Zuul API Gateway example

A service API gateway acts as an intermediary between the service client and a service being invoked. The service client talks only to a single URL managed by the service gateway. The service API gateway sits as the gatekeeper for all inbound traffic to microservice calls within your application.

With a service gateway in place, your service clients never directly call the URL of an individual service, but instead place all calls to the service gateway.

You can implement API gateway using Netflix open source project Zuul. Zuul is a services gateway that's extremely easy to set up. Below example shows how to configure service registgry, write a simple microservice application (Book Service) and then build a reverse proxy application that uses Netflix Zuul to forward requests to the book service application.

<parent>
    <groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-parent</artifactId>
	<version>2.6.3</version>
	<relativePath/> 
</parent>
        
<dependencies>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-eureka-server</artifactId>
    </dependency>
</dependencies>

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-dependencies</artifactId>
            <version>2021.0.3</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>        

Step 2) Create EurekaServiceApplication class to launch Eureka Service registry

package com.example.hello;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.netflix.eureka.server.EnableEurekaServer;

@EnableEurekaServer
@SpringBootApplication
public class EurekaServiceApplication {

  public static void main(String[] args) {
    SpringApplication.run(EurekaServiceApplication.class, args);
  }
}
  

Step 3) Create application.yml file under src/main/resources

server:
  port: 8761
  
spring:  
  application:
    name: discovery-service

eureka:
  client:
    register-with-eureka: false
    fetch-registry: false    

<parent>
    <groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-parent</artifactId>
	<version>2.6.3</version>
	<relativePath/> 
</parent>
        
<dependencies>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>

</dependencies>

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-dependencies</artifactId>
            <version>2021.0.3</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>        

package com.example.demo;

    public class Book {
        
        private String title;
        private String author;
        private String genre;
    
    //removed getters, setters for brevity      
}   

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.netflix.eureka.EnableEurekaClient;

@EnableEurekaClient
@SpringBootApplication
public class BookApplication {

    public static void main(String[] args) {
        SpringApplication.run(BookApplication.class, args);
    }
}   

package com.example.demo;

import java.util.ArrayList;
import java.util.List;

import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class BookRestController {
    
    List<Book> books = new ArrayList<>();
        
    public BookRestController() {
        books.add(new Book("The Fountainhead", "Ayn Rand", "Philosophical"));
        books.add(new Book("Murder on the Orient Express", "Agatha Christie", "Thriller"));
        books.add(new Book("The Silence of the Lambs", "Thomas Harris", "Thriller"));    }

    @GetMapping("/{id}") 
    public Book get(@PathVariable("id") int id) {
        return books.get(id);
    }
}   

spring.application.name=book-service
eureka.client.serviceUrl.defaultZone=http://localhost:8761/eureka/
    

<parent>
    <groupId>org.springframework.boot</groupId>
	<artifactId>spring-boot-starter-parent</artifactId>
	<version>2.2.1.RELEASE</version>
	<relativePath/> 
</parent>

        
<dependencies>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-zuul</artifactId>
    </dependency>

</dependencies>

<dependencyManagement>
    <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-dependencies</artifactId>
            <version>Hoxton.RELEASE</version>
            <type>pom</type>
            <scope>import</scope>
        </dependency>
    </dependencies>
</dependencyManagement>
        

package com.example.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.cloud.netflix.zuul.EnableZuulProxy;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
@EnableDiscoveryClient
@EnableZuulProxy
public class ZuulApplication {

    public static void main(String[] args) { 
        SpringApplication.run(ZuulApplication.class, args);
    }

    @Bean
    public SimpleFilter simpleFilter() {
        return new SimpleFilter();
    }

}   

package com.example.demo;

import javax.servlet.http.HttpServletRequest;
import com.netflix.zuul.context.RequestContext;
import com.netflix.zuul.ZuulFilter;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class SimpleFilter extends ZuulFilter {

    private static Logger log = LoggerFactory.getLogger(SimpleFilter.class);

    @Override
    public String filterType() {
        return "pre";
    }

    @Override
    public int filterOrder() {
        return 1;
    }

    @Override
    public boolean shouldFilter() {
        return true;
    }

    @Override
    public Object run() {
        RequestContext ctx = RequestContext.getCurrentContext();
        HttpServletRequest request = ctx.getRequest();

        log.info(String.format("%s request to %s", request.getMethod(), request
                .getRequestURL().toString()));

        return null;
    }

}
   

spring:  
  application:
    name: zuul-gateway
server:
  port: 9999    

eureka:
  client:
    serviceUrl:
      defaultZone: http://localhost:8761/eureka