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Spring / Spring Webflux Interview questions

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What is Reactive Programming?

Reactive programming is a programming paradigm that deals with asynchronous data streams and the specific propagation of change, which means it implements modifications to the execution environment in a certain order.

The primary benefits of reactive programming are, better utilization of computing resources on multicore and multi-CPU hardware and better performance by truncating serialization.

What is Spring Webflux?

Spring WebFlux is part of Spring 5, it is a parallel version of Spring MVC and supports fully non-blocking reactive streams. It supports the backpressure concept and uses Netty as the inbuilt server to run reactive applications.

Is Spring WebClient thread safe?

Yes, WebClient is thread-safe since it is immutable.

Is Spring RestTemplate deprecated?

RestTemplate allows you to consume Rest resources in a synchronous manner, which means it will block the thread before it receives a response. RestTemplate has been deprecated since Spring 5.

What are the benefits of using Spring Webflux?

The spring-webflux is not necessarily faster but it is better in dealing with the issues of scalability and efficient use of hardware resources. It is also better at dealing with the issues of latency.

What is bodyToMono?

The method bodyToMono() is extracts the body to a Mono instance. The method Mono.block() subscribes to this Mono instance and blocks until the response is received while Mono.subscribe() method is non-blocking.

Difference between Mono and Flux.

A Mono, which can either return zero or one result before completing,

And a Flux, which can return zero to many, possibly infinite, results before completing.

How to handle errors in Spring WebFlux?

Handle Errors With,

  • onErrorReturn (), to return a static default value whenever an error occurs.
  • onErrorResume (), to compute a dynamic fallback value, (or) execute an alternative path with a fallback method, (or) Catch, wrap and re-throw an error.
  • Global Level by customizing the Global Error Response Attributes and implement the Global Error Handler.
Difference between Spring MVC @async and Spring Webflux?

Spring Async I/O model during its communication with the client is blocking while Spring Webflux doesn't block.

Spring WebFlux supports more Web/App servers such as Netty, Undertow.

Processing the request body in Spring Async is blocking while it is non-blocking with Spring Webflux.

What is Reactor netty?

Reactor Netty is an asynchronous event-driven network application framework. It provides non-blocking and backpressure-ready TCP, HTTP, and UDP clients and servers. As the name implies, it's based on the Netty framework.

What are router functions?

RouterFunction is a functional alternative to the @RequestMapping and @Controller annotation style used in standard Spring MVC.

We can use it to route requests to the handler functions.

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@Bean
public RouterFunction<ServerResponse> listEmployees(EmployeeService es) {
    return route().GET("/employee", req -> ok().body(es.findAll()))
      .build();
}

What does Mono.defer() do?

Mono. defer(monoSupplier) lets you provide the whole expression that supplies the resulting Mono instance. The evaluation of this expression is deferred until somebody subscribes. Inside of this expression you can additionally use control structures like Mono.

When you run Mono.just() it creates immediately an Observable(Mono)and reuses it but when you use defer it doesn't create it immediately it creates a new Observable in every subscribe.

When should we choose Reactor over RxJava?

RxJava support project which is based on JDK8- and Project Reactor supports JDK 8+. RxJava has too many problems which can cause Out of Memory if you can't use it well. It is preferred to use Reactor for JDK8+ projects.

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