#297 Create API Gateway pattern

api-gateway/pom.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+  <parent>
+    <artifactId>java-design-patterns</artifactId>
+    <groupId>com.iluwatar</groupId>
+    <version>1.10.0-SNAPSHOT</version>
+  </parent>
+  <modelVersion>4.0.0</modelVersion>
+  <artifactId>api-gateway</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

api-gateway/src/main/java/com/iluwatar/api/gateway/ApiGateway.java

@@ -0,0 +1,31 @@
+package com.iluwatar.api.gateway;
+
+/**
+ * The ApiGateway aggregates calls to microservices based on the needs of the individual clients.
+ */
+public class ApiGateway {
+
+  private ImageService imageService = new ImageService();
+  private PriceService priceService = new PriceService();
+
+  /**
+   * Retrieves product information that desktop clients need
+   * @return Product information for clients on a desktop
+   */
+  public DesktopProduct getProductDesktop() {
+    DesktopProduct desktopProduct = new DesktopProduct();
+    desktopProduct.setImagePath(imageService.getImagePath());
+    desktopProduct.setPrice(priceService.getPrice());
+    return desktopProduct;
+  }
+
+  /**
+   * Retrieves product information that mobile clients need
+   * @return Product information for clients on a mobile device
+   */
+  public MobileProduct getProductMobile() {
+    MobileProduct mobileProduct = new MobileProduct();
+    mobileProduct.setPrice(priceService.getPrice());
+    return mobileProduct;
+  }
+}

api-gateway/src/main/java/com/iluwatar/api/gateway/App.java

@@ -0,0 +1,46 @@
+package com.iluwatar.api.gateway;
+
+/**
+ * With the Microservices pattern, a client may need data from multiple different microservices.
+ * If the client called each microservice directly, that could contribute to longer load times,
+ * since the client would have to make a network request for each microservice called. Moreover,
+ * having the client call each microservice directly ties the client to that microservice - if the
+ * internal implementations of the microservices change (for example, if two microservices are
+ * combined sometime in the future) or if the location (host and port) of a microservice changes,
+ * then every client that makes use of those microservices must be updated.
+ *
+ * <p>
+ * The intent of the API Gateway pattern is to alleviate some of these issues. In the API Gateway
+ * pattern, an additional entity (the API Gateway) is placed between the client and the
+ * microservices. The job of the API Gateway is to aggregate the calls to the microservices.
+ * Rather than the client calling each microservice individually, the client calls the API Gateway
+ * a single time. The API Gateway then calls each of the microservices that the client needs.
+ *
+ * <p>
+ * This implementation shows what the API Gateway pattern could look like for an e-commerce site.
+ * In this case, the (@link ImageService) and (@link PriceService) represent our microservices.
+ * Customers viewing the site on a desktop device can see both price information and an image of
+ * a product, so the (@link ApiGateway) calls both of the microservices and aggregates the data in
+ * the (@link DesktopProduct) model. However, mobile users only see price information; they do not
+ * see a product image. For mobile users, the (@link ApiGateway) only retrieves price information,
+ * which it uses to populate the (@link MobileProduct).
+ */
+public class App {
+
+  /**
+   * Program entry point
+   *
+   * @param args
+   *          command line args
+   */
+  public static void main(String[] args) {
+    ApiGateway apiGateway = new ApiGateway();
+
+    DesktopProduct desktopProduct = apiGateway.getProductDesktop();
+    System.out.println(String.format("Desktop Product \nPrice: %s\nImage path: %s",
+        desktopProduct.getPrice(), desktopProduct.getImagePath()));
+
+    MobileProduct mobileProduct = apiGateway.getProductMobile();
+    System.out.println(String.format("Mobile Product \nPrice: %s", mobileProduct.getPrice()));
+  }
+}

api-gateway/src/main/java/com/iluwatar/api/gateway/DesktopProduct.java

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+package com.iluwatar.api.gateway;
+
+/**
+ * Encapsulates all of the information that a desktop client needs to display a product.
+ */
+public class DesktopProduct {
+
+  private String price;
+  private String imagePath;
+
+  public String getPrice() {
+    return price;
+  }
+
+  public void setPrice(String price) {
+    this.price = price;
+  }
+
+  public String getImagePath() {
+    return imagePath;
+  }
+
+  public void setImagePath(String imagePath) {
+    this.imagePath = imagePath;
+  }
+}

api-gateway/src/main/java/com/iluwatar/api/gateway/ImageService.java

@@ -0,0 +1,15 @@
+package com.iluwatar.api.gateway;
+
+/**
+ * Represents a microservice used to retrieve image data.
+ */
+public class ImageService {
+
+  /**
+   * Gets the image path
+   * @return the image path
+   */
+  public String getImagePath() {
+    return "/product-image.png";
+  }
+}

api-gateway/src/main/java/com/iluwatar/api/gateway/MobileProduct.java

@@ -0,0 +1,17 @@
+package com.iluwatar.api.gateway;
+
+/**
+ * Encapsulates all of the information that mobile client needs to display a product.
+ */
+public class MobileProduct {
+
+  private String price;
+
+  public String getPrice() {
+    return price;
+  }
+
+  public void setPrice(String price) {
+    this.price = price;
+  }
+}

api-gateway/src/main/java/com/iluwatar/api/gateway/PriceService.java

@@ -0,0 +1,15 @@
+package com.iluwatar.api.gateway;
+
+/**
+ * Represents a microservice used to retrieve price data.
+ */
+public class PriceService {
+
+  /**
+   * Gets the price
+   * @return the price
+   */
+  public String getPrice() {
+    return "20";
+  }
+}

pom.xml

@@ -93,6 +93,7 @@
 		<module>publish-subscribe</module>
 		<module>delegation</module>
 		<module>event-driven-architecture</module>
+		<module>api-gateway</module>
 	</modules>
 
 	<dependencyManagement>