Question 1

What is Project Reactor?

Accepted Answer

Project Reactor is a reactive programming library for building non-blocking applications on the JVM. It implements the Reactive Streams specification and provides two core types: Mono (0-1 elements) and Flux (0-N elements) for composing asynchronous data pipelines.

Question 2

What is the difference between Mono and Flux?

Accepted Answer

Mono represents a stream of 0 or 1 element, ideal for single-value operations like database lookups or HTTP requests. Flux represents a stream of 0 to N elements, suitable for collections, event streams, or any sequence of multiple values.

Question 3

What is backpressure in reactive programming?

Accepted Answer

Backpressure is a flow control mechanism where subscribers can signal to publishers how many elements they can handle. It prevents fast producers from overwhelming slow consumers, avoiding memory issues and ensuring system stability.

Question 4

What is the difference between map and flatMap in Reactor?

Accepted Answer

map transforms each element synchronously using a function that returns a plain value. flatMap transforms each element into a Publisher (Mono/Flux) and flattens the results, enabling asynchronous transformations and nested reactive operations.

Question 5

What is the difference between subscribeOn and publishOn?

Accepted Answer

subscribeOn affects the subscription and upstream operations, determining where the source emits. publishOn affects downstream operations, switching the execution context for subsequent operators. subscribeOn placement doesn't matter; publishOn placement does.

Question 6

How do you handle errors in Project Reactor?

Accepted Answer

Use onErrorReturn for fallback values, onErrorResume to switch to alternative publishers, onErrorMap to transform exceptions, and retry/retryWhen for retry logic. doOnError allows side effects without changing the error signal.

Question 7

What is the difference between block() and subscribe()?

Accepted Answer

subscribe() is non-blocking and starts the reactive pipeline asynchronously, returning a Disposable. block() is blocking and waits for the result, converting reactive to imperative. Avoid block() in reactive code paths; use it only at application boundaries.

Question 8

What are Schedulers in Project Reactor?

Accepted Answer

Schedulers provide execution contexts for reactive operations. Schedulers.parallel() is for CPU-bound tasks, Schedulers.boundedElastic() for blocking I/O, and Schedulers.single() for sequential execution. They enable thread switching in reactive pipelines.

Question 9

How do you test reactive streams with StepVerifier?

Accepted Answer

StepVerifier validates reactive streams by asserting expected elements, errors, and completion. Use expectNext() for values, expectError() for exceptions, expectComplete() for completion, and verify() to execute the test. It supports virtual time for time-based testing.

Question 10

What is the difference between Spring MVC and Spring WebFlux?

Accepted Answer

Spring MVC uses blocking I/O with thread-per-request model, suitable for traditional applications. Spring WebFlux uses non-blocking I/O with event loop model, handling more concurrent connections with fewer threads, ideal for reactive and high-concurrency scenarios.

Question 11

How does Project Reactor integrate with Spring WebFlux?

Accepted Answer

Spring WebFlux uses Project Reactor as its reactive foundation. Controllers return Mono/Flux, which WebFlux handles non-blockingly. Request/response bodies are processed as reactive streams, enabling end-to-end non-blocking I/O from client to database.

Question 12

What is the Stale-While-Revalidate pattern in Reactor?

Accepted Answer

This caching pattern serves cached data immediately while fetching fresh data in the background. In Reactor, implement with cache operators and background refresh using subscribeOn to update cached Mono/Flux without blocking the main response.

Question 13

What is R2DBC and how does it work with Reactor?

Accepted Answer

R2DBC (Reactive Relational Database Connectivity) provides non-blocking database access. It returns Mono/Flux from queries, enabling reactive database operations. Spring Data R2DBC integrates with WebFlux for end-to-end reactive applications.

Question 14

What is the zip operator in Reactor?

Accepted Answer

The zip operator combines elements from multiple publishers by position, waiting for all sources to emit before combining. It's useful for parallel operations where you need all results together, like fetching data from multiple services simultaneously.

Question 15

How do you handle blocking operations in reactive code?

Accepted Answer

Wrap blocking calls with Mono.fromCallable() or Flux.fromIterable() and use subscribeOn(Schedulers.boundedElastic()) to execute on a dedicated thread pool. This prevents blocking the event loop while integrating legacy blocking APIs.

Project Reactor Fundamentals(5 questions)

What is Project Reactor and how does it relate to Reactive Streams?

What is the difference between reactive and imperative programming?

What are the core types in Project Reactor (Mono and Flux)?

What is backpressure and why is it important in reactive programming?

How does Project Reactor differ from RxJava?

Mono and Flux(5 questions)

What is a Mono and when would you use it?

What is a Flux and when would you use it?

How do you create a Mono or Flux from different data sources?

What is the difference between Mono.empty() and Mono.just(null)?

How do you convert between Mono and Flux?

Operators(6 questions)

What are the most commonly used transformation operators in Reactor?

What is the difference between map and flatMap in Reactor?

When would you use flatMapMany vs flatMap?

What is the filter operator and how does it work?

What is the zip operator and when would you use it?

How does the merge operator differ from concat?

Subscription and Lifecycle(5 questions)

How do you subscribe to a Mono or Flux?

What is the subscribe() method and its overloaded variants?

What is the difference between block() and subscribe()?

When is it appropriate to use block() in reactive code?

What is a Disposable and how do you manage subscriptions?

Error Handling(4 questions)

How do you handle errors in Project Reactor?

What is the difference between onErrorReturn, onErrorResume, and onErrorMap?

How do you implement retry logic in Reactor?

What is doOnError and when would you use it?

Schedulers and Threading(4 questions)

What are Schedulers in Project Reactor?

What is the difference between subscribeOn and publishOn?

What are the different built-in Schedulers (parallel, boundedElastic, single)?

How do you handle blocking operations in reactive code?

Testing(4 questions)

How do you test reactive streams with StepVerifier?

What is the purpose of expectNext, expectComplete, and verify?

How do you test time-dependent reactive streams?

How do you test error scenarios in reactive code?

Spring WebFlux Integration(5 questions)

How does Project Reactor integrate with Spring WebFlux?

What is the difference between Spring MVC and Spring WebFlux?

How do you handle request/response bodies as Mono and Flux?

What are the best practices for building reactive REST APIs?

How do you handle database access reactively with R2DBC?