API Gateway

// API Gateway architecture
//
//  Mobile App    Web App    3rd-party
//       │           │          │
//       └─────┬─────┴──────────┘
//             ▼
//     ┌──────────────────┐
//     │   API Gateway     │  ← routing, auth, rate limit, aggregation
//     │  (Kong / NGINX)   │
//     └──┬──┬──┬──┬──────┘
//        │  │  │  │
//        ▼  ▼  ▼  ▼
//    User  Order  Payment  Search
//    Svc   Svc    Svc      Svc

// Gateway responsibilities:
// 1. Routing:      /api/users/** → User Service
// 2. Auth:         Validate JWT, attach user context
// 3. Rate limit:   100 req/min per API key
// 4. Aggregation:  /api/dashboard → calls 3 services, merges response
// 5. Transformation: REST → gRPC (for internal services)
// 6. SSL termination: HTTPS at gateway, HTTP internally
// 7. Logging:      Centralised access logs, request tracing

// Spring Cloud Gateway — route configuration
spring:
  cloud:
    gateway:
      routes:
        - id: user-service
          uri: lb://USER-SERVICE       # service discovery
          predicates:
            - Path=/api/v1/users/**
          filters:
            - StripPrefix=0
            - name: CircuitBreaker
              args:
                name: userServiceCB
                fallbackUri: forward:/fallback/users

        - id: order-service
          uri: lb://ORDER-SERVICE
          predicates:
            - Path=/api/v1/orders/**
          filters:
            - name: RequestRateLimiter
              args:
                redis-rate-limiter.replenishRate: 10
                redis-rate-limiter.burstCapacity: 20
                key-resolver: "#{@userKeyResolver}"

// JWT authentication filter
@Component
public class JwtAuthFilter implements GlobalFilter {
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        String token = exchange.getRequest().getHeaders().getFirst("Authorization");
        if (token == null || !jwtUtil.isValid(token)) {
            exchange.getResponse().setStatusCode(HttpStatus.UNAUTHORIZED);
            return exchange.getResponse().setComplete();
        }
        return chain.filter(exchange);
    }
}