Spring MVC (async) vs Spring WebFlux

Servlet API is blocking I/O which requires 1 thread per HTTP request. Spring MVC async relies on Servlet APIs which only provides async behavior between container threads and request processing threads but not end to end.

Spring WebFlux on the other hand achieves concurrency by a fixed number of threads by using HTTP sockets and pushing chunks of data at a time through the sockets. This mechanism is called event loop, an idea made popular by Node.js. Such an approach is scalable and resilient. Spring 5's spring-webflux uses the event loop approach to provide async behavior.

More can be read from

• Servlet vs. Reactive
• Spring Boot performance battle
• Comparing WebFlux with Spring Web MVC

The Servlet async model introduces an async boundary between the container threads (1 Servlet request/thread model) and the processing of the request in your application. Processing can happen on a different thread or wait. In the end, you have to dispatch back to a container thread and read/write in a blocking way (InputStream and OutputStream are inherently blocking APIs).

With that model, you need many threads to achieve concurrency (because many of those can be blocked waiting for I/O). This costs resources and it can be a tradeoff, depending on your use case.

With non-blocking code, you only need a few threads to process a lot of requests concurrently. This is a different concurrency model; like any model, there are benefits and tradeoffs coming with it.

For more information about that comparison, this Servlet vs. Reactive stacks talk should be of interest.