Backend for Frontend (BFF) — Deep Dive

graph TD
    M[Mobile App] --> MBFF[Mobile BFF]
    W[Web App] --> WBFF[Web BFF]
    P[Partner API] --> PBFF[Partner BFF]
    MBFF --> OS[Order Service]
    WBFF --> OS
    PBFF --> OS
    MBFF --> US[User Service]
    WBFF --> US
    WBFF --> RS[Recommendation Service]

graph TD
    subgraph Clients
        M[Mobile]
        W[Web]
        TV[Smart TV]
    end
    subgraph BFFs
        MBFF[Mobile BFF]
        WBFF[Web BFF]
        TVBFF[TV BFF]
    end
    subgraph Services
        OS[Orders]
        US[Users]
        PS[Products]
    end
    M --> MBFF
    W --> WBFF
    TV --> TVBFF
    MBFF --> OS
    MBFF --> US
    WBFF --> OS
    WBFF --> US
    WBFF --> PS
    TVBFF --> PS

graph TD
    M[Mobile] -->|REST| MBFF[Mobile BFF]
    W[Web] -->|GraphQL| GQL[GraphQL BFF]
    MBFF --> Services
    GQL --> Services

@RestController
@RequestMapping("/api/v1/dashboard")
public class MobileDashboardController {

    private final OrderClient orderClient;
    private final UserClient userClient;

    @GetMapping
    public MobileDashboardResponse getDashboard(@RequestHeader("X-User-Id") String userId) {
        // Parallel calls
        CompletableFuture<List<Order>> ordersFuture = 
            CompletableFuture.supplyAsync(() -> orderClient.getRecentOrders(userId, 5));
        CompletableFuture<User> userFuture = 
            CompletableFuture.supplyAsync(() -> userClient.getUser(userId));

        // Wait and combine
        List<Order> orders = ordersFuture.join();
        User user = userFuture.join();

        // Transform for mobile — compact payload
        return MobileDashboardResponse.builder()
            .userName(user.getFirstName())
            .orderCount(orders.size())
            .orders(orders.stream()
                .map(o -> MobileOrderSummary.builder()
                    .id(o.getId())
                    .status(o.getStatus())
                    .total(o.getTotal())
                    .build())
                .toList())
            .build();
    }
}

@RestController
@RequestMapping("/api/v1/dashboard")
public class WebDashboardController {

    @GetMapping
    public WebDashboardResponse getDashboard(@RequestHeader("X-User-Id") String userId) {
        // More data for web
        CompletableFuture<List<Order>> ordersFuture = 
            CompletableFuture.supplyAsync(() -> orderClient.getOrders(userId, 20));
        CompletableFuture<User> userFuture = 
            CompletableFuture.supplyAsync(() -> userClient.getUserWithPreferences(userId));
        CompletableFuture<List<Product>> recommendationsFuture = 
            CompletableFuture.supplyAsync(() -> recommendationClient.getForUser(userId));

        // Rich response for web
        return WebDashboardResponse.builder()
            .user(userFuture.join())  // Full user object
            .orders(ordersFuture.join().stream()
                .map(this::toDetailedOrder)  // Includes items, addresses
                .toList())
            .recommendations(recommendationsFuture.join())
            .analytics(getAnalytics(userId))
            .build();
    }
}

graph TD
    subgraph Edge
        GW[API Gateway]
    end
    subgraph Mesh
        MBFF[Mobile BFF + Envoy]
        OS[Order Service + Envoy]
        US[User Service + Envoy]
    end
    GW --> MBFF
    MBFF --> OS
    MBFF --> US

@Pact(consumer = "mobile-bff", provider = "order-service")
public RequestResponsePact getOrdersPact(PactDslWithProvider builder) {
    return builder
        .given("orders exist for user 123")
        .uponReceiving("a request for recent orders")
        .path("/orders")
        .query("userId=123&limit=5")
        .method("GET")
        .willRespondWith()
        .status(200)
        .body(newJsonArrayMinLike(1, order -> 
            order.stringType("id")
                 .stringType("status")))
        .toPact();
}