Spring Cloud Gateway: Building an API Gateway for Microservices (with Eureka) – Microservices Essentials

In a microservices architecture, each service usually has its own endpoint. Managing all these endpoints manually is tedious and error-prone. That’s where Spring Cloud Gateway steps in as a powerful, scalable, and simple solution for routing requests to various services via a single entry point.

In this article, you will:

API Gateway serves as the single entry point. It forwards client requests to appropriate services, handles
– Routing,
– Load-balancing,
– Authentication, and more.

Case: There is many microservices having different domain/ip/port, which one to call exactly it is very complex if we handle it manually. To resolve this api gateway comes into the picture.

We will build:

• ✅ eureka-server: Service registry
• ✅ api-gateway: API Gateway with Spring Cloud Gateway
• ✅ user-service: A simple microservice behind the gateway

🔧 Tech Stack

Technology	Version ( use as per latest)
Spring Boot	3.1.5
Spring Cloud	2022.0.4
Java	17
Maven	Yes

✅ Architecture Overview

               [ API Gateway (Spring Cloud Gateway) ]
                           |
         -----------------------------------------
         |                       |               |
 [User-Service]        [Order-Service]   [Other microservices...]
         |
   [Eureka Discovery Server (Registry)]

🗂️ Project Structure

microservices-eureka-server/
├── eureka-server/
├── api-gateway/
└── user-service/

1️⃣ Step 1: Eureka Discovery Server

📁 Project: eureka-server

📄 pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.0.1</version>
    </parent>

    <groupId>com.example</groupId>
    <artifactId>eureka-server</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>eureka-server</name>

    <properties>
        <java.version>17</java.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-netflix-eureka-server</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>2022.0.4</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

Note:

📄 EurekaServerApplication.java

package com.example.eurekaserver;

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

@SpringBootApplication
@EnableEurekaServer
public class EurekaServerApplication {
    public static void main(String[] args) {
        SpringApplication.run(EurekaServerApplication.class, args);
    }
}

📄 application.properties

server.port=8761
spring.application.name=eureka-server

eureka.client.register-with-eureka=false
eureka.client.fetch-registry=false

➡️ Run this server and open http://localhost:8761 to see the Eureka dashboard.

2️⃣ Step 2: User Service (Microservice)

📁 Project: user-service

📄 pom.xml

Same setup as Eureka, but include the Eureka client.

<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>

📄 UserServiceApplication.java

package com.example.userservice;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

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

📄 UserController.java

com.example.userservice.controller;

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

@RestController
public class UserController {

    @GetMapping("/users")
    public String getUsers() {
        return "User service response";
    }
}

📄 application.properties

server.port=8081
spring.application.name=user-service

eureka.client.service-url.defaultZone=http://localhost:8761/eureka

➡️ Run this server and open http://localhost:8081 .

3️⃣ Step 3: API Gateway (Spring Cloud Gateway)

📁 Project: api-gateway

📄 pom.xml

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

<!-- ✅ Correct Gateway dependency -->
<dependency>
	<groupId>org.springframework.cloud</groupId>
	<artifactId>spring-cloud-starter-gateway-server-webmvc</artifactId>
</dependency>

📄 ApiGatewayApplication.java

package com.example.apigateway;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

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

📄 application.properties

spring.application.name=api-gateway
server.port=8080

# Register this service with Eureka
eureka.client.service-url.defaultZone=http://localhost:8761/eureka

#  Required for reactive Spring Gateway (to avoid auto-detection of MVC)
spring.main.web-application-type=reactive

# (Optional) Gateway route example
spring.cloud.gateway.routes[0].id=user-service
spring.cloud.gateway.routes[0].uri=http://localhost:8081
spring.cloud.gateway.routes[0].predicates[0]=Path=/users/**

lb://user-service means the request will be load balanced to the registered service via Eureka.

🧪 Testing Everything

1. Start eureka-server (localhost:8761)
2. Start user-service (localhost:8081) → should show up on Eureka dashboard
3. Start api-gateway (localhost:8080)
4. Open: http://localhost:8080/users → should return "User service response" via gateway

discovery server / eureka showing both services:

githib code link: https://github.com/infotechseo/microservices-api-gateway

This is the foundation for a powerful microservices architecture. You can add more services and configure routing, filters, authentication, rate-limiting, etc. using this gateway.