Real World Example

Apps like Swiggy, Myntra and Paytm talk to one API host, while hundreds of services run behind it. Netflix built Zuul for this; Spring Cloud Gateway is the Spring successor. The ShopEase gateway setup:

With the Resilience4j starter (spring-cloud-starter-circuitbreaker-reactor-resilience4j) the gateway can return a friendly response instead of a 500 when product-service is down:

spring:
  cloud:
    gateway:
      routes:
        - id: product-service
          uri: lb://product-service
          predicates:
            - Path=/api/products/**
          filters:
            - name: CircuitBreaker
              args:
                name: productsCB
                fallbackUri: forward:/fallback/products

@RestController
class FallbackController {

    @GetMapping("/fallback/products")
    ResponseEntity<Map<String, Object>> productsFallback() {
        return ResponseEntity.status(HttpStatus.SERVICE_UNAVAILABLE).body(Map.of(
                "message", "Products are taking longer than usual. Please try again in a minute.",
                "products", List.of()));
    }
}

curl -i localhost:8080/api/products   # product-service stopped

HTTP/1.1 503 Service Unavailable
{"message":"Products are taking longer than usual. Please try again in a minute.","products":[]}