Microservices Communication with Spring Boot’s WebClient: A Step-by-Step Guide

Microservices architecture has gained popularity due to its ability to break down large applications into smaller, independent services. In this tutorial, we will explore how to establish communication between microservices using WebClient in a Spring Boot application. WebClient is a powerful and non-blocking HTTP client provided by Spring WebFlux, allowing for efficient communication between microservices. Follow along to learn how to set up the project and implement microservices communication using WebClient.

Prerequisites:

1. Basic knowledge of Spring Boot and Spring WebFlux.
2. Java Development Kit (JDK) 8 or higher installed.
3. Maven or Gradle installed.

Step 1: Set up a Spring Boot project

1. Create a new Spring Boot project using your preferred IDE or the Spring Initializr (https://start.spring.io/).
2. Add the necessary dependencies:
   • spring-boot-starter-webflux: for Spring WebFlux and WebClient.
   • spring-boot-starter-test: for testing.

<?xml version="1.0" encoding="UTF-8"?>
<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>3.1.1</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>
	<groupId>com.example</groupId>
	<artifactId>demo</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<name>demo</name>
	<description>Demo project for Spring Boot</description>
	<properties>
		<java.version>17</java.version>
	</properties>
	<dependencies>
		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-webflux</artifactId>
		</dependency>

		<dependency>
			<groupId>org.springframework.boot</groupId>
			<artifactId>spring-boot-starter-test</artifactId>
			<scope>test</scope>
		</dependency>
		<dependency>
			<groupId>io.projectreactor</groupId>
			<artifactId>reactor-test</artifactId>
			<scope>test</scope>
		</dependency>
	</dependencies>

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

</project>

Step 2: Create Microservice A

1. Create a new package called com.example.microservicea.
2. Inside the package, create a new class MicroserviceAController.
3. Annotate the class with @RestController to make it a RESTful controller.
4. Define a simple GET endpoint to retrieve data from Microservice B:

@RestController
public class MicroserviceAController {
    
    private final WebClient webClient;

    public MicroserviceAController(WebClient.Builder webClientBuilder) {
        this.webClient = webClientBuilder.baseUrl("http://localhost:8081").build();
    }

    @GetMapping("/data")
    public Mono<String> getDataFromMicroserviceB() {
        return webClient.get().uri("/data").retrieve().bodyToMono(String.class);
    }
}

Step 3: Create Microservice B

1. Create a new package called com.example.microserviceb.
2. Inside the package, create a new class MicroserviceBController.
3. Annotate the class with @RestController.
4. Define a simple GET endpoint to return data to Microservice A:

@RestController
public class MicroserviceBController {
    
    @GetMapping("/data")
    public Mono<String> getData() {
        return Mono.just("Data from Microservice B");
    }
}

Step 4: Run the application

1. Start both Microservice A and Microservice B.
2. Access the endpoint of Microservice A by navigating to http://localhost:8080/data in your web browser or using a tool like cURL or Postman.

Congratulations! You have successfully implemented microservices communication using WebClient in Spring Boot.

Explanation:

1. In Microservice A, we create a WebClient instance and set the base URL of Microservice B using baseUrl().
2. In the getDataFromMicroserviceB() method, we use the webClient to send a GET request to Microservice B’s /data endpoint.
3. We use the retrieve() method to retrieve the response and bodyToMono() to convert the response body to a Mono, which represents a single result or an error signal in Reactor.
4. In Microservice B, we define a simple endpoint /data that returns a Mono containing the string “Data from Microservice B”.

Note: This tutorial provides a basic example of microservices communication using WebClient. In a real-world scenario, you would typically have more complex interactions and handle error scenarios appropriately.

Feel free to explore further by adding more endpoints, implementing other HTTP methods, or integrating other features provided by WebClient, such as request headers, query parameters, or handling response errors.