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iluwatar#590 explanation for API Gateway
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‎api-gateway/README.md

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permalink: /patterns/api-gateway/
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categories: Architectural
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tags:
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- Cloud distributed
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- Decoupling
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- Cloud distributed
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- Decoupling
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- Microservices
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---
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## Intent
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Aggregate calls to microservices in a single location: the API Gateway. The user makes a single
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call to the API Gateway, and the API Gateway then calls each relevant microservice.
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Aggregate calls to microservices in a single location: the API Gateway. The user makes a single call to the API Gateway,
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and the API Gateway then calls each relevant microservice.
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## Explanation
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With the Microservices pattern, a client may need data from multiple different microservices. If the client called each
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microservice directly, that could contribute to longer load times, since the client would have to make a network request
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for each microservice called. Moreover, having the client call each microservice directly ties the client to that
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microservice - if the 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, then every client that
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makes use of those microservices must be updated.
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The intent of the API Gateway pattern is to alleviate some of these issues. In the API Gateway pattern, an additional
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entity (the API Gateway) is placed between the client and the microservices. The job of the API Gateway is to aggregate
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the calls to the microservices. Rather than the client calling each microservice individually, the client calls the
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API Gateway a single time. The API Gateway then calls each of the microservices that the client needs.
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Real world example
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> We are implementing microservices and API Gateway pattern for an e-commerce site. In this system the API Gateway makes
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calls to the Image and Price microservices.
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In plain words
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> For a system implemented using microservices architecture, API Gateway is the single entry point that aggregates the
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calls to the individual microservices.
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Wikipedia says
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> API Gateway is a server that acts as an API front-end, receives API requests, enforces throttling and security
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policies, passes requests to the back-end service and then passes the response back to the requester. A gateway often
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includes a transformation engine to orchestrate and modify the requests and responses on the fly. A gateway can also
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provide functionality such as collecting analytics data and providing caching. The gateway can provide functionality to
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support authentication, authorization, security, audit and regulatory compliance.
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**Programmatic Example**
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This implementation shows what the API Gateway pattern could look like for an e-commerce site. The `ApiGateway` makes
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calls to the Image and Price microservices using the `ImageClientImpl` and `PriceClientImpl` respectively. Customers
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viewing the site on a desktop device can see both price information and an image of a product, so the `ApiGateway` calls
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both of the microservices and aggregates the data in the `DesktopProduct` model. However, mobile users only see price
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information; they do not see a product image. For mobile users, the `ApiGateway` only retrieves price information, which
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it uses to populate the `MobileProduct`.
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Here's the Image microservice implementation.
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```java
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public interface ImageClient {
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String getImagePath();
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}
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public class ImageClientImpl implements ImageClient {
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@Override
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public String getImagePath() {
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var httpClient = HttpClient.newHttpClient();
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var httpGet = HttpRequest.newBuilder()
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.GET()
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.uri(URI.create("http://localhost:50005/image-path"))
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.build();
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try {
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var httpResponse = httpClient.send(httpGet, BodyHandlers.ofString());
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return httpResponse.body();
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} catch (IOException | InterruptedException e) {
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e.printStackTrace();
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}
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return null;
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}
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}
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```
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Here's the Price microservice implementation.
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```java
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public interface PriceClient {
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String getPrice();
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}
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public class PriceClientImpl implements PriceClient {
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@Override
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public String getPrice() {
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var httpClient = HttpClient.newHttpClient();
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var httpGet = HttpRequest.newBuilder()
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.GET()
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.uri(URI.create("http://localhost:50006/price"))
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.build();
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try {
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var httpResponse = httpClient.send(httpGet, BodyHandlers.ofString());
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return httpResponse.body();
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} catch (IOException | InterruptedException e) {
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e.printStackTrace();
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}
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return null;
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}
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}
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```
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And here we can see how API Gateway maps the requests to the microservices.
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```java
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public class ApiGateway {
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@Resource
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private ImageClient imageClient;
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@Resource
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private PriceClient priceClient;
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@RequestMapping(path = "/desktop", method = RequestMethod.GET)
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public DesktopProduct getProductDesktop() {
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var desktopProduct = new DesktopProduct();
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desktopProduct.setImagePath(imageClient.getImagePath());
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desktopProduct.setPrice(priceClient.getPrice());
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return desktopProduct;
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}
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@RequestMapping(path = "/mobile", method = RequestMethod.GET)
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public MobileProduct getProductMobile() {
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var mobileProduct = new MobileProduct();
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mobileProduct.setPrice(priceClient.getPrice());
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return mobileProduct;
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}
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}
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```
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## Class diagram
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![alt text](./etc/api-gateway.png "API Gateway")
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Use the API Gateway pattern when
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* you're also using the Microservices pattern and need a single point of aggregation for your
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microservice calls
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* You're using microservices architecture and need a single point of aggregation for your microservice calls.
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## Credits
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* [microservices.io - API Gateway](http://microservices.io/patterns/apigateway.html)
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* [NGINX - Building Microservices: Using an API Gateway](https://www.nginx.com/blog/building-microservices-using-an-api-gateway/)
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* [Microservices Patterns: With examples in Java](https://www.amazon.com/gp/product/1617294543/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1617294543&linkId=ac7b6a57f866ac006a309d9086e8cfbd)
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* [Building Microservices: Designing Fine-Grained Systems](https://www.amazon.com/gp/product/1491950358/ref=as_li_qf_asin_il_tl?ie=UTF8&tag=javadesignpat-20&creative=9325&linkCode=as2&creativeASIN=1491950358&linkId=4c95ca9831e05e3f0dadb08841d77bf1)

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