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‎api-gateway/README.md‎

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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 call to the API Gateway,
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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
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to the API Gateway, 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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With the Microservices pattern, a client may need data from multiple different microservices. If the
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client called each microservice directly, that could contribute to longer load times, since the
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client would have to make a network request for each microservice called. Moreover, having the
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client call each microservice directly ties the client to that microservice - if the internal
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implementations of the microservices change (for example, if two microservices are combined sometime
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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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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 microservices.
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The job of the API Gateway is to aggregate the calls to the microservices. Rather than the client
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calling each microservice individually, the client calls the API Gateway a single time. The API
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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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> We are implementing microservices and API Gateway pattern for an e-commerce site. In this system
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> the API Gateway makes 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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> For a system implemented using microservices architecture, API Gateway is the single entry point
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> that aggregates the 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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> API Gateway is a server that acts as an API front-end, receives API requests, enforces throttling
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> and security policies, passes requests to the back-end service and then passes the response back
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> to the requester. A gateway often includes a transformation engine to orchestrate and modify the
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> requests and responses on the fly. A gateway can also provide functionality such as collecting
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> analytics data and providing caching. The gateway can provide functionality to support
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> 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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This implementation shows what the API Gateway pattern could look like for an e-commerce site. The
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`ApiGateway` makes calls to the Image and Price microservices using the `ImageClientImpl` and
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`PriceClientImpl` respectively. Customers viewing the site on a desktop device can see both price
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information and an image of a product, so the `ApiGateway` calls both of the microservices and
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aggregates the data in the `DesktopProduct` model. However, mobile users only see price information;
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they do not see a product image. For mobile users, the `ApiGateway` only retrieves price
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information, which it uses to populate the `MobileProduct`.
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Here's the Image microservice implementation.
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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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}
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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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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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