You're tasked with building a non-blocking, reactive REST API that can handle a high volume of concurrent requests efficiently. Describe how you would use Spring WebFlux to achieve this.

Question

Accepted Answer

Spring WebFlux is Spring's reactive web framework designed for building non-blocking and event-driven applications that can efficiently handle a large number of concurrent requests using a small number of threads.

Key Points:
• Spring WebFlux uses non-blocking I/O to improve scalability and resource utilization.
• Reactive types such as Mono and Flux represent asynchronous streams of data.
• Reactive repositories and drivers ensure end-to-end non-blocking communication.

Example:
Consider a stock market application receiving thousands of requests per second.

Traditional Blocking Approach:

Request 1 → Thread 1 → Waiting for Database
Request 2 → Thread 2 → Waiting for Database
Request 3 → Thread 3 → Waiting for Database

Result:
• Large number of threads required.
• Increased memory consumption.
• Reduced scalability.

Reactive Approach with WebFlux:

Request 1
Request 2
Request 3
    ↓
Event Loop Threads
    ↓
Non-Blocking Processing
    ↓
Database Response
    ↓
Return Result

Result:
• Fewer threads.
• Better CPU utilization.
• Higher throughput.

Code Example:

```
@RestController
@RequestMapping("/products")
public class ProductController {

@GetMapping("/{id}")
    public Mono getProduct(
            @PathVariable Long id) {
```
        return productService
```
                .findById(id);
    }

@GetMapping
    public Flux getAllProducts() {
```
        return productService
```
                .findAll();
    }
}
```
Explanation:

Mono
• Represents zero or one result.

Examples:
• Fetch user by ID.
• Retrieve a single order.

Flux
• Represents zero to many results.

Examples:
• Product list.
• Transaction history.
• Live event streams.

Reactive Data Access:

Use:

ReactiveCrudRepository

Example:

public interface ProductRepository
        extends ReactiveCrudRepository
```
         {
}
```
This ensures database operations remain non-blocking.

Supported Reactive Databases:

• MongoDB Reactive Driver
• R2DBC
• Cassandra Reactive Driver
• Redis Reactive Driver

Dependency Required:

spring-boot-starter-webflux

Embedded Server Options:

• Netty (default)
• Undertow
• Servlet Containers with adapters

Advantages of WebFlux:

• Handles thousands of concurrent requests.
• Lower memory usage.
• Improved scalability.
• Better performance for I/O intensive applications.

Ideal Use Cases:

• Streaming APIs
• Chat Applications
• Notification Systems
• IoT Platforms
• Real-Time Dashboards
• Microservices Communication

When Not to Use WebFlux:

• CPU-intensive processing.
• Traditional blocking database drivers.
• Small applications with low concurrency requirements.

Real-World Example:

Live Stock Trading Platform:

Market Updates
       ↓
WebFlux API
       ↓
Reactive Database
       ↓
Thousands of Concurrent Clients

The application can process massive traffic using minimal resources.

Interview Tip:
A concise interview answer is: To build a non-blocking REST API in Spring Boot, I would use Spring WebFlux with spring-boot-starter-webflux, return Mono and Flux from controller methods, use reactive repositories such as ReactiveCrudRepository, and leverage Netty's event-driven architecture to efficiently handle a high volume of concurrent requests with fewer threads and better scalability.