Spring Boot Quiz: Blocking Calls in Reactive WebFlux

Question

What happens if you call a blocking DB method inside a reactive WebFlux endpoint?

A. It works fine
B. Non-blocking benefit is lost
C. Reactor detects and throws an error
D. App crashes

Answer

B. Non-blocking benefit is lost

Why?

If we call a blocking method (ex., JDBC call) inside a reactive webflux endpoint, the call will block the event-loop thread that is supposed to handle many requests concurrently.

• It still works because the blocking call returns eventually. So the request completes.

• But the scalability benefit of WebFlux is gone, since that thread is occupied and can’t process other requests while waiting.

• The reactor doesn’t automatically detect or throw an error. It just runs the blocking call on the event loop.

• The app doesn’t crash, but under the load, performance can degrade severely.

Best Practice

If we must call a blocking call a blocking API, we should wrap it boundedElastic Scheduler.

Mono.fromCallable(() -> repository.findById(id))
    .subscribeOn(Schedulers.boundedElastic());

This way, blocking work runs on a dedicated thread pool, and the event-loop remains non-blocking.