Interview FAQs of Spring Web Flux Reactive Programming

Q-1). What is Spring WebFlux?

A-1). Spring WebFlux is a part of the Spring Framework that supports reactive programming for web applications. It provides a non-blocking, reactive programming model that can efficiently handle large numbers of concurrent connections.

Q-2). What is reactive programming?

A-2). Reactive programming is a programming paradigm focused on asynchronous data streams and change propagation. It enables building non-blocking, event-driven applications that can scale with fewer threads.

Q-3). What are the core components of Spring WebFlux?

A-3). The core components include are as follows:

1. Flux and Mono – These are reactive types from Project Reactor.
2. WebClient – A non-blocking, reactive web client.
3. RouterFunction and HandlerFunction – This creates functional-style routing and request handling.

Q-4). What is the difference between Mono and Flux?

A-4). The differences between Mono and Flux are as follows:

Mono	Flux
Represents a single element or no element which means empty.	Represents a sequence of 0 to N elements.

Q-5). Explain the difference between Spring MVC and Spring WebFlux?

A-5). The differences between Spring MVC and Spring WebFlux are as follows:

Spring MVC	Spring WebFlux
It is based on traditional Servlet API	It is built on a non-blocking, reactive programming model.
Likewise, it uses a blocking I/O model.	It can run on non-blocking servers like Netty using Servlet 3.1 containers with non-blocking I/O.

Q-6). How does Spring WebFlux handle backpressure?

A-6). Spring WebFlux handles backpressure using the Publisher and Subscriber interfaces from the Reactive Streams specification. It allows the consumer (subscriber) to request only the amount of data it can handle, and the producer (publisher) respects that demand, thereby preventing the consumer from being overwhelmed.

Q-7). What is Schedulers.parallel() in Spring WebFlux?

A-7). Schedulers.parallel() provides a Scheduler that can be used to run tasks in a parallel thread pool. It’s commonly used for CPU-intensive tasks in a reactive pipeline to avoid blocking the main event loop.

Q-8). Explain the role of WebClient in Spring WebFlux?

A-8). WebClient is a non-blocking, reactive web client provided by Spring WebFlux. It is used to perform HTTP requests asynchronously and supports both synchronous and asynchronous response handling, with built-in support for reactive types like Mono and Flux.

Q-9). How does Spring WebFlux ensure thread safety in a multi-threaded environment?

A-9). Spring WebFlux ensures thread safety by following the reactive programming model, where data transformations are handled by thread-safe operators. Also, immutable data structures and thread-local storage are used to avoid shared mutable state and race conditions.

Q-10). Describe the role of DiapatcherHandler in Spring WebFlux?

A-10). DispatcherHandler in Spring WebFlux is analogous to DispatcherServlet in Spring MVC. It is responsible for dispatching requests to the appropriate handler functions or controllers. It acts as the main entry point for processing web requests in a reactive manner.

Q-11). What is the use of @Conteoller and @RestController annotations in Spring WebFlux?

A-11). The use of @Controller and @RestController annotations are as follows:

1. @Controller – Marks a class as a Spring MVC controller, typically returning a View in response.
2. @RestController – A specialized version of @Controller that combines @Controller and @ResponseBody. It returns the response body directly instead of a view.

Q-12). How can you handle exceptions globally in Spring WebFlux?

A-12). You can handle exceptions globally in Spring WebFlux using @ControllerAdvice and @ExceptionHandler annotations. Another approach is using WebExceptionHandler interface to define a custom exception handler.

Q-13). Explain the internal working of the reactive WebClient and its integration with Netty in Spring WebFlux?

A-13). WebClient in Spring WebFlux is built on top of the Reactor Netty HTTP client. It uses the event-driven, non-blocking I/O model provided by Netty to send and receive HTTP requests and responses. WebClient integrates with the reactive streams model, converting HTTP response data into reactive streams (Mono or Flux) for further processing.

Q-14). How do you test reactive endpoints in Spring WebFlux?

A-14). Reactive endpoints in Spring WebFlux can be tested using WebTestClient, which allows for end-to-end testing of WebFlux controllers and functional endpoints. You can also use StepVerifier from Project Reactor to test the reactive streams by asserting expected behaviors.

Q-15). What are the benefits and drawbacks of using Spring WebFlux over Spring MVC?

A-15). The benefits and drawbacks of using Spring WebFlux over Spring MVC are as follows:

Benefits

1. Non-blocking I/O leads to better scalability with fewer resources.
2. Suitable for handling large numbers of concurrent connections.
3. Better performance for I/O-bound operations.

Drawbacks

1. Increased complexity in handling reactive streams.
2. More difficult debugging and profiling.
3. Not always necessary for applications with lower concurrency requirements.

Q-16). How does reactive programming improve the efficiency of I/O-bound tasks in Spring WebFlux?

A-16). Reactive programming improves the efficiency of I/O-bound tasks by using a non-blocking, event-driven model. It allows the application to handle multiple I/O operations concurrently without blocking threads, thus utilizing fewer resources and increasing throughput.

Q-17). What are the differences between Publisher, Subscriber, and Processor in Reactive Streams?

A-17). The differences between Publisher, Subscriber, and Processor are as follows:

Publisher	Subscriber	Processor
Emits items to Subscribers according to the demand signaled by the subscribers.	Consumes items emitted by a Publisher.	A special type of Subscriber that can also act as a Publisher, bridging the gap between upstream and downstream.

Q-18). What is the role of ReactiveAdapterResgistry in Spring WebFlux?

A-18). ReactiveAdapterRegistry is a registry that provides conversion between different reactive libraries (like RxJava, and Project Reactor) and Spring’s Publisher types. It allows you to use multiple reactive types in your application seamlessly.

Q-19). How does Spring WebFlux handle content negotiation?

A-19). Spring WebFlux uses RequestMappingHandlerMapping and RequestMappingHandlerAdapter for annotation-based controllers to determine which method to invoke based on the request, including handling content negotiation using ContentNegotiationManager.

Q-20). How does Spring WebFlux support streaming responses?

A-20). Spring WebFlux supports streaming responses by using Mono or Flux return types in controller methods. The Flux type is beneficial for streaming multiple chunks of data to the client as they become available.

Q-21). Explain the use of ServerSentEvent in Spring WebFlux?

A-21). ServerSentEvent is used to send server-sent events (SSE) to clients in a streaming fashion. It is a one-way communication from the server to the client, useful for pushing real-time updates. In Spring WebFlux, you can use Flux<ServerSentEvent> to stream these events.

Q-22). How can you implement security in a Spring WebFlux application?

A-22). Security in Spring WebFlux can be implemented using Spring Security’s reactive support (spring-security-webflux). It provides non-blocking, reactive security features like authentication and authorization. You can use SecurityWebFilterChain to configure security filters for reactive web applications.

Q-23). What is Context in Project Reactor, and how is it used in Spring WebFlux?

A-23). Context in Project Reactor is a mechanism for passing contextual data along the reactive stream, similar to thread-local storage but in a non-blocking, reactive way. It can be used in Spring WebFlux to carry request-specific information, such as authentication data, throughout the reactive pipeline.

Q-24). How does Spring Data R2DBC integrate with Spring WebFlux?

A-24). Spring Data R2DBC provides reactive database access support for relational databases using the R2DBC specification. It integrates with Spring WebFlux by allowing you to use Mono and Flux types to perform non-blocking database operations, making it suitable for reactive web applications.

Q-25). How can you handle timeouts in Spring WebFlux?

A-25). Timeouts in Spring WebFlux can be handled using the timeout operator provided by Project Reactor. You can set a timeout duration on Mono or Flux, and if the operation exceeds the specified time, a TimeoutException is thrown, which can be handled accordingly.

Q-26). How would you design a reactive microservices architecture using Spring WebFlux?

A-26). A reactive microservices architecture using Spring WebFlux would include the below following points:

1. Each microservice is built with Spring WebFlux to handle non-blocking I/O.
2. Reactive communication between services using WebClient or messaging systems like Kafka, RabbitMQ.
3. Use of Circuit Breaker patterns for fault tolerance.
4. Reactive data access using Spring Data R2DBC for relational databases or reactive drivers for NoSQL databases.
5. Integration with reactive APIs for external services.

Q-27). How do you handle stateful interactions in a stateless Spring WebFlux application?

A-27). Stateful interactions in a stateless Spring WebFlux application can be managed using tokens, session IDs in cookies, or context propagation with reactive types. For instance, a token can be stored in the Context of a reactive pipeline and propagated downstream for each interaction.

Q-28). What strategies can be used to manage long-running tasks in Spring WebFlux?

A-28). Strategies for managing long-running tasks are as follows:

1. Breaking down the task into smaller chunks processed asynchronously.
2. Using Schedulers to offload the task to a separate thread pool.
3. Utilizing message queues to decouple long-running tasks from the main request-response cycle.
4. Streaming partial results back to the client using Flux.