Reactor Core

Non-Blocking Reactive Streams Foundation for the JVM both implementing a [Reactive Extensions] (http://reactivex.io) inspired API and efficient message-passing support.

Getting it

2.5 requires Java 8 or + to run.

With Gradle from repo.spring.io or Maven Central repositories (stable releases only):

    repositories {
      //maven { url 'http://repo.spring.io/snapshot' }
      mavenCentral()
    }

    dependencies {
      //compile "io.projectreactor:reactor-core:2.5.0.BUILD-SNAPSHOT"
      compile "io.projectreactor:reactor-core:2.5.0.M2"
    }

Getting Started

New to Reactive Programming or bored of reading already ? Try the [Introduction to Reactor Core hands-on](https://github .com/reactor/lite-rx-api-hands-on) !

Flux

A Reactive Streams Publisher with basic Rx operators.

• Static factories on Flux allow for source generation from arbitrary callbacks types.
• Instance methods allows operational building, materialized on each Flux#subscribe(), Flux#consume() or multicasting operations such as Flux#publish and Flux#publishNext.

Flux in action :

Flux.fromIterable(getSomeLongList())
    .mergeWith(Flux.interval(100))
    .doOnNext(serviceA::someObserver)
    .map(d -> d * 2)
    .take(3)
    .onErrorResumeWith(errorHandler::fallback)
    .doAfterTerminate(serviceM::incrementTerminate)
    .consume(System.out::println);

Mono

A Reactive Streams Publisher constrained to ZERO or ONE element with appropriate operators.

• Static factories on Mono allow for deterministic zero or one sequence generation from arbitrary callbacks types.
• Instance methods allows operational building, materialized on each Mono#subscribe() or Mono#get() eventually called.

Mono in action :

Mono.fromCallable(System::currentTimeMillis)
    .then(time -> Mono.any(serviceA.findRecent(time), serviceB.findRecent(time)))
    .timeout(Duration.ofSeconds(3), errorHandler::fallback)
    .doOnSuccess(r -> serviceM.incrementSuccess())
    .consume(System.out::println);

Blocking Mono result :

Tuple2<Long, Long> nowAndLater = 
        Mono.when(
                Mono.just(System.currentTimeMillis()),
                Flux.just(1).delay(1).map(i -> System.currentTimeMillis()))
            .get();

Schedulers

Create and Reuse scheduling resources over multiple Subscribers with adapted concurrency strategy for producing flows (publishOn) or receiving flows (dispatchOn) :

SchedulerGroup async = SchedulerGroup.async();
SchedulerGroup io = SchedulerGroup.io();

Flux.create( sub -> sub.onNext(System.currentTimeMillis()) )
    .dispatchOn(async)
    .log("foo.bar")
    .flatMap(time ->
        Mono.fromCallable(() -> { Thread.sleep(1000); return time; })
            .publishOn(io)
    , 8) //maxConcurrency 8
    .subscribe();

//... a little later
async.forceShutdown()
     .subscribe(Runnable::run);
     
io.shutdown();

Hot Publishing : SignalEmitter

To bridge a Subscriber or Processor into an outside context that is taking care of producing non concurrently, use SignalEmitter.create(Subscriber), the common FluxProcessor.startEmitter() or Flux.yield(emitter -> {}) :

Flux.yield(sink -> {
        while(sink.hasRequested()){
            Emission status = sink.emit("Non blocking and returning emission status");
            long latency = sink.submit("Blocking until emitted and returning latency");
        }
        sink.finish();
    })
    .timeout(3)
    .doOnComplete(() -> System.out.println("completed!"))
    .consume(System.out::println)

Hot Publishing : Processors

The 3 main processor implementations are message relays using 0 (EmitterProcessor) or N threads (TopicProcessor and WorkQueueProcessor). They also use bounded and replayable buffers, aka RingBuffer.

Pub-Sub : EmitterProcessor

A signal broadcaster that will safely handle asynchronous boundaries between N Subscribers (asynchronous or not) and a parent producer.

EmitterProcessor<Integer> emitter = EmitterProcessor.create();
SignalEmitter<Integer> sink = emitter.startEmitter();
sink.submit(1);
sink.submit(2);
emitter.consume(System.out::println);
sink.submit(3); //output : 3
sink.finish();

Replay capacity in action:

EmitterProcessor<Integer> replayer = EmitterProcessor.replay();
SignalEmitter<Integer> sink = replayer.startEmitter();
sink.submit(1);
sink.submit(2);
replayer.consume(System.out::println); //output 1, 2
sink.submit(3); //output : 3
sink.finish();

Async Pub-Sub : TopicProcessor

An asynchronous signal broadcaster dedicating an event loop thread per subscriber and maxing out producing/consuming rate with temporary tolerance to latency peaks. Also supports multi-producing and emission without onSubscribe.

TopicProcessor<Integer> topic = TopicProcessor.create();
topic.consume(System.out::println);
topic.onNext(1); //output : ...1
topic.onNext(2); //output : ...2
topic.consume(System.out::println); //output : ...1, 2
topic.onNext(3); //output : ...3 ...3
topic.onComplete();

Async Distributed : WorkQueueProcessor

Similar to TopicProcessor regarding thread per subscriber but this time exclusively distributing the input data signal to the next available Subscriber. WorkQueueProcessor is also able to replay detected dropped data downstream (error or cancel) to any Subscriber ready.

WorkQueueProcessor<Integer> queue = WorkQueueProcessor.create();
queue.consume(System.out::println);
queue.consume(System.out::println);
queue.onNext(1); //output : ...1
queue.onNext(2); //output : .... ...2
queue.onNext(3); //output : ...3 
queue.onComplete();

The Backpressure Thing

Most of this cool stuff uses bounded ring buffer implementation under the hood to mitigate signal processing difference between producers and consumers. Now, the operators and processors or any standard reactive stream component working on the sequence will be instructed to flow in when these buffers have free room AND only then. This means that we make sure we both have a deterministic capacity model (bounded buffer) and we never block (request more data on write capacity). Yup, it's not rocket science after all, the boring part is already being worked by us in collaboration with Reactive Streams Commons on going research effort.

What's more in it ?

"Operator Fusion" (flow optimizers), health state observers, TestSubscriber, helpers to build custom reactive components, bounded queue generator, hash-wheel timer, converters from/to RxJava1, Java 9 Flow.Publisher and Java 8 CompletableFuture.

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Reference

http://projectreactor.io/core/docs/reference/

Javadoc

http://projectreactor.io/core/docs/api/

Getting started with Flux and Mono

https://github.com/reactor/lite-rx-api-hands-on

Beyond Reactor Core

• Everything to jump outside the JVM with the non-blocking drivers from Reactor IO.
• Reactor Addons include Bus and Pipes event routers plus a handful of extra reactive modules.

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