---
layout: pattern
title: API Gateway
folder: api-gateway
permalink: /patterns/api-gateway/
categories: Architectural
tags:
  - Cloud distributed
  - Decoupling
  - Microservices
---

## Intent

Aggregate calls to microservices in a single location, the API Gateway. The user makes a single call 
to the API Gateway, and the API Gateway then calls each relevant microservice.

## Explanation

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.

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.

Real world example

> We are implementing microservices and API Gateway pattern for an e-commerce site. In this system 
> the API Gateway makes calls to the Image and Price microservices.

In plain words

> For a system implemented using microservices architecture, API Gateway is the single entry point 
> that aggregates the calls to the individual microservices. 

Wikipedia says

> API Gateway is a server that acts as an API front-end, receives API requests, enforces throttling 
> and security policies, passes requests to the back-end service and then passes the response back 
> to the requester. A gateway often includes a transformation engine to orchestrate and modify the 
> requests and responses on the fly. A gateway can also provide functionality such as collecting 
> analytics data and providing caching. The gateway can provide functionality to support 
> authentication, authorization, security, audit and regulatory compliance.

**Programmatic Example**

This implementation shows what the API Gateway pattern could look like for an e-commerce site. The 
`ApiGateway` makes calls to the Image and Price microservices using the `ImageClientImpl` and 
`PriceClientImpl` respectively. Customers viewing the site on a desktop device can see both price 
information and an image of a product, so the `ApiGateway` calls both of the microservices and 
aggregates the data in the `DesktopProduct` model. However, mobile users only see price information; 
they do not see a product image. For mobile users, the `ApiGateway` only retrieves price 
information, which it uses to populate the `MobileProduct`.

Here's the Image microservice implementation.

```java
public interface ImageClient {
  String getImagePath();
}

public class ImageClientImpl implements ImageClient {
  @Override
  public String getImagePath() {
    var httpClient = HttpClient.newHttpClient();
    var httpGet = HttpRequest.newBuilder()
        .GET()
        .uri(URI.create("http://localhost:50005/image-path"))
        .build();

    try {
      var httpResponse = httpClient.send(httpGet, BodyHandlers.ofString());
      return httpResponse.body();
    } catch (IOException | InterruptedException e) {
      e.printStackTrace();
    }

    return null;
  }
}
```

Here's the Price microservice implementation.

```java
public interface PriceClient {
  String getPrice();
}

public class PriceClientImpl implements PriceClient {

  @Override
  public String getPrice() {
    var httpClient = HttpClient.newHttpClient();
    var httpGet = HttpRequest.newBuilder()
        .GET()
        .uri(URI.create("http://localhost:50006/price"))
        .build();

    try {
      var httpResponse = httpClient.send(httpGet, BodyHandlers.ofString());
      return httpResponse.body();
    } catch (IOException | InterruptedException e) {
      e.printStackTrace();
    }

    return null;
  }
}
```

Here we can see how API Gateway maps the requests to the microservices.

```java
public class ApiGateway {

  @Resource
  private ImageClient imageClient;

  @Resource
  private PriceClient priceClient;

  @RequestMapping(path = "/desktop", method = RequestMethod.GET)
  public DesktopProduct getProductDesktop() {
    var desktopProduct = new DesktopProduct();
    desktopProduct.setImagePath(imageClient.getImagePath());
    desktopProduct.setPrice(priceClient.getPrice());
    return desktopProduct;
  }

  @RequestMapping(path = "/mobile", method = RequestMethod.GET)
  public MobileProduct getProductMobile() {
    var mobileProduct = new MobileProduct();
    mobileProduct.setPrice(priceClient.getPrice());
    return mobileProduct;
  }
}
```

## Class diagram
![alt text](./etc/api-gateway.png "API Gateway")

## Applicability

Use the API Gateway pattern when

* You're using microservices architecture and need a single point of aggregation for your microservice calls.

## Credits

* [microservices.io - API Gateway](http://microservices.io/patterns/apigateway.html)
* [NGINX - Building Microservices: Using an API Gateway](https://www.nginx.com/blog/building-microservices-using-an-api-gateway/)
* [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)
* [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)
