Service-to-service communication in Spring Boot - Mini Project: Build & Apply

Service-to-service communication with Spring Boot

📖 Scenario: You are building two small Spring Boot services that need to talk to each other. The first service will call the second service to get a greeting message.This is like when you ask a friend to get information for you from another friend. Here, one service asks another service for data.

🎯 Goal: Create two Spring Boot services where ServiceA calls ServiceB using RestTemplate to get a greeting message and display it.

📋 What You'll Learn

Create a Spring Boot application for ServiceB that returns a greeting string at /greeting endpoint

Create a Spring Boot application for ServiceA that calls ServiceB's /greeting endpoint using RestTemplate

Configure the base URL of ServiceB in ServiceA as a variable

Display the greeting message received from ServiceB in ServiceA's /show-greeting endpoint

💡 Why This Matters

🌍 Real World

Microservices often need to communicate with each other to share data or trigger actions. This project shows the basic way to do that using REST calls in Spring Boot.

💼 Career

Understanding service-to-service communication is essential for backend developers working with microservices architecture, enabling them to build scalable and modular applications.

1

Create ServiceB with a greeting endpoint

Create a Spring Boot controller class called GreetingController in ServiceB with a method getGreeting() mapped to /greeting that returns the string "Hello from ServiceB!".

Spring Boot

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

@RestController
public class GreetingController {
    // Your code here
}

Hint

Use @RestController and @GetMapping annotations to create the endpoint.

2

Configure ServiceB base URL in ServiceA

In ServiceA, create a String variable called serviceBBaseUrl and set it to "http://localhost:8081" which is the base URL of ServiceB.

Spring Boot

// Your code here

Hint

This variable will hold the address where ServiceB is running.

3

Use RestTemplate in ServiceA to call ServiceB's greeting

In ServiceA, create a RestTemplate object called restTemplate. Then write a method fetchGreeting() that uses restTemplate.getForObject to call serviceBBaseUrl + "/greeting" and returns the greeting string.

Spring Boot

String serviceBBaseUrl = "http://localhost:8081";
// Your code here

Hint

Use new RestTemplate() to create the client and getForObject to call the URL.

4

Create endpoint in ServiceA to show greeting from ServiceB

In ServiceA, create a Spring Boot controller class called ShowGreetingController with a method showGreeting() mapped to /show-greeting. This method should call fetchGreeting() and return the greeting string.

Spring Boot

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

// Your code here

Hint

Use @RestController and @GetMapping to create the endpoint that calls fetchGreeting().

Practice

1. What is the main purpose of service-to-service communication in Spring Boot microservices?

easy

A. To create user interfaces for microservices

B. To allow different microservices to exchange data and work together

C. To store data in a database

D. To compile Java code faster

Solution

1. Step 1: Understand microservices architecture

   Microservices are small services that work independently but often need to share data or trigger actions in other services.

2. Step 2: Identify the role of service-to-service communication

   This communication allows microservices to interact and cooperate by exchanging data or requests.

3. Final Answer:

   To allow different microservices to exchange data and work together -> Option B

4. Quick Check:

   Service communication = microservices working together [OK]

Hint: Microservices talk to each other to share data [OK]

Common Mistakes:

• Confusing service communication with UI creation
• Thinking it manages database storage
• Assuming it speeds up code compilation

2. Which of the following is the correct way to create a RestTemplate bean in Spring Boot for service-to-service calls?

easy

A. @Component
public void restTemplate() {
return new RestTemplate();
}

B. @Service
public RestTemplate restTemplate() {
return new RestTemplate();
}

C. @Bean
public RestTemplate restTemplate() {
return new RestTemplate();
}

D. RestTemplate restTemplate = new RestTemplate();

Solution

1. Step 1: Understand Spring bean creation

   To create a reusable RestTemplate, define a method annotated with @Bean inside a @Configuration class.

2. Step 2: Check the correct syntax

   @Bean public RestTemplate restTemplate() { return new RestTemplate(); } correctly uses @Bean and returns a new RestTemplate instance.

3. Final Answer:

   @Bean public RestTemplate restTemplate() { return new RestTemplate(); } -> Option C

4. Quick Check:

   @Bean method returns RestTemplate instance [OK]

Hint: Use @Bean to create reusable RestTemplate [OK]

Common Mistakes:

• Using @Service instead of @Bean
• Not returning RestTemplate instance
• Missing @Bean annotation

3. Given the following Spring Boot code snippet using WebClient, what will be the output if the called service returns "Hello from Service B"?

WebClient client = WebClient.create("http://service-b/api/greet");
String response = client.get()
    .retrieve()
    .bodyToMono(String.class)
    .block();
System.out.println(response);

medium

A. "Hello from Service B"

B. null

C. An exception is thrown

D. "Error: Service not found"

Solution

1. Step 1: Understand WebClient call

   The WebClient sends a GET request to the URL and retrieves the response body as a String.

2. Step 2: Analyze the response handling

   The block() method waits for the response synchronously and returns the body content.

3. Final Answer:

   "Hello from Service B" -> Option A

4. Quick Check:

   WebClient returns response body string [OK]

Hint: block() waits and returns response body string [OK]

Common Mistakes:

• Assuming asynchronous call returns immediately
• Expecting null without response
• Confusing error message with normal output

4. Identify the error in this Spring Boot service-to-service call using RestTemplate:

@Autowired
private RestTemplate restTemplate;

public String callService() {
    String url = "http://service-c/api/data";
    ResponseEntity<String> response = restTemplate.getForEntity(url, String.class);
    return response.getBody();
}

medium

A. getForEntity method does not exist

B. URL string is missing protocol

C. ResponseEntity cannot hold String type

D. RestTemplate bean is not defined in the configuration

Solution

1. Step 1: Check RestTemplate injection

   The RestTemplate must be defined as a bean for @Autowired to inject it properly.

2. Step 2: Verify URL and method usage

   The URL includes protocol and getForEntity is a valid method returning ResponseEntity<String>.

3. Final Answer:

   RestTemplate bean is not defined in the configuration -> Option D

4. Quick Check:

   Missing RestTemplate bean causes injection error [OK]

Hint: Always define RestTemplate as a @Bean before autowiring [OK]

Common Mistakes:

• Forgetting to create RestTemplate bean
• Using incomplete URL
• Misunderstanding getForEntity method

5. You want to call Service D from Service E using WebClient with a timeout of 2 seconds and handle errors gracefully. Which code snippet correctly implements this?

hard

A. WebClient client = WebClient.create("http://service-d/api");
String result = client.get()
.retrieve()
.bodyToMono(String.class)
.timeout(Duration.ofSeconds(2))
.onErrorReturn("Timeout or error")
.block();

B. WebClient client = WebClient.create();
String result = client.get()
.uri("http://service-d/api")
.retrieve()
.bodyToMono(String.class)
.block(Duration.ofSeconds(2));

C. RestTemplate restTemplate = new RestTemplate();
restTemplate.setTimeout(2000);
String result = restTemplate.getForObject("http://service-d/api", String.class);

D. WebClient client = WebClient.builder()
.baseUrl("http://service-d/api")
.build();
String result = client.get()
.retrieve()
.bodyToMono(String.class)
.block();

Solution

1. Step 1: Setup WebClient with timeout and error handling

   WebClient client = WebClient.create("http://service-d/api"); String result = client.get() .retrieve() .bodyToMono(String.class) .timeout(Duration.ofSeconds(2)) .onErrorReturn("Timeout or error") .block(); uses timeout(Duration.ofSeconds(2)) to limit wait time and onErrorReturn to provide fallback on errors.

2. Step 2: Verify other options

   WebClient client = WebClient.create(); String result = client.get() .uri("http://service-d/api") .retrieve() .bodyToMono(String.class) .block(Duration.ofSeconds(2)); but uses block(Duration) which times out and throws an exception instead of providing a fallback; RestTemplate restTemplate = new RestTemplate(); restTemplate.setTimeout(2000); String result = restTemplate.getForObject("http://service-d/api", String.class); tries to set timeout on RestTemplate incorrectly; WebClient client = WebClient.builder() .baseUrl("http://service-d/api") .build(); String result = client.get() .retrieve() .bodyToMono(String.class) .block(); lacks timeout and error handling.

3. Final Answer:

   WebClient client = WebClient.create("http://service-d/api"); String result = client.get() .retrieve() .bodyToMono(String.class) .timeout(Duration.ofSeconds(2)) .onErrorReturn("Timeout or error") .block(); -> Option A

4. Quick Check:

   Timeout + onErrorReturn = safe WebClient call [OK]

Hint: Use timeout() and onErrorReturn() for safe WebClient calls [OK]

Common Mistakes:

• Using block(Duration) which is invalid
• Trying to set timeout directly on RestTemplate
• Ignoring error handling in WebClient calls