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Reactive interfaces for Java.

Reakt is reactive interfaces for Java which includes:

The emphasis is on defining interfaces that enable lambda expressions, and fluent APIs for asynchronous programming for Java.

Note: This mostly just provides the interfaces not the implementations. There are some starter implementations but the idea is that anyone can implement this. It is all about interfaces. There will be adapters for Vertx, RxJava, Reactive Streams, etc. There is support for Guava Async (used by Cassandra) and the QBit microservices lib. Elekt uses Reakt for its reactive leadership election.

Have a question?

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Getting started

Using from maven

Reakt is published in the maven public repo.


Using from gradle

compile 'io.advantageous.reakt:reakt:3.0.4.RELEASE'

Fluent Promise API

                .then(e -> saveEmployee(e))
                .catchError(error -> logger.error("Unable to lookup employee", error))

Promises are both a callback and a Result; however, you can work with Callbacks directly.

Using Result and callback directly

        employeeService.lookupEmployee(33, result -> {
            result.then(e -> saveEmployee(e))
                  .catchError(error -> logger.error("Unable to lookup", error));

In both of these examples, lookupEmployee would look like:

Using Result and callback directly

   public void lookupEmployee(long employeeId, Callback<Employee> callback){...}

You can use Promises to transform into other promises.

Transforming into another type of promise using thenMap

        Promise<Employee> employeePromise = Promises.<Employee>blockingPromise();

        Promise<Sheep> sheepPromise = employeePromise
                .thenMap(employee1 -> new Sheep(employee1.getId()));

The thenMap will return a new type of Promise.

You can find more examples in the reakt wiki.

We also support working with streams.

Promise concepts

This has been adapted from this article on ES6 promises. A promise can be:

  • fulfilled The callback/action relating to the promise succeeded
  • rejected The callback/action relating to the promise failed
  • pending The callback/action has not been fulfilled or rejected yet
  • completed The callback/action has been fulfilled/resolved or rejected

Java is not single threaded, meaning that two bits of code can run at the same time, so the design of this promise and streaming library takes that into account.

There are three types of promises:

  • Callback promises
  • Blocking promises (for testing and legacy integration)
  • Replay promises (allow promises to be handled on the same thread as caller)

Replay promises are the most like their JS cousins. Replay promises are usually managed by the Reakt Reactor and supports environments like Vert.x and QBit. See the wiki for more details on Replay promises.

It is common to make async calls to store data in a NoSQL store or to call a remote REST interface or deal with a distributed cache or queue. Also Java is strongly typed so the library that mimics JS promises is going to look a bit different. We tried to use similar terminology where it makes sense.

Events and Streams are great for things that can happen multiple times on the same object — keyup, touchstart, or event a user action stream from Kafka, etc.

With those events you don't really care about what happened before when you attached the listener.

But often times when dealing with services and data repositories, you want to handle a response with a specific next action, and a different action if there was an error or timeout from the responses. You essentially want to call and handle a response asynchronously and that is what promises allow.

This is not our first time to bat with Promises. QBit has had Promises for a few years now. We just called them CallbackBuilders instead. We wanted to use more standard terminology and wanted to use the same terminology and modeling on projects that do not use QBit like Conekt, Vert.x, RxJava, and reactive streams.

At their most basic level, promises are like event listeners except:

A promise can only succeed or fail once. A promise cannot succeed or fail twice, neither can it switch from success to failure. Once it enters its completed state, then it is done.


Reakt Guava Bridge which allows libs that use Guava async support to now have a modern Java feel.

Cassandra Reakt example

register(session.executeAsync("SELECT release_version FROM system.local"), 
  promise().thenExpect(expected -> 
     gui.setMessage("Cassandra version is " +
  ).catchError(error -> 
     gui.setMessage("Error while reading Cassandra version: " 
     + error.getMessage())

QBit 1 ships with a bridge and QBit 2will use Reakt as its primary reactive callback mechanism.

Conekt, a slimmed down fork of Vert.x, will also use Reakt.

See QBit microservices lib for more details.

See our wiki for more details on Reakt.

Further reading

What is Microservices Architecture?

QBit Java Micorservices lib tutorials

The Java microservice lib. QBit is a reactive programming lib for building microservices - JSON, HTTP, WebSocket, and REST. QBit uses reactive programming to build elastic REST, and WebSockets based cloud friendly, web services. SOA evolved for mobile and cloud. ServiceDiscovery, Health, reactive StatService, events, Java idiomatic reactive programming for Microservices.

Find more tutorial on QBit.

Reactive Programming, Java Microservices, Rick Hightower

High-speed microservices consulting firm and authors of QBit with lots of experience with Vertx - Mammatus Technology

Highly recommended consulting and training firm who specializes in microservices architecture and mobile development that are already very familiar with QBit and Vertx as well as iOS and Android - About Objects

Java Microservices Architecture

[Microservice Service Discovery with Consul] (

Microservices Service Discovery Tutorial with Consul

[Reactive Microservices] (

[High Speed Microservices] (

Java Microservices Consulting

Microservices Training

[Reactive Microservices Tutorial, using the Reactor] (

QBit is mentioned in the Restlet blog

All code is written using JetBrains Idea - the best IDE ever!


Fluent Java Reactive interfaces. Promises, Streams, Callbacks, Async results, interfaces for Java 8 that are lambda expression friendly.




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