Introduction to Spring Cloud Gateway

Spring Cloud Gateway serves as an API gateway built on the Spring ecosystem, leveraging technologies like Spring 5, Spring Boot 2.0, and Project Reactor. It provides a straightforward approach to route APIs while offering robust filter capabilities including circuit breaking, rate limiting, and retries.

Core Components

1. Routing Fundamentals

A route consists of three key elements:

• ID: Unique route identifier
• Target URI: Destination endpoint
• Predicates: Matching conditinos for requests
• Filters: Request/response modifiers

2. Request Processing Flow

1. Client request enters Gateway
2. Handler mapping finds matching route
3. Request pases through filter chain
4. Proxy request sent to backend service
5. Response processed through post-filters

Basic Configuration Example

spring:
  cloud:
    gateway:
      routes:
        - id: payment_route
          uri: lb://payment-service
          predicates:
            - Path=/payment/**
          filters:
            - AddRequestHeader=X-Request-Red, Blue

Predicate Factories

Gateway provides multiple built-in predicate factories:

Path Predicate

predicates:
  - Path=/api/v1/**

Header Predicate

predicates:
  - Header=X-Request-Id, \d+

Filter Factories

Gateway offers various filter factories to modify requests/responses:

AddRequestHeader Filter

filters:
  - AddRequestHeader=X-Request-Red, Blue

RewritePath Filter

filters:
  - RewritePath=/service/(?<segment>.*), /$\{segment}

Global Filters

Filters applied to all routes:

@Component
public class CustomGlobalFilter implements GlobalFilter, Ordered {
  @Override
  public Mono<Void> filter(ServerWebExchange exchange, 
                         GatewayFilterChain chain) {
    // Pre-processing logic
    return chain.filter(exchange).then(Mono.fromRunnable(() -> {
      // Post-processing logic
    }));
  }
}

Rate Limiting

Redis-based rate limiting configuration:

filters:
  - name: RequestRateLimiter
    args:
      redis-rate-limiter.replenishRate: 10
      redis-rate-limiter.burstCapacity: 20

Load Balancing

Service discovery integration:

spring:
  cloud:
    gateway:
      discovery:
        locator:
          enabled: true

Circuit Breaker

filters:
  - name: CircuitBreaker
    args:
      name: myCircuitBreaker
      fallbackUri: forward:/fallback

WebSocket Support

routes:
  - id: websocket_route
    uri: ws://service:8080
    predicates:
      - Path=/ws/**

Monitoring Endpoints

Actuator endpoints for monitoring:

• /actuator/gateway/routes - List all routes
• /actuator/gateway/globalfilters - Show global filters
• /actuator/gateway/routefilters - Display route filters

Custom Filter Implementation

@Component
public class CustomFilterFactory extends 
    AbstractGatewayFilterFactory<CustomFilterFactory.Config> {
  
  @Override
  public GatewayFilter apply(Config config) {
    return (exchange, chain) -> {
      // Filter logic
      return chain.filter(exchange);
    };
  }
  
  public static class Config {
    // Configuration properties
  }
}

TLS/SSL Configuration

server:
  ssl:
    enabled: true
    key-store: classpath:keystore.p12
    key-store-password: changeit