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iluwatar#297 Create API Gateway pattern
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‎api-gateway/pom.xml‎

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

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

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

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

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

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

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

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

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

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

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

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

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

‎pom.xml‎

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<module>publish-subscribe</module>
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<module>delegation</module>
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<module>event-driven-architecture</module>
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<module>api-gateway</module>
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</modules>
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<dependencyManagement>

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