Build Reactive REST APIs with Spring WebFlux – Part2

Reactive Streams, Project Reactor, Spring WebFlux, Spring, Spring Boot
Photo by Edgar Chaparro on Unsplash

                            In continuation of the last post, in this article, we will see the reactive streams specification and one of its implementation called Project Reactor. Reactive Streams specification has the following interfaces defined. Let us see the details of those interfaces.

  • Publisher → A Publisher is a provider of a potentially unlimited number of sequenced elements, publishing them as requested by its Subscriber(s)

public interface Publisher<T> {
public void subscribe(Subscriber<? super T> s);
view raw Publisher hosted with ❤ by GitHub

  • Subscriber A Subscriber is a consumer of a potentially unbounded number of sequenced elements.

public interface Subscriber<T> {
public void onSubscribe(Subscription s);
public void onNext(T t);
public void onError(Throwable t);
public void onComplete();
view raw Subscriber hosted with ❤ by GitHub

  • Subscription A Subscription represents a one-to-one lifecycle of a Subscriber subscribing to a Publisher.

public interface Subscription {
public void request(long n);
public void cancel();
view raw Subscription hosted with ❤ by GitHub

  • Processor A Processor represents a processing stage — which is both a Subscriber and a Publisher and obeys the contracts of both.

The class diagram of the reactive streams specification is given below.

Class digram of Reactive Streams

The reactive streams specification has many implementations. Project Reactor is one of the implementations. The Reactor is fully non-blocking and provides efficient demand management. The Reactor offers two reactive and composable APIs, Flux [N] and Mono [0|1], which extensively implement Reactive Extensions. Reactor offers Non-Blocking, backpressure-ready network engines for HTTP (including Websockets), TCP, and UDP. It is well-suited for a microservices architecture.

  • Flux → It is a Reactive Streams Publisher with rx operators that emits 0 to N elements, and then complete (successfully or with an error). The marble diagram of the Flux is represented below.
  • Mono It is a Reactive Streams Publisher with basic rx operators that completes successfully by emitting 0 to 1 element, or with an error. The marble diagram of the Mono is represented below.

As Spring 5.x comes with Reactor implementation, if we want to build REST APIs using imperative style programming with Spring servlet stack, it still supports. Below is the diagram which explains how Spring supports both reactive and servlet stack implementations.

Spring MVC vs Spring Reactive
Image Credit:

In the coming article, we will see an example application with reactive APIs. Until then, Happy Learning!!


Siva Janapati is an Architect with experience in building Cloud Native Microservices architectures, Reactive Systems, Large scale distributed systems, and Serverless Systems. Siva has hands-on in architecture, design, and implementation of scalable systems using Cloud, Java, Go lang, Apache Kafka, Apache Solr, Spring, Spring Boot, Lightbend reactive tech stack, APIGEE edge & on-premise and other open-source, proprietary technologies. Expertise working with and building RESTful, GraphQL APIs. He has successfully delivered multiple applications in retail, telco, and financial services domains. He manages the GitHub( where he put the source code of his work related to his blog posts.

Tagged with: , , , , ,
Posted in Java, Microservices, Reactive Programming, Spring, Spring Boot
One comment on “Build Reactive REST APIs with Spring WebFlux – Part2
  1. Aravind says:

    Nice article

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.


Java Code Geeks
Java Code Geeks
%d bloggers like this: