spring-cloud-api-gateway

In a microservices architecture, clients would need to know the location of every service, handle authentication for each service, and manage cross-cutting concerns (rate limiting, logging) individually. This creates tight coupling between clients and services, and duplicates security/observability logic.

Spring Cloud Gateway is an API Gateway built on Spring WebFlux (Reactive). It acts as a single entry point for all client requests, routing them to the appropriate microservice based on path, headers, or other predicates. It handles cross-cutting concerns — authentication, rate limiting, header transformation — in one place.

1. Route definition: Routes match incoming requests using predicates (path, method, header, query param)
2. Predicate matching: Path=/users/** matches all requests starting with /users
3. Filter chains: Before routing to the service, the request passes through a filter chain: authentication, rate limiting, header modification
4. Service routing: Gateway forwards the request to the target service (discovered via Eureka or configured URI)
5. Response filters: On the return path, filters modify the response (add headers, transform body)
6. WebFlux: Built on reactive Netty, not Tomcat — non-blocking I/O for high throughput

• Reactive: Built on Spring WebFlux, not Servlet API — non-blocking, high concurrency
• Route predicates: Path, Method, Header, Query, Cookie, Host, RemoteAddr, Weight
• Filters: AddRequestHeader, AddResponseHeader, CircuitBreaker, Retry, RateLimiter, RequestRateLimiter
• Service discovery integration: Automatically resolves service names via Eureka / LoadBalancer
• Custom filters: Implement GatewayFilter or GlobalFilter interfaces for custom logic
• Circuit breaking: Integrates with Resilience4j for downstream service failure protection

• Built from: Spring Cloud — Gateway is a Spring Cloud module for edge services
• Built from: Spring Cloud Service Discovery — Gateway uses Eureka to find service instances
• Related: Spring Boot — Gateway is auto-configured via Spring Boot
• Builds into: Java Microservices — API Gateway is a core component of microservice architecture
• Contrasts with: Servlets — Servlets are blocking I/O; Gateway uses reactive non-blocking WebFlux

• WebFlux vs MVC: Gateway runs on Netty (WebFlux) — can’t use @Controller, RestTemplate; use WebClient instead
• WebSocket support: Gateway supports WebSocket proxying via ws: / wss: route URIs
• CORS: Gateway must handle CORS at the edge — configure spring.cloud.gateway.globalcors
• Latency: Each filter adds latency — keep filter chains lean
• Gateway as SPOF: The gateway is a single point of entry — deploy multiple instances behind a load balancer