feat: Add Backends For Frontends (BFF) pattern (#300) (#3543)

* Add Backends For Frontends pattern (#300)

Implements the BFF pattern with two client-specific gateways
(MobileBff, DesktopBff) aggregating shared downstream services
(AuthService, CartService, OrderService, SupplierService) into
client-tailored response shapes.

Closes #300

* Rename SupplierService lookup key to productName for clarity

The service is looked up by product name throughout the codebase, not
an actual product ID as the previous naming implied.

* Address review feedback: remove unnecessary compact constructors, add InMemory* service tests

backends-for-frontends/src/main/java/com/iluwatar/bff/App.java

@@ -0,0 +1,109 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff;
+
+import com.iluwatar.bff.bff.DesktopBff;
+import com.iluwatar.bff.bff.MobileBff;
+import com.iluwatar.bff.model.CartItem;
+import com.iluwatar.bff.model.Order;
+import com.iluwatar.bff.model.Product;
+import com.iluwatar.bff.model.SupplierRecord;
+import com.iluwatar.bff.model.User;
+import com.iluwatar.bff.service.impl.InMemoryAuthService;
+import com.iluwatar.bff.service.impl.InMemoryCartService;
+import com.iluwatar.bff.service.impl.InMemoryOrderService;
+import com.iluwatar.bff.service.impl.InMemorySupplierService;
+import java.util.List;
+import java.util.Map;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * {@link App} demonstrates the Backends For Frontends (BFF) pattern.
+ *
+ * <p>A single set of downstream microservices (customer authentication, cart, order and supplier
+ * services) is shared by every client. Two different client-facing gateways -- {@link MobileBff}
+ * for the mobile apps and {@link DesktopBff} for the intranet desktop/chatbot clients -- each call
+ * a different subset of those services and reshape the results into a response tailored to what
+ * their own client actually needs, rather than exposing one one-size-fits-all API to every client.
+ */
+public final class App {
+
+  /** Logger for this class. */
+  private static final Logger LOGGER = LoggerFactory.getLogger(App.class);
+
+  /** User id used in the demonstration. */
+  private static final String USER_ID = "u-1";
+
+  /** Product id used in the demonstration. */
+  private static final String PRODUCT_ID = "p-42";
+
+  /** Unit price of the demo product in US dollars. */
+  private static final double DEMO_PRICE_USD = 79.99;
+
+  /** Supplier stock level used in the demonstration. */
+  private static final int DEMO_STOCK_LEVEL = 120;
+
+  private App() {
+    // utility class
+  }
+
+  /**
+   * Program entry point.
+   *
+   * @param args no argument sent
+   */
+  public static void main(final String[] args) {
+    // shared downstream microservices, as drawn in the pattern diagram
+    var authService = new InMemoryAuthService(Map.of(USER_ID, new User(USER_ID, "Alice", "GOLD")));
+
+    var product = new Product(PRODUCT_ID, "Wireless Headphones", DEMO_PRICE_USD);
+    var cartService = new InMemoryCartService(Map.of(USER_ID, List.of(new CartItem(product, 2))));
+
+    var orderService =
+        new InMemoryOrderService(
+            Map.of(
+                USER_ID,
+                List.of(
+                    new Order("o-1", "Wireless Headphones", "DELIVERED"),
+                    new Order("o-2", "USB-C Cable", "IN_TRANSIT"))));
+
+    var supplierService =
+        new InMemorySupplierService(
+            Map.of(
+                "Wireless Headphones",
+                List.of(new SupplierRecord(PRODUCT_ID, "Acme Audio Co.", DEMO_STOCK_LEVEL))));
+
+    // client-specific BFFs, each calling only the services their client needs
+    var mobileBff = new MobileBff(authService, cartService, orderService);
+    var desktopBff = new DesktopBff(authService, orderService, supplierService);
+
+    var mobileResponse = mobileBff.getDashboard(USER_ID);
+    LOGGER.info("Mobile BFF response: {}", mobileResponse);
+
+    var desktopResponse = desktopBff.getDashboard(USER_ID);
+    LOGGER.info("Desktop BFF response: {}", desktopResponse);
+  }
+}

backends-for-frontends/src/main/java/com/iluwatar/bff/bff/ClientBff.java

@@ -0,0 +1,44 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.bff;
+
+/**
+ * Common contract every client-specific Backend For Frontend implements: given a user id, build
+ * whatever response shape ({@code T}) that particular client needs. Each implementation is free to
+ * call a different subset of downstream services and aggregate them differently -- that freedom is
+ * the entire point of the pattern.
+ *
+ * @param <T> the response DTO shape this BFF returns to its client
+ */
+public interface ClientBff<T> {
+
+  /**
+   * Builds the dashboard response for a given user, tailored to this BFF's client.
+   *
+   * @param userId identifier of the user requesting their dashboard
+   * @return the client-specific response
+   */
+  T getDashboard(String userId);
+}

backends-for-frontends/src/main/java/com/iluwatar/bff/bff/DesktopBff.java

@@ -0,0 +1,94 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.bff;
+
+import com.iluwatar.bff.dto.DesktopDashboardResponse;
+import com.iluwatar.bff.service.AuthService;
+import com.iluwatar.bff.service.OrderService;
+import com.iluwatar.bff.service.SupplierService;
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * Backend For Frontend serving the intranet clients (desktop app, chatbot) from the diagram. It
+ * aggregates the auth, order and supplier services and reshapes the results into a richer,
+ * back-office style payload. Unlike {@link MobileBff}, it does not touch the cart service at all
+ * and it does reach into the intranet-only supplier service, matching the fan-out drawn in the
+ * pattern diagram.
+ */
+public final class DesktopBff implements ClientBff<DesktopDashboardResponse> {
+
+  /** The customer authentication service. */
+  private final AuthService authService;
+
+  /** The order service. */
+  private final OrderService orderService;
+
+  /** The supplier service (intranet-only). */
+  private final SupplierService supplierService;
+
+  /**
+   * Creates a desktop BFF wired to the downstream services it depends on.
+   *
+   * @param auth the customer authentication service
+   * @param orders the order service
+   * @param suppliers the supplier service
+   */
+  public DesktopBff(
+      final AuthService auth, final OrderService orders, final SupplierService suppliers) {
+    this.authService = auth;
+    this.orderService = orders;
+    this.supplierService = suppliers;
+  }
+
+  @Override
+  public DesktopDashboardResponse getDashboard(final String userId) {
+    var user = authService.getUser(userId);
+    var orders = orderService.getOrders(userId);
+
+    var orderStatuses =
+        orders.stream()
+            .map(order -> order.id() + ": " + order.productName() + " [" + order.status() + "]")
+            .toList();
+
+    var supplierStockSummaries = new ArrayList<String>();
+    for (var order : orders) {
+      for (var supplierRecord : supplierService.getSupplierRecords(order.productName())) {
+        supplierStockSummaries.add(
+            supplierRecord.supplierName()
+                + ": "
+                + supplierRecord.stockLevel()
+                + " units of "
+                + order.productName());
+      }
+    }
+
+    return new DesktopDashboardResponse(
+        "Welcome back, " + user.displayName(),
+        user.loyaltyTier(),
+        List.copyOf(orderStatuses),
+        List.copyOf(supplierStockSummaries));
+  }
+}

backends-for-frontends/src/main/java/com/iluwatar/bff/bff/MobileBff.java

@@ -0,0 +1,86 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.bff;
+
+import com.iluwatar.bff.dto.MobileDashboardResponse;
+import com.iluwatar.bff.service.AuthService;
+import com.iluwatar.bff.service.CartService;
+import com.iluwatar.bff.service.OrderService;
+import java.util.List;
+
+/**
+ * Backend For Frontend serving the mobile clients (iOS app, Android app) from the diagram. It
+ * aggregates the auth, cart and order services and reshapes the results into a small payload suited
+ * to a phone screen and a limited-bandwidth connection. It never calls the supplier service: a
+ * mobile shopper has no use for back-office stock data, so this BFF simply does not expose it,
+ * rather than sending it and letting the client ignore it.
+ */
+public final class MobileBff implements ClientBff<MobileDashboardResponse> {
+
+  /** Maximum number of recent order summaries to include in the mobile response. */
+  private static final int MAX_RECENT_ORDERS = 3;
+
+  /** The customer authentication service. */
+  private final AuthService authService;
+
+  /** The cart service. */
+  private final CartService cartService;
+
+  /** The order service. */
+  private final OrderService orderService;
+
+  /**
+   * Creates a mobile BFF wired to the downstream services it depends on.
+   *
+   * @param auth the customer authentication service
+   * @param cart the cart service
+   * @param orders the order service
+   */
+  public MobileBff(final AuthService auth, final CartService cart, final OrderService orders) {
+    this.authService = auth;
+    this.cartService = cart;
+    this.orderService = orders;
+  }
+
+  @Override
+  public MobileDashboardResponse getDashboard(final String userId) {
+    var user = authService.getUser(userId);
+    var cart = cartService.getCart(userId);
+    var orders = orderService.getOrders(userId);
+
+    var cartTotal = cart.stream().mapToDouble(item -> item.lineTotal()).sum();
+    var recentOrderSummaries =
+        orders.stream()
+            .limit(MAX_RECENT_ORDERS)
+            .map(order -> order.productName() + " (" + order.status() + ")")
+            .toList();
+
+    return new MobileDashboardResponse(
+        "Hi " + user.displayName() + "!",
+        cart.size(),
+        cartTotal,
+        List.copyOf(recentOrderSummaries));
+  }
+}

backends-for-frontends/src/main/java/com/iluwatar/bff/bff/package-info.java

@@ -0,0 +1,29 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+/**
+ * Client-specific Backend For Frontend implementations: one per client type (mobile, desktop) that
+ * each aggregate a different subset of downstream services.
+ */
+package com.iluwatar.bff.bff;

backends-for-frontends/src/main/java/com/iluwatar/bff/dto/DesktopDashboardResponse.java

@@ -0,0 +1,42 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.dto;
+
+import java.util.List;
+
+/**
+ * The shape of data the desktop back-office client renders: richer than the mobile payload,
+ * including full order status and supplier stock levels that a mobile shopper never needs.
+ *
+ * @param greeting a short personalized greeting for the user
+ * @param loyaltyTier the user's loyalty program tier
+ * @param orderStatuses detailed "id: productName [status]" lines for every order
+ * @param supplierStockSummaries "supplierName: stockLevel units" lines for relevant products
+ */
+public record DesktopDashboardResponse(
+    String greeting,
+    String loyaltyTier,
+    List<String> orderStatuses,
+    List<String> supplierStockSummaries) {}

backends-for-frontends/src/main/java/com/iluwatar/bff/dto/MobileDashboardResponse.java

@@ -0,0 +1,39 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.dto;
+
+import java.util.List;
+
+/**
+ * The shape of data the mobile (iOS/Android) client actually renders: a lean payload with just
+ * enough for a small screen, deliberately excluding fields the desktop client needs.
+ *
+ * @param greeting a short personalized greeting for the user
+ * @param cartItemCount number of items currently in the user's cart
+ * @param cartTotalUsd total value of the user's cart in US dollars
+ * @param recentOrderSummaries short human-readable summaries of the user's most recent orders
+ */
+public record MobileDashboardResponse(
+    String greeting, int cartItemCount, double cartTotalUsd, List<String> recentOrderSummaries) {}

backends-for-frontends/src/main/java/com/iluwatar/bff/dto/package-info.java

@@ -0,0 +1,28 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+/**
+ * Response DTOs shaped by each BFF for its specific client (mobile dashboard, desktop dashboard).
+ */
+package com.iluwatar.bff.dto;

backends-for-frontends/src/main/java/com/iluwatar/bff/model/CartItem.java

@@ -0,0 +1,43 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.model;
+
+/**
+ * A single line item inside a user's shopping cart, as returned by the cart service API.
+ *
+ * @param product the product being purchased
+ * @param quantity number of units of the product
+ */
+public record CartItem(Product product, int quantity) {
+
+  /**
+   * Computes the line total for this cart item.
+   *
+   * @return quantity multiplied by unit price
+   */
+  public double lineTotal() {
+    return quantity * product.priceUsd();
+  }
+}

backends-for-frontends/src/main/java/com/iluwatar/bff/model/Order.java

@@ -0,0 +1,34 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.model;
+
+/**
+ * A past order returned by the order service API.
+ *
+ * @param id order identifier
+ * @param productName name of the ordered product
+ * @param status current fulfillment status, e.g. "DELIVERED", "IN_TRANSIT"
+ */
+public record Order(String id, String productName, String status) {}

backends-for-frontends/src/main/java/com/iluwatar/bff/model/Product.java

@@ -0,0 +1,35 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.model;
+
+/**
+ * Simple product record returned by the downstream Cart/Order services. Represents the data a
+ * catalog or cart entry carries before each BFF trims or reshapes it for its own client.
+ *
+ * @param id product identifier
+ * @param name display name of the product
+ * @param priceUsd unit price in US dollars
+ */
+public record Product(String id, String name, double priceUsd) {}

backends-for-frontends/src/main/java/com/iluwatar/bff/model/SupplierRecord.java

@@ -0,0 +1,35 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.model;
+
+/**
+ * A supplier stock record returned by the supplier service API, used by the desktop back-office
+ * client to show inventory information that mobile customers never need to see.
+ *
+ * @param productId identifier of the product this record refers to
+ * @param supplierName name of the supplying vendor
+ * @param stockLevel units currently available from this supplier
+ */
+public record SupplierRecord(String productId, String supplierName, int stockLevel) {}

backends-for-frontends/src/main/java/com/iluwatar/bff/model/User.java

@@ -0,0 +1,35 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.model;
+
+/**
+ * Authenticated user profile returned by the customer authentication service API. Kept
+ * intentionally minimal; each BFF decides which of these fields its client actually needs.
+ *
+ * @param id unique user identifier
+ * @param displayName human-readable name of the user
+ * @param loyaltyTier loyalty program tier, e.g. "GOLD", "SILVER", "STANDARD"
+ */
+public record User(String id, String displayName, String loyaltyTier) {}

backends-for-frontends/src/main/java/com/iluwatar/bff/model/package-info.java

@@ -0,0 +1,30 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+/**
+ * Domain model records shared across downstream services: User, Product, CartItem, Order, and
+ * SupplierRecord. Each BFF consumes these raw domain objects and reshapes them into its own
+ * client-specific DTO.
+ */
+package com.iluwatar.bff.model;

backends-for-frontends/src/main/java/com/iluwatar/bff/package-info.java

@@ -0,0 +1,29 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+/**
+ * Backends For Frontends pattern: a dedicated gateway per client type (mobile, desktop) that
+ * aggregates only the downstream services each client actually needs.
+ */
+package com.iluwatar.bff;

backends-for-frontends/src/main/java/com/iluwatar/bff/service/AuthService.java

@@ -0,0 +1,43 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.service;
+
+import com.iluwatar.bff.model.User;
+
+/**
+ * Represents the customer authentication service API from the diagram: a downstream microservice
+ * shared by every client-specific BFF. Each BFF calls this the same way; only what they do with the
+ * result differs.
+ */
+public interface AuthService {
+
+  /**
+   * Looks up the authenticated user profile for the given id.
+   *
+   * @param userId identifier of the user to look up
+   * @return the matching {@link User}
+   */
+  User getUser(String userId);
+}

backends-for-frontends/src/main/java/com/iluwatar/bff/service/CartService.java

@@ -0,0 +1,43 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.service;
+
+import com.iluwatar.bff.model.CartItem;
+import java.util.List;
+
+/**
+ * Represents the cart service API from the diagram. In this example only the mobile and android
+ * clients need cart data, so only {@link com.iluwatar.bff.bff.MobileBff} calls this service.
+ */
+public interface CartService {
+
+  /**
+   * Retrieves the current cart contents for a user.
+   *
+   * @param userId identifier of the user whose cart is requested
+   * @return the list of items currently in the user's cart
+   */
+  List<CartItem> getCart(String userId);
+}

backends-for-frontends/src/main/java/com/iluwatar/bff/service/OrderService.java

@@ -0,0 +1,43 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.service;
+
+import com.iluwatar.bff.model.Order;
+import java.util.List;
+
+/**
+ * Represents the order service API from the diagram. This downstream service is shared by every
+ * client-specific BFF, matching the fan-out shown for "order service API" in the pattern diagram.
+ */
+public interface OrderService {
+
+  /**
+   * Retrieves the order history for a user.
+   *
+   * @param userId identifier of the user whose orders are requested
+   * @return the list of past orders for the user
+   */
+  List<Order> getOrders(String userId);
+}

backends-for-frontends/src/main/java/com/iluwatar/bff/service/SupplierService.java

@@ -0,0 +1,43 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.service;
+
+import com.iluwatar.bff.model.SupplierRecord;
+import java.util.List;
+
+/**
+ * Represents the supplier service API from the diagram, reachable only from the intranet services
+ * server. Only back-office style clients (desktop app, chatbot) call this service.
+ */
+public interface SupplierService {
+
+  /**
+   * Retrieves supplier stock records for a product.
+   *
+   * @param productName name of the product to check stock for
+   * @return the list of supplier stock records for the product
+   */
+  List<SupplierRecord> getSupplierRecords(String productName);
+}

backends-for-frontends/src/main/java/com/iluwatar/bff/service/impl/InMemoryAuthService.java

@@ -0,0 +1,58 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.service.impl;
+
+import com.iluwatar.bff.model.User;
+import com.iluwatar.bff.service.AuthService;
+import java.util.Map;
+
+/**
+ * In-memory stand-in for the real customer authentication service API. Real deployments would
+ * replace this with an HTTP/gRPC client; the BFFs are written against the {@link AuthService}
+ * interface, so swapping the implementation later requires no change to any BFF.
+ */
+public final class InMemoryAuthService implements AuthService {
+
+  /** Users stored in memory, keyed by user id. */
+  private final Map<String, User> users;
+
+  /**
+   * Creates the service with a fixed backing map of users, keyed by user id.
+   *
+   * @param userData the user data this service serves
+   */
+  public InMemoryAuthService(final Map<String, User> userData) {
+    this.users = userData;
+  }
+
+  @Override
+  public User getUser(final String userId) {
+    var user = users.get(userId);
+    if (user == null) {
+      throw new IllegalArgumentException("Unknown user id: " + userId);
+    }
+    return user;
+  }
+}

backends-for-frontends/src/main/java/com/iluwatar/bff/service/impl/InMemoryCartService.java

@@ -0,0 +1,51 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.service.impl;
+
+import com.iluwatar.bff.model.CartItem;
+import com.iluwatar.bff.service.CartService;
+import java.util.List;
+import java.util.Map;
+
+/** In-memory stand-in for the real cart service API. */
+public final class InMemoryCartService implements CartService {
+
+  /** Carts stored in memory, keyed by user id. */
+  private final Map<String, List<CartItem>> cartsByUserId;
+
+  /**
+   * Creates the service with a fixed backing map of carts, keyed by user id.
+   *
+   * @param carts the cart data this service serves
+   */
+  public InMemoryCartService(final Map<String, List<CartItem>> carts) {
+    this.cartsByUserId = carts;
+  }
+
+  @Override
+  public List<CartItem> getCart(final String userId) {
+    return cartsByUserId.getOrDefault(userId, List.of());
+  }
+}

backends-for-frontends/src/main/java/com/iluwatar/bff/service/impl/InMemoryOrderService.java

@@ -0,0 +1,51 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.service.impl;
+
+import com.iluwatar.bff.model.Order;
+import com.iluwatar.bff.service.OrderService;
+import java.util.List;
+import java.util.Map;
+
+/** In-memory stand-in for the real order service API. */
+public final class InMemoryOrderService implements OrderService {
+
+  /** Order histories stored in memory, keyed by user id. */
+  private final Map<String, List<Order>> ordersByUserId;
+
+  /**
+   * Creates the service with a fixed backing map of order histories, keyed by user id.
+   *
+   * @param orders the order data this service serves
+   */
+  public InMemoryOrderService(final Map<String, List<Order>> orders) {
+    this.ordersByUserId = orders;
+  }
+
+  @Override
+  public List<Order> getOrders(final String userId) {
+    return ordersByUserId.getOrDefault(userId, List.of());
+  }
+}

backends-for-frontends/src/main/java/com/iluwatar/bff/service/impl/InMemorySupplierService.java

@@ -0,0 +1,51 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package com.iluwatar.bff.service.impl;
+
+import com.iluwatar.bff.model.SupplierRecord;
+import com.iluwatar.bff.service.SupplierService;
+import java.util.List;
+import java.util.Map;
+
+/** In-memory stand-in for the real supplier service API, reachable only from the intranet. */
+public final class InMemorySupplierService implements SupplierService {
+
+  /** Supplier records stored in memory, keyed by product name. */
+  private final Map<String, List<SupplierRecord>> recordsByProductName;
+
+  /**
+   * Creates the service with a fixed backing map of supplier records, keyed by product name.
+   *
+   * @param records the supplier data this service serves
+   */
+  public InMemorySupplierService(final Map<String, List<SupplierRecord>> records) {
+    this.recordsByProductName = records;
+  }
+
+  @Override
+  public List<SupplierRecord> getSupplierRecords(final String productName) {
+    return recordsByProductName.getOrDefault(productName, List.of());
+  }
+}

backends-for-frontends/src/main/java/com/iluwatar/bff/service/impl/package-info.java

@@ -0,0 +1,29 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+/**
+ * In-memory implementations of the downstream service API interfaces, used for the pattern
+ * demonstration. Production deployments would replace these with real HTTP/gRPC clients.
+ */
+package com.iluwatar.bff.service.impl;

backends-for-frontends/src/main/java/com/iluwatar/bff/service/package-info.java

@@ -0,0 +1,29 @@
+/*
+ * This project is licensed under the MIT license. Module model-view-viewmodel is using ZK framework licensed under LGPL (see lgpl-3.0.txt).
+ *
+ * The MIT License
+ * Copyright © 2014-2022 Ilkka Seppälä
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+/**
+ * Downstream service API interfaces shared by all BFFs: AuthService, CartService, OrderService, and
+ * SupplierService. Each BFF wires in only the services its client requires.
+ */
+package com.iluwatar.bff.service;