What is REST API in Microservices?

REST API is one of the most commonly used communication mechanisms in Microservices Architecture.

REST stands for:

Representational State Transfer

A REST API allows microservices to communicate with each other using HTTP protocols such as:

• GET
• POST
• PUT
• DELETE

In simple words:

REST API is a way for one microservice to send requests and receive responses from another microservice over HTTP.

Simple Real-Time Understanding

Imagine ordering food through a mobile app.

• You place order using app
• App sends request to restaurant server
• Restaurant server processes request
• Response is returned to app

This request-response communication works similar to REST APIs.

Why REST API is Important in Microservices

In Microservices Architecture:

• Applications are split into multiple services
• Services must communicate with each other

REST APIs enable this communication.

Example Microservices

• Order Service
• Payment Service
• Notification Service
• Inventory Service

REST API Communication Flow

Order Service
      |
HTTP Request
      |
      v
Payment Service
      |
HTTP Response
      |
      v
Order Service

Real-Time Example

Suppose customer places an order.

Order Service must:

• Call Payment Service
• Verify payment
• Continue order processing

REST API Request Example

POST /payments

REST API Response Example

{
   "status": "SUCCESS"
}

How REST APIs Work

1. Client sends HTTP request
2. Server receives request
3. Server processes business logic
4. Server returns HTTP response

REST Architecture Flow

Client
   |
HTTP Request
   |
   v
REST API
   |
Business Logic
   |
Database
   |
HTTP Response
   |
   v
Client

Main Components of REST API

• Client
• Server
• HTTP Methods
• Endpoints
• Request Body
• Response Body
• Status Codes

1. Client

Client sends request to API.

Examples

• Frontend application
• Mobile app
• Another microservice

2. Server

Server processes request and returns response.

Example

Payment Service

3. Endpoint

Endpoint is API URL exposed by service.

Example

/payments

/orders

/users

4. HTTP Methods

REST APIs use HTTP methods to perform operations.

Main HTTP Methods

GET Method Example

Retrieve all courses.

GET /courses

POST Method Example

Create new order.

POST /orders

PUT Method Example

Update student details.

PUT /students/1

DELETE Method Example

Delete user record.

DELETE /users/1

5. Request Body

Request body contains data sent to server.

Example JSON Request

{
   "name": "Naresh",
   "course": "Spring Boot"
}

6. Response Body

Server returns response body to client.

Example Response

{
   "message": "Course Created Successfully"
}

7. HTTP Status Codes

Status codes indicate request result.

Common Status Codes

Spring Boot REST API Example

Create REST Controller

@RestController
@RequestMapping("/courses")

public class CourseController {

    @GetMapping
    public List<String> getCourses() {

        return List.of(
            "Java",
            "Spring Boot",
            "Microservices"
        );
    }
}

API Call Example

GET /courses

Response

[
   "Java",
   "Spring Boot",
   "Microservices"
]

POST API Example

@PostMapping

public String createCourse(
    @RequestBody Course course
) {

    return "Course Created";
}

REST API in Microservices Communication

Microservices commonly communicate using REST APIs.

Example

Order Service
      |
HTTP Request
      |
      v
Payment Service

Calling REST API Using RestTemplate

RestTemplate restTemplate =
    new RestTemplate();

String response =
    restTemplate.getForObject(
        "http://payment-service/pay",
        String.class
    );

Calling REST API Using Feign Client

@FeignClient(name = "payment-service")

public interface PaymentClient {

    @GetMapping("/pay")
    String pay();
}

Feign Client Flow

Order Service
      |
Feign Client
      |
      v
Payment Service

REST API Best Practices

• Use proper HTTP methods
• Use meaningful URLs
• Return proper status codes
• Secure APIs using JWT
• Use versioning

Example of Good REST API URL

/api/v1/courses

REST API Versioning

Versioning prevents breaking existing clients.

Example

/api/v1/orders

/api/v2/orders

Security in REST APIs

REST APIs should be secured.

Common Security Mechanisms

• JWT Authentication
• OAuth2
• HTTPS
• Role-based authorization

JWT Example

Authorization:
Bearer eyJhbGciOi...

REST API Statelessness

REST APIs are usually:

Stateless

Meaning

Server does not store client session between requests.

Advantages

• Better scalability
• Independent requests
• Cloud-friendly architecture

REST API Advantages

1. Simplicity

Easy to understand and implement.

2. Language Independent

Any programming language can use REST APIs.

3. Scalability

Supports distributed systems.

4. Stateless Architecture

Improves scalability and reliability.

5. Wide Industry Adoption

Used by almost all modern applications.

Disadvantages of REST APIs

1. Increased Latency

HTTP communication introduces network delay.

2. Tight Coupling

Services depend on each other.

3. Cascading Failures

Failure in one service may affect others.

Real-Time Example

Order Service
      |
      v
Payment Service (Slow)

Order Service also becomes slow.

How to Improve REST API Reliability

• Circuit Breaker
• Retry Mechanism
• Fallback Methods
• Load Balancing
• Caching

Circuit Breaker Example

@CircuitBreaker(
    name = "paymentService",
    fallbackMethod = "fallback"
)

REST API vs gRPC

REST API in My Project

In my project:

• API Gateway routed requests
• Microservices communicated using REST APIs
• Feign Client simplified communication
• JWT secured REST endpoints
• Nginx handled HTTPS routing

Project Architecture

Client
   |
HTTPS
   |
   v
Nginx
   |
   v
API Gateway
   |
--------------------------------------------------
|               |               |                |
v               v               v                v

Interview     Payment       Internship      Notification
Service       Service       Service         Service

Real-Time Company Usage

• Amazon uses REST APIs for order systems
• Netflix uses REST APIs for streaming services
• Banking applications use REST APIs for transactions
• E-Commerce systems use REST APIs for payments and orders

Professional Interview Answer

REST API stands for Representational State Transfer and is one of the most commonly used communication mechanisms in Microservices Architecture. REST APIs allow microservices to communicate using HTTP methods such as GET, POST, PUT, and DELETE. REST APIs are stateless, scalable, language-independent, and easy to integrate. In microservices, REST APIs are commonly implemented using Spring Boot controllers, RestTemplate, Feign Client, or WebClient. In my project, microservices communicated through REST APIs using API Gateway, JWT security, Feign Client, and HTTPS routing with Nginx.

Why Interviewers Like This Answer

• Explains REST APIs clearly
• Includes Spring Boot examples
• Covers real-time communication
• Shows distributed systems understanding
• Includes security concepts
• Demonstrates production-level knowledge

Frequently Asked Questions

What does REST stand for?

Representational State Transfer.

Why REST APIs are used in microservices?

To enable service-to-service communication over HTTP.

What are common HTTP methods?

GET, POST, PUT, DELETE.

Why REST APIs are stateless?

Server does not store client session between requests.

What are popular REST communication tools in Spring Boot?

RestTemplate, Feign Client, and WebClient.