Example of reactive web application. Java. Spring 5. Reactive Streams. Docker.

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This project is intended to demonstrate best practices for building a reactive web application with Spring 5 platform.

Table of Contents

Reactive programming and Reactive systems

In plain terms reactive programming is about non-blocking applications that are asynchronous and message-driven and require a small number of threads to scale vertically (i.e. within the JVM) rather than horizontally (i.e. through clustering).

A key aspect of reactive applications is the concept of backpressure which is a mechanism to ensure producers don’t overwhelm consumers. For example in a pipeline of reactive components extending from the database to the HTTP response when the HTTP connection is too slow the data repository can also slow down or stop completely until network capacity frees up.

Reactive programming also leads to a major shift from imperative to declarative async composition of logic. It is comparable to writing blocking code vs using the CompletableFuture from Java 8 to compose follow-up actions via lambda expressions.

For a longer introduction check the blog series “Notes on Reactive Programming” by Dave Syer.

“We look at Reactive Programming as one of the methodologies or pieces of the puzzle for Reactive [Systems] as a broader term.” Please read the ‘Reactive Manifesto’ and ‘Reactive programming vs. Reactive systems’ for more informations.

Why now?

What is driving the rise of Reactive in Enterprise Java? Well, it’s not (all) just a technology fad — people jumping on the bandwagon with the shiny new toys. The driver is efficient resource utilization, or in other words, spending less money on servers and data centres. The promise of Reactive is that you can do more with less, specifically you can process higher loads with fewer threads. This is where the intersection of Reactive and non-blocking, asynchronous I/O comes to the foreground. For the right problem, the effects are dramatic. For the wrong problem, the effects might go into reverse (you actually make things worse). Also remember, even if you pick the right problem, there is no such thing as a free lunch, and Reactive doesn’t solve the problems for you, it just gives you a toolbox that you can use to implement solutions.

Spring WebFlux (web reactive) module

Spring Framework 5 includes a new spring-webflux module. The module contains support for reactive HTTP and WebSocket clients as well as for reactive server web applications including REST, HTML browser, and WebSocket style interactions.

Server side

On the server-side WebFlux supports 2 distinct programming models:

Annotation based
public class BlogPostController {

	private final BlogPostRepository blogPostRepository;

	public BlogPostController(BlogPostRepository blogPostRepository) {
		this.blogPostRepository = blogPostRepository;

	Mono<Void> create(@RequestBody Publisher<BlogPost> blogPostStream) {

	Flux<BlogPost> list() {
		return this.blogPostRepository.findAll();

	Mono<BlogPost> findById(@PathVariable String id) {
		return this.blogPostRepository.findOne(id);

Functional programming model is not implemented within this application. I am not sure if it is posible to have both models in one application.

Both programming models are executed on the same reactive foundation that adapts non-blocking HTTP runtimes to the Reactive Streams API.

Client side

WebFlux includes a functional, reactive WebClient that offers a fully non-blocking and reactive alternative to the RestTemplate. It exposes network input and output as a reactive ClientHttpRequest and ClientHttpRespones where the body of the request and response is a Flux rather than an InputStream and OutputStream. In addition it supports the same reactive JSON, XML, and SSE serialization mechanism as on the server side so you can work with typed objects.

@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
public class ApplicationIntegrationTest {

	WebTestClient webTestClient;

	List<BlogPost> expectedBlogPosts;
	List<Project> expectedProjects;

	BlogPostRepository blogPostRepository;

	ProjectRepository projectRepository;

	public void setup() {
		webTestClient = WebTestClient.bindToController(new BlogPostController(blogPostRepository), new ProjectController(projectRepository)).build();

		expectedBlogPosts = blogPostRepository.findAll().collectList().block();
		expectedProjects = projectRepository.findAll().collectList().block();


	public void listAllBlogPostsIntegrationTest() {

	public void listAllProjectsIntegrationTest() {

	public void streamAllBlogPostsIntegrationTest() throws Exception {
		FluxExchangeResult<BlogPost> result = this.webTestClient.get()

			.expectNext(expectedBlogPosts.get(0), expectedBlogPosts.get(1))
			.consumeNextWith(blogPost -> assertThat(blogPost.getAuthorId(), endsWith("4")))


Please note that webClient is requesting Server-Sent Events (text/event-stream). We could stream individual JSON objects (application/stream+json) but that would not be a valid JSON document as a whole and a browser client has no way to consume a stream other than using Server-Sent Events or WebSocket.

Spring Reactive data

Spring Data Kay M1 is the first release ever that comes with support for reactive data access. Its initial set of supported stores — MongoDB, Apache Cassandra and Redis

The repositories programming model is the most high-level abstraction Spring Data users usually deal with. They’re usually comprised of a set of CRUD methods defined in a Spring Data provided interface and domain-specific query methods.

In contrast to the traditional repository interfaces, a reactive repository uses reactive types as return types and can do so for parameter types, too.

public interface BlogPostRepository extends ReactiveSortingRepository<BlogPost, String>{

	Flux<BlogPost> findByTitle(Mono<String> title);


CI with Travis

The application is build by Travis. Pipeline is triggered on every push to master branch.

Running instructions

Run the application by maven:

This application is using embedded mongo database. You do not have to install and run mongo database before you run the application locally.

You can use NON-embedded version of mongo by setting scope of ‘de.flapdoodle.embed.mongo’ to ‘test’. In this case you have to install mongo server locally:

$ brew install mongodb
$ brew services start mongodb

Run it:

$ cd reactive-company
$ ./mvnw spring-boot:run

Run the application on Cloud Foundry

Run application on local workstation with PCF Dev

You can adopt any CI pipeline you have to deploy your application on any cloud foundry instance, for example:

mvn cf:push [-Dcf.appname] [-Dcf.path] [-Dcf.url] [-Dcf.instances] [-Dcf.memory] []

Run the application by Docker

I am running Docker Community Edition, version: 17.05.0-ce-mac11 (Channel: edge).

A swarm is a cluster of Docker engines, or nodes, where you deploy services. The Docker Engine CLI and API include commands to manage swarm nodes (e.g., add or remove nodes), and deploy and orchestrate services across the swarm. By running script bellow you will initialize a simple swarm with one node, and you will install services:

$ cd reactive-company
$ ./

Manage docker swarm with Portainer

Portainer is a simple management solution for Docker, and is really simple to deploy:

$ docker service create \
    --name portainer \
    --publish 9000:9000 \
    --constraint 'node.role == manager' \
    --mount type=bind,src=/var/run/docker.sock,dst=/var/run/docker.sock \
    portainer/portainer \
    -H unix:///var/run/docker.sock

Visit http://localhost:9000

Manage docker swarm with CLI

List docker services
$ docker service ls
Scale docker services
$ docker service scale stack_reactive-company=2

Now you have two tasks/containers running for this service.

Browse docker service logs
$ docker service logs stack_reactive-company -f

You will be able to determine what task/container handled the request.

Swarm mode load balancer

When using HTTP/1.1, by default, the TCP connections are left open for reuse. Docker swarm load balancer will not work as expected in this case. You will get routed to the same task of the service every time.

You can use ‘curl’ command line tool (NOT BROWSER) to avoid this problem.

The Swarm load balancer is a basic Layer 4 (TCP) load balancer. Many applications require additional features, like these, to name just a few:

Browse the application:

Index page

Open your browser and navigate to http://localhost:8080

The response is resolved by and home.html.

Server-Sent Events page

Open your browser and navigate to http://localhost:8080/stream

This view is resolved by and sse.html template.

@GetMapping(value = "/stream/blog")
public String blog(final Model model) {
	final Flux<BlogPost> blogPostStream = this.blogPostRepository.findAll().log();
	model.addAttribute("blogPosts", new ReactiveDataDriverContextVariable(blogPostStream, 1000));
	return "sse :: #blogTableBody";
@GetMapping(value = "/stream/project")
public String project(final Model model) {
	final Flux<Project> projectStream = this.projectRepository.findAll().log();
	model.addAttribute("projects", new ReactiveDataDriverContextVariable(projectStream, 1000));
	return "sse :: #projectTableBody";
Flux<BlogPost> tail() {"Received request: BlogPost - Tail");
	try {
		// Using tailable cursor
		return this.blogPostRepository.findBy().log();
	} finally {"Request pocessed: BlogPost - Tail");

Blog posts (REST API):

$ curl http://localhost:8080/blogposts


$ curl -v -H "Accept: text/event-stream" http://localhost:8080/blogposts

Projects (REST API):

$ curl http://localhost:8080/projects


$ curl -v -H "Accept: text/event-stream" http://localhost:8080/projects

Blog posts - tial (REST API)

$ curl -v -H "Accept: text/event-stream" http://localhost:8080/tail/blogposts

Load testing with Gatling

Run application first (by maven or docker)

$ ./mvnw gatling:execute

By default src/main/test/scala/com/idugalic/RecordedSimulation.scala will be run. The reports will be available in the console and in *html files within the ‘target/gatling/results’ folder

Log output

A possible log output we could see is: Log - Reactive

As we can see the output of the controller method is evaluated after its execution in a different thread too!

Flux<BlogPost> list() {"Received request: BlogPost - List");
	try {
		return this.blogPostRepository.findAll().log();
	} finally {"Request pocessed: BlogPost - List");

We can no longer think in terms of a linear execution model where one request is handled by one thread. The reactive streams will be handled by a lot of threads in their lifecycle. This complicates things when we migrate from the old MVC framework. We no longer can rely on thread affinity for things like the security context or transaction handling.


References and further reading