateway", "true")
                    .circuitBreaker(c -> c
                        .setName("userServiceCB")
                        .setFallbackUri("forward:/fallback/users")
                    )
                )
                .uri("lb://user-service")
            )
            
            // Route Product Service
            .route("product-service", r -> r
                .path("/api/products/**")
                .and()
                .method("GET", "POST", "PUT", "DELETE")
                .filters(f -> f
                    .rewritePath("/api/products/(?<segment>.*)", "/products/${segment}")
                    .retry(config -> config
                        .setRetries(3)
                        .setStatuses(HttpStatus.BAD_GATEWAY)
                    )
                )
                .uri("lb://product-service")
            )
            
            // Route with Rate Limiting
            .route("order-service", r -> r
                .path("/api/orders/**")
                .filters(f -> f
                    .requestRateLimiter(config -> config
                        .setRateLimiter(redisRateLimiter())
                        .setKeyResolver(userKeyResolver())
                    )
                )
                .uri("lb://order-service")
            )
            
            .build();
    }
}

---

9. Custom Filters

9.1 Global Filter (All Routes)

package com.example.gateway.filter;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.cloud.gateway.filter.GatewayFilterChain;
import org.springframework.cloud.gateway.filter.GlobalFilter;
import org.springframework.core.Ordered;
import org.springframework.http.server.reactive.ServerHttpRequest;
import org.springframework.stereotype.Component;
import org.springframework.web.server.ServerWebExchange;
import reactor.core.publisher.Mono;

import java.util.UUID;

@Component
public class LoggingFilter implements GlobalFilter, Ordered {
    
    private static final Logger log = LoggerFactory.getLogger(LoggingFilter.class);
    
    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        // PRE-FILTER: Before sending to the service
        String requestId = UUID.randomUUID().toString();
        long startTime = System.currentTimeMillis();
        
        ServerHttpRequest request = exchange.getRequest().mutate()
            .header("X-Request-Id", requestId)
            .build();
        
        log.info(">>> Request: {} {} | RequestId: {}", 
            request.getMethod(), 
            request.getURI().getPath(),
            requestId
        );
        
        // Modify the exchange with the new request
        ServerWebExchange modifiedExchange = exchange.mutate()
            .request(request)
            .build();
        
        return chain.filter(modifiedExchange)
            .then(Mono.fromRunnable(() -> {
                // POST-FILTER: After the response
                long duration = System.currentTimeMillis() - startTime;
                log.info("<<< Response: {} | Duration: {}ms | RequestId: {}",
                    exchange.getResponse().getStatusCode(),
                    duration,
                    requestId
                );
            }));
    }
    
    @Override
    public int getOrder() {
        return -1; // Executed first
    }
}

9.2 GatewayFilter (Specific Route)

package com.example.gateway.filter;

import org.springframework.cloud.gateway.filter.GatewayFilter;
import org.springframework.cloud.gateway.filter.factory.AbstractGatewayFilterFactory;
import org.springframework.http.HttpStatus;
import org.springframework.stereotype.Component;

@Component
public class AuthenticationFilter extends AbstractGatewayFilterFactory<AuthenticationFilter.Config> {
    
    public AuthenticationFilter() {
        super(Config.class);
    }
    
    @Override
    public GatewayFilter apply(Config config) {
        return (exchange, chain) -> {
            String authHeader = exchange.getRequest()
                .getHeaders()
                .getFirst("Authorization");
            
            if (authHeader == null || !authHeader.startsWith("Bearer ")) {
                exchange.getResponse().setStatusCode(HttpStatus.UNAUTHORIZED);
                return exchange.getResponse().setComplete();
            }
            
            String token = authHeader.substring(7);
            
            // Validate the token (simplified)
            if (!isValidToken(token)) {
                exchange.getResponse().setStatusCode(HttpStatus.UNAUTHORIZED);
                return exchange.getResponse().setComplete();
            }
            
            return chain.filter(exchange);
        };
    }
    
    private boolean isValidToken(String token) {
        // Implement your JWT validation logic
        return token != null && token.length() > 10;
    }
    
    public static class Config {
        // Filter configuration if needed
    }
}

Usage in YAML:

filters:
  - Authentication

---

10. Fallback and Circuit Breaker

10.1 Fallback Configuration

spring:
  cloud:
    gateway:
      routes:
        - id: user-service
          uri: lb://user-service
          predicates:
            - Path=/api/users/**
          filters:
            - name: CircuitBreaker
              args:
                name: userServiceCB
                fallbackUri: forward:/fallback/users

10.2 Fallback Controller

package com.example.gateway.controller;

import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import java.util.Map;

@RestController
@RequestMapping("/fallback")
public class FallbackController {
    
    @GetMapping("/users")
    public ResponseEntity<Map<String, String>> userServiceFallback() {
        return ResponseEntity
            .status(HttpStatus.SERVICE_UNAVAILABLE)
            .body(Map.of(
                "message", "User Service is currently unavailable",
                "status", "FALLBACK"
            ));
    }
    
    @GetMapping("/products")
    public ResponseEntity<Map<String, String>> productServiceFallback() {
        return ResponseEntity
            .status(HttpStatus.SERVICE_UNAVAILABLE)
            .body(Map.of(
                "message", "Product Service is currently unavailable",
                "status", "FALLBACK"
            ));
    }
    
    @GetMapping("/orders")
    public ResponseEntity<Map<String, String>> orderServiceFallback() {
        return ResponseEntity
            .status(HttpStatus.SERVICE_UNAVAILABLE)
            .body(Map.of(
                "message", "Order Service is currently unavailable",
                "status", "FALLBACK"
            ));
    }
}

10.3 Resilience4j Configuration

resilience4j:
  circuitbreaker:
    configs:
      default:
        slidingWindowSize: 10
        failureRateThreshold: 50
        waitDurationInOpenState: 10000
        permittedNumberOfCallsInHalfOpenState: 3
        automaticTransitionFromOpenToHalfOpenEnabled: true
    instances:
      userServiceCB:
        baseConfig: default
      productServiceCB:
        baseConfig: default
        failureRateThreshold: 30

---

11. CORS Configuration

package com.example.gateway.config;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.cors.CorsConfiguration;
import org.springframework.web.cors.reactive.CorsWebFilter;
import org.springframework.web.cors.reactive.UrlBasedCorsConfigurationSource;

import java.util.Arrays;
import java.util.List;

@Configuration
public class CorsConfig {
    
    @Bean
    public CorsWebFilter corsWebFilter() {
        CorsConfiguration corsConfig = new CorsConfiguration();
        corsConfig.setAllowedOrigins(List.of(
            "http://localhost:3000",
            "https://myapp.com"
        ));
        corsConfig.setAllowedMethods(Arrays.asList(
            "GET", "POST", "PUT", "DELETE", "OPTIONS"
        ));
        corsConfig.setAllowedHeaders(List.of("*"));
        corsConfig.setAllowCredentials(true);
        corsConfig.setMaxAge(3600L);
        
        UrlBasedCorsConfigurationSource source = new UrlBasedCorsConfigurationSource();
        source.registerCorsConfiguration("/**", corsConfig);
        
        return new CorsWebFilter(source);
    }
}

Or via YAML:

spring:
  cloud:
    gateway:
      globalcors:
        cors-configurations:
          '[/**]':
            allowedOrigins: "http://localhost:3000"
            allowedMethods:
              - GET
              - POST
              - PUT
              - DELETE
            allowedHeaders: "*"
            allowCredentials: true

---

12. Monitoring and Actuator

management:
  endpoints:
    web:
      exposure:
        include: health,info,gateway
  endpoint:
    gateway:
      enabled: true

Useful Endpoints

# List of routes
curl http://localhost:8080/actuator/gateway/routes

# Refresh routes
curl -X POST http://localhost:8080/actuator/gateway/refresh

# Global filters
curl http://localhost:8080/actuator/gateway/globalfilters

# Route filters
curl http://localhost:8080/actuator/gateway/routefilters

---

13. Summary

Key Points

✅ API Gateway = Single entry point
✅ Routes = Define where to send requests
✅ Predicates = Matching conditions
✅ Filters = Modify request/response
✅ lb:// = Load Balancing via Eureka
✅ CircuitBreaker = Protection against failures
✅ RateLimiter = Protection against abuse

Production Type Configuration

spring:
  cloud:
    gateway:
      routes:
        - id: user-service
          uri: lb://user-service
          predicates:
            - Path=/api/users/**
          filters:
            - StripPrefix=1
            - name: CircuitBreaker
              args:
                name: userServiceCB
                fallbackUri: forward:/fallback/users
            - name: RequestRateLimiter
              args:
                redis-rate-limiter.replenishRate: 100
                redis-rate-limiter.burstCapacity: 200

---

Next Chapter: Inter-Service Communication →

Back to Module 13