= Vert.x RxJava
:toc: left

== Vert.x API for RxJava

https://github.com/ReactiveX/RxJava[RxJava] is a popular library for composing asynchronous and event based programs using
observable sequences for the Java VM. Vert.x integrates naturally with RxJava, allowing to use
observable wherever you can use streams or asynchronous results.

=== Using Vert.x API for RxJava1

To use Vert.x API for RxJava1, add the following dependency to the _dependencies_ section of your build descriptor:

* Maven (in your `pom.xml`):

[source,xml,subs="+attributes"]
----
<dependency>
 <groupId>io.vertx</groupId>
 <artifactId>vertx-rx-java</artifactId>
 <version>3.8.1</version>
</dependency>
----

* Gradle (in your `build.gradle` file):

[source,groovy,subs="+attributes"]
----
compile 'io.vertx:vertx-rx-java:3.8.1'
----

There are two ways for using the RxJava API with Vert.x:

- via the original Vert.x API with the `link:../../apidocs/io/vertx/rx/java/RxHelper.html[RxHelper]` helper class that provides static
methods for converting objects between Vert.x core API and RxJava API.
- via the _Rxified_ Vert.x API enhancing the core Vert.x API.

=== Read stream support

RxJava `Observable` is a perfect match for Vert.x `ReadStream` class : both provide a flow of items.

The `link:../../apidocs/io/vertx/rx/java/RxHelper.html#toObservable-io.vertx.core.streams.ReadStream-[RxHelper.toObservable]` static methods convert
a Vert.x read stream to an `rx.Observable`:

[source,java]
----
FileSystem fileSystem = vertx.fileSystem();
fileSystem.open("/data.txt", new OpenOptions(), result -> {
  AsyncFile file = result.result();
  Observable<Buffer> observable = RxHelper.toObservable(file);
  observable.forEach(data -> System.out.println("Read data: " + data.toString("UTF-8")));
});
----

The _Rxified_ Vert.x API provides a `link:../../apidocs/io/vertx/rxjava/core/streams/ReadStream.html#toObservable--[toObservable]`  method on
`link:../../apidocs/io/vertx/rxjava/core/streams/ReadStream.html[ReadStream]`:

[source,java]
----
FileSystem fs = vertx.fileSystem();
fs.open("/data.txt", new OpenOptions(), result -> {
  AsyncFile file = result.result();
  Observable<Buffer> observable = file.toObservable();
  observable.forEach(data -> System.out.println("Read data: " + data.toString("UTF-8")));
});
----

Such observables arehot* observables, i.e. they will produce notifications regardless of subscriptions because
a `ReadStream` can potentially emit items spontaneously or not, depending on the implementation:

At subscription time, the adapter calls `link:../../apidocs/io/vertx/core/streams/ReadStream.html#handler-io.vertx.core.Handler-[handler]`
to set its own handler.

Some `ReadStream` implementations can start to emit events after this call, others will emit events whether an
handler is set or not:

- `AsyncFile` produces buffer events after the handler is set
- `HttpServerRequest` produces events independantly of the handler (i.e buffer may be lost if no handler is set)

In both cases, subscribing to the `Observable` in the same call is safe because the event loop or the worker
verticles cannot be called concurrently, so the subscription will always happens before the handler starts emitting
data.

When you need to delay the subscription, you need to `pause` the `ReadStream` and then `resume` it, which is what
you would do with a `ReadStream`.

[source,java]
----
server.requestHandler(request -> {
  if (request.method() == HttpMethod.POST) {

    // Stop receiving buffers
    request.pause();

    checkAuth(res -> {

      // Now we can receive buffers again
      request.resume();

      if (res.succeeded()) {
        Observable<Buffer> observable = request.toObservable();
        observable.subscribe(buff -> {
          // Get buffers
        });
      }
    });
  }
});
----

Likewise it is possible to turn an existing `Observable` into a Vert.x `ReadStream`.

The `link:../../apidocs/io/vertx/rx/java/RxHelper.html#toReadStream-rx.Observable-[RxHelper.toReadStream]`  static methods convert
an `rx.Observable` to a Vert.x read stream:

[source,java]
----
Observable<Buffer> observable = getObservable();
ReadStream<Buffer> readStream = RxHelper.toReadStream(observable);
Pump pump = Pump.pump(readStream, response);
pump.start();
----

=== Write stream support

A `link:../../apidocs/io/vertx/core/streams/WriteStream.html[WriteStream]`, like a `rx.Subscriber`, consumes items, and, when it can't keep-up, collaborates with the producer to avoid an ever-growing backlog.

Vert.x provides the `link:../../apidocs/io/vertx/rx/java/WriteStreamSubscriber.html[WriteStreamSubscriber]` adapter that you can use to send `Observable` items to any `link:../../apidocs/io/vertx/core/streams/WriteStream.html[WriteStream]`:

.Sending buffers to an HTTP server response
[source,java]
----
response.setChunked(true);
WriteStreamSubscriber<io.vertx.core.buffer.Buffer> subscriber = io.vertx.rx.java.RxHelper.toSubscriber(response);
observable.subscribe(subscriber);
----

If you are progamming with the _Rxified_ Vert.x API, the `link:../../apidocs/io/vertx/rxjava/core/streams/WriteStream.html[WriteStream]` implementations provide a `toSubscriber` method.
The previous example then becomes even more straightforward:

[source,java]
----
response.setChunked(true);
observable.subscribe(response.toSubscriber());
----

NOTE: When the `Observable` terminates successfully, the adapter invokes the `link:../../apidocs/io/vertx/rxjava/core/streams/WriteStream.html#end--[end]` method.

CAUTION: The adapter sets the `link:../../apidocs/io/vertx/rxjava/core/streams/WriteStream.html[WriteStream]` `drain` and `exception` handlers, so don't use them after subscribing.

The `link:../../apidocs/io/vertx/rx/java/WriteStreamSubscriber.html[WriteStreamSubscriber]` adapter is able to invoke callbacks when:

* the `Observable` completes, or
* the `Observable` terminates with an error, or
* the `link:../../apidocs/io/vertx/rxjava/core/streams/WriteStream.html[WriteStream]` fails (e.g. HTTP connection is closed or filesystem is full)

This allows for a more robust program design, as well as scheduling other tasks after the stream has been handled:

[source,java]
----
response.setChunked(true);

WriteStreamSubscriber<Buffer> subscriber = response.toSubscriber();

subscriber.onError(throwable -> {
  if (!response.headWritten() && response.closed()) {
    response.setStatusCode(500).end("oops");
  } else {
    // log error
  }
});

subscriber.onWriteStreamError(throwable -> {
  // log error
});

subscriber.onComplete(() -> {
  // log end of transaction to audit system...
});

observable.subscribe(subscriber);
----

NOTE: If the `link:../../apidocs/io/vertx/rxjava/core/streams/WriteStream.html[WriteStream]` fails, the adapter unsubscribes from the `Observable`.

=== Handler support

The `link:../../apidocs/io/vertx/rx/java/RxHelper.html[RxHelper]` can create an `link:../../apidocs/io/vertx/rx/java/ObservableHandler.html[ObservableHandler]`: an `Observable` with a
`link:../../apidocs/io/vertx/rx/java/ObservableHandler.html#toHandler--[toHandler]` method returning an `Handler<T>` implementation:

[source,java]
----
ObservableHandler<Long> observable = RxHelper.observableHandler();
observable.subscribe(id -> {
  // Fired
});
vertx.setTimer(1000, observable.toHandler());
----

The _Rxified_ Vert.x API does not provide a specific API for handler.

=== Async result support

You can create an RxJava `Subscriber` from an existing Vert.x `Handler<AsyncResult<T>>` and subscribe
it to an `Observable` or a `Single`:

[source,java]
----
observable.subscribe(RxHelper.toSubscriber(handler1));

// Subscribe to a Single
single.subscribe(RxHelper.toSubscriber(handler2));
----

The Vert.x `Handler<AsyncResult<T>>` construct occuring as last parameter of an asynchronous method can
be mapped to an observable of a single element:

- when the callback is a success, the observer `onNext` method is called with the item
and the `onComplete` method is immediately invoked after
- when the callback is a failure, the observer `onError` method is called

The `link:../../apidocs/io/vertx/rx/java/RxHelper.html#observableFuture--[RxHelper.observableFuture]` method creates an `link:../../apidocs/io/vertx/rx/java/ObservableFuture.html[ObservableFuture]`:
an `Observable` with a `link:../../apidocs/io/vertx/rx/java/ObservableFuture.html#toHandler--[toHandler]` method returning a `Handler<AsyncResult<T>>`
implementation:

[source,java]
----
ObservableFuture<HttpServer> observable = RxHelper.observableFuture();
observable.subscribe(
    server -> {
      // Server is listening
    },
    failure -> {
      // Server could not start
    }
);
vertx.createHttpServer(new HttpServerOptions().
    setPort(1234).
    setHost("localhost")
).listen(observable.toHandler());
----

The `ObservableFuture<Server>` will get a single `HttpServer` object, if the `listen` operation fails,
the subscriber will be notified with the failure.

The `link:../../apidocs/io/vertx/rx/java/RxHelper.html#toHandler-rx.Observer-[RxHelper.toHandler]` method adapts an existing `Observer` into an handler:

[source,java]
----
Observer<HttpServer> observer = new Observer<HttpServer>() {
  @Override
  public void onNext(HttpServer o) {
  }
  @Override
  public void onError(Throwable e) {
  }
  @Override
  public void onCompleted() {
  }
};
Handler<AsyncResult<HttpServer>> handler = RxHelper.toFuture(observer);
----

It also works with just actions:

[source,java]
----
Action1<HttpServer> onNext = httpServer -> {};
Action1<Throwable> onError = httpServer -> {};
Action0 onComplete = () -> {};

Handler<AsyncResult<HttpServer>> handler1 = RxHelper.toFuture(onNext);
Handler<AsyncResult<HttpServer>> handler2 = RxHelper.toFuture(onNext, onError);
Handler<AsyncResult<HttpServer>> handler3 = RxHelper.toFuture(onNext, onError, onComplete);
----

The _Rxified_ Vert.x API duplicates each such method with the `rx` prefix that returns an RxJava `Single`:

[source,java]
----
Single<HttpServer> single = vertx
  .createHttpServer()
  .rxListen(1234, "localhost");

// Subscribe to bind the server
single.
    subscribe(
        server -> {
          // Server is listening
        },
        failure -> {
          // Server could not start
        }
    );
----

Such single arecold* singles, and the corresponding API method is called on subscribe.

NOTE: the `rx*` methods replace the `*Observable` of the previous _Rxified_ versions with a semantic
change to be more in line with RxJava.

=== Scheduler support

The reactive extension sometimes needs to schedule actions, for instance `Observable#timer` creates and returns
a timer that emit periodic events. By default, scheduled actions are managed by RxJava, it means that the
timer threads are not Vert.x threads and therefore not executing in a Vert.x event loop.

When an RxJava method deals with a scheduler, it accepts an overloaded method accepting an extra `rx.Scheduler`,
the `link:../../apidocs/io/vertx/rx/java/RxHelper.html#scheduler-io.vertx.core.Vertx-[RxHelper.scheduler]`  method will return a scheduler that can be used
in such places.

[source,java]
----
Scheduler scheduler = RxHelper.scheduler(vertx);
Observable<Long> timer = Observable.timer(100, 100, TimeUnit.MILLISECONDS, scheduler);
----

For blocking scheduled actions, a scheduler can be created with the `link:../../apidocs/io/vertx/rx/java/RxHelper.html#blockingScheduler-io.vertx.core.Vertx-[RxHelper.blockingScheduler]`
method:

[source,java]
----
Scheduler scheduler = RxHelper.blockingScheduler(vertx);
Observable<Integer> obs = blockingObservable.observeOn(scheduler);
----

RxJava can also be reconfigured to use the Vert.x scheduler, thanks to the scheduler hook created with
`link:../../apidocs/io/vertx/rx/java/RxHelper.html#schedulerHook-io.vertx.core.Vertx-[RxHelper.schedulerHook]`, the returned scheduler hook
uses a blocking scheduler for IO actions:

[source,java]
----
RxJavaSchedulersHook hook = RxHelper.schedulerHook(vertx);
RxJavaHooks.setOnIOScheduler(f -> hook.getIOScheduler());
RxJavaHooks.setOnNewThreadScheduler(f -> hook.getNewThreadScheduler());
RxJavaHooks.setOnComputationScheduler(f -> hook.getComputationScheduler());
----

The _Rxified_ Vert.x API provides also similar method on the `link:../../apidocs/io/vertx/rxjava/core/RxHelper.html[RxHelper]` class:

[source,java]
----
Scheduler scheduler = io.vertx.rxjava.core.RxHelper.scheduler(vertx);
Observable<Long> timer = Observable.interval(100, 100, TimeUnit.MILLISECONDS, scheduler);
----

[source,java]
----
RxJavaSchedulersHook hook = io.vertx.rxjava.core.RxHelper.schedulerHook(vertx);
  RxJavaHooks.setOnIOScheduler(f -> hook.getIOScheduler());
  RxJavaHooks.setOnNewThreadScheduler(f -> hook.getNewThreadScheduler());
  RxJavaHooks.setOnComputationScheduler(f -> hook.getComputationScheduler());
----

It is also possible to create a scheduler backed by a named worker pool. This can be useful if you want to re-use
the specific thread pool for scheduling blocking actions:

[source,java]
----
Scheduler scheduler = io.vertx.rxjava.core.RxHelper.scheduler(workerExecutor);
Observable<Long> timer = Observable.interval(100, 100, TimeUnit.MILLISECONDS, scheduler);
----

=== Json unmarshalling

The `link:../../apidocs/io/vertx/rxjava/core/RxHelper.html#unmarshaller-java.lang.Class-[RxHelper.unmarshaller]` creates an `rx.Observable.Operator` that
transforms an `Observable<Buffer>` in json format into an object observable:

[source,java]
----
fileSystem.open("/data.txt", new OpenOptions(), result -> {
  AsyncFile file = result.result();
  Observable<Buffer> observable = RxHelper.toObservable(file);
  observable.lift(RxHelper.unmarshaller(MyPojo.class)).subscribe(
      mypojo -> {
        // Process the object
      }
  );
});
----

The same can be done with the _Rxified_ helper:

[source,java]
----
fileSystem.open("/data.txt", new OpenOptions(), result -> {
  AsyncFile file = result.result();
  Observable<Buffer> observable = file.toObservable();
  observable.lift(io.vertx.rxjava.core.RxHelper.unmarshaller(MyPojo.class)).subscribe(
      mypojo -> {
        // Process the object
      }
  );
});
----

=== Deploying a Verticle

The Rxified API cannot deploy an existing Verticle instance, the helper `link:../../apidocs/io/vertx/rx/java/RxHelper.html#observableFuture--[RxHelper.observableFuture]` method
provides a solution to that.

The `link:../../apidocs/io/vertx/rxjava/core/RxHelper.html#deployVerticle-io.vertx.rxjava.core.Vertx-io.vertx.core.Verticle-[RxHelper.deployVerticle]` does it automatically
for you, it deploys a `Verticle` and returns an `Observable<String>` of the deployment ID.

[source,java]
----
Observable<String> deployment = RxHelper.deployVerticle(vertx, verticle);

deployment.subscribe(id -> {
  // Deployed
}, err -> {
  // Could not deploy
});
----

=== HttpClient GET on subscription

The `link:../../apidocs/io/vertx/rxjava/core/RxHelper.html#get-io.vertx.rxjava.core.http.HttpClient-int-java.lang.String-java.lang.String-[RxHelper.get]`
is a convenient helper method that performs an HTTP GET upon subscription:

[source,java]
----
Observable<HttpClientResponse> get = RxHelper.get(client, "http://the-server");

// Do the request
get.subscribe(resp -> {
  // Got response
}, err -> {
  // Something went wrong
});
----

WARNING: this API is different from the HttpClient that performs the GET request when the method is called and returns
a one shot `Observable`.

= Rxified API

The _Rxified_ API is a code generated version of the Vert.x API, just like the _JavaScript_ or _Groovy_
language. The API uses the `io.vertx.rxjava` prefix, for instance the `io.vertx.core.Vertx` class is
translated to the `link:../../apidocs/io/vertx/rxjava/core/Vertx.html[Vertx]` class.

=== Embedding Rxfified Vert.x

Just use the `link:../../apidocs/io/vertx/rxjava/core/Vertx.html#vertx--[Vertx.vertx]` methods:

[source,java]
----
Vertx vertx = io.vertx.rxjava.core.Vertx.vertx();
----

=== As a Verticle

Extend the `link:../../apidocs/io/vertx/rxjava/core/AbstractVerticle.html[AbstractVerticle]` class, it will wrap it for you:

[source,java]
----
class MyVerticle extends io.vertx.rxjava.core.AbstractVerticle {
  public void start() {
    // Use Rxified Vertx here
  }
}
----

Deploying an RxJava verticle is still performed by the Java deployer and does not need a specified
deployer.

Verticles having an asynchronous start can override instead the `rxStart` method and return a `Completable`:

[source,java]
----
class MyVerticle extends io.vertx.rxjava.core.AbstractVerticle {
  public Completable rxStart() {
    return vertx.createHttpServer()
      .requestHandler(req -> req.response().end("Hello World"))
      .rxListen()
      .toCompletable();
  }
}
----

== Api examples

Let's study now a few examples of using Vert.x with RxJava.

=== EventBus message stream

The event bus `link:../../apidocs/io/vertx/rxjava/core/eventbus/MessageConsumer.html[MessageConsumer]` provides naturally an `Observable<Message<T>>`:

[source,java]
----
EventBus eb = vertx.eventBus();
MessageConsumer<String> consumer = eb.<String>consumer("the-address");
Observable<Message<String>> observable = consumer.toObservable();
Subscription sub = observable.subscribe(msg -> {
  // Got message
});

// Unregisters the stream after 10 seconds
vertx.setTimer(10000, id -> {
  sub.unsubscribe();
});
----

The `link:../../apidocs/io/vertx/rxjava/core/eventbus/MessageConsumer.html[MessageConsumer]` provides a stream of `link:../../apidocs/io/vertx/rxjava/core/eventbus/Message.html[Message]`.
The `link:../../apidocs/io/vertx/rxjava/core/eventbus/Message.html#body--[body]` gives access to a new stream of message bodies if needed:

[source,java]
----
EventBus eb = vertx.eventBus();
MessageConsumer<String> consumer = eb.<String>consumer("the-address");
Observable<String> observable = consumer.bodyStream().toObservable();
----

RxJava map/reduce composition style can then be used:

[source,java]
----
Observable<Double> observable = vertx.eventBus().
    <Double>consumer("heat-sensor").
    bodyStream().
    toObservable();

observable.
    buffer(1, TimeUnit.SECONDS).
    map(samples -> samples.
        stream().
        collect(Collectors.averagingDouble(d -> d))).
    subscribe(heat -> {
      vertx.eventBus().send("news-feed", "Current heat is " + heat);
    });
----

=== Timers

Timer task can be created with `link:../../apidocs/io/vertx/rxjava/core/Vertx.html#timerStream-long-[timerStream]`:

[source,java]
----
vertx.timerStream(1000).
    toObservable().
    subscribe(
        id -> {
          System.out.println("Callback after 1 second");
        }
    );
----

Periodic task can be created with `link:../../apidocs/io/vertx/rxjava/core/Vertx.html#periodicStream-long-[periodicStream]`:

[source,java]
----
vertx.periodicStream(1000).
    toObservable().
    subscribe(
        id -> {
          System.out.println("Callback every second");
        }
    );
----

The observable can be cancelled with an unsubscription:

[source,java]
----
vertx.periodicStream(1000).
    toObservable().
    subscribe(new Subscriber<Long>() {
      public void onNext(Long aLong) {
        // Callback
        unsubscribe();
      }
      public void onError(Throwable e) {}
      public void onCompleted() {}
    });
----

=== Http client requests

`link:../../apidocs/io/vertx/rxjava/core/http/HttpClientRequest.html#toObservable--[toObservable]` provides a one shot callback with the
`link:../../apidocs/io/vertx/core/http/HttpClientResponse.html[HttpClientResponse]` object. The observable reports a request failure.

[source,java]
----
HttpClient client = vertx.createHttpClient(new HttpClientOptions());
HttpClientRequest request = client.request(HttpMethod.GET, 8080, "localhost", "/the_uri");
request.toObservable().subscribe(
    response -> {
      // Process the response
    },
    error -> {
      // Could not connect
    }
);
request.end();
----

The response can be processed as an `Observable<Buffer>` with the
`link:../../apidocs/io/vertx/rxjava/core/http/HttpClientResponse.html#toObservable--[toObservable]` method:

[source,java]
----
request.toObservable().
    subscribe(
        response -> {
          Observable<Buffer> observable = response.toObservable();
          observable.forEach(
              buffer -> {
                // Process buffer
              }
          );
        }
    );
----

The same flow can be achieved with the `flatMap` operation:

[source,java]
----
request.toObservable().
    flatMap(HttpClientResponse::toObservable).
    forEach(
        buffer -> {
          // Process buffer
        }
    );
----

We can also unmarshall the `Observable<Buffer>` into an object using the `link:../../apidocs/io/vertx/rxjava/core/RxHelper.html#unmarshaller-java.lang.Class-[RxHelper.unmarshaller]`
static method. This method creates an `Rx.Observable.Operator` unmarshalling buffers to an object:

[source,java]
----
request.toObservable().
    flatMap(HttpClientResponse::toObservable).
    lift(io.vertx.rxjava.core.RxHelper.unmarshaller(MyPojo.class)).
    forEach(
        pojo -> {
          // Process pojo
        }
    );
----

=== Http server requests

The `link:../../apidocs/io/vertx/rxjava/core/http/HttpServer.html#requestStream--[requestStream]` provides a callback for each incoming
request:

[source,java]
----
Observable<HttpServerRequest> requestObservable = server.requestStream().toObservable();
requestObservable.subscribe(request -> {
  // Process request
});
----

The `link:../../apidocs/io/vertx/core/http/HttpServerRequest.html[HttpServerRequest]` can then be adapted to an `Observable<Buffer>`:

[source,java]
----
Observable<HttpServerRequest> requestObservable = server.requestStream().toObservable();
requestObservable.subscribe(request -> {
  Observable<Buffer> observable = request.toObservable();
});
----

The `link:../../apidocs/io/vertx/rxjava/core/RxHelper.html#unmarshaller-java.lang.Class-[RxHelper.unmarshaller]` can be used to parse and map
a json request to an object:

[source,java]
----
Observable<HttpServerRequest> requestObservable = server.requestStream().toObservable();
requestObservable.subscribe(request -> {
  Observable<MyPojo> observable = request.
      toObservable().
      lift(io.vertx.rxjava.core.RxHelper.unmarshaller(MyPojo.class));
});
----

=== Websocket client

The `link:../../apidocs/io/vertx/rxjava/core/http/HttpClient.html#rxWebSocket-int-java.lang.String-java.lang.String-[rxWebSocket]` provides a single callback when the websocket
connects, otherwise a failure:

[source,java]
----
HttpClient client = vertx.createHttpClient(new HttpClientOptions());
client.rxWebSocket(8080, "localhost", "/the_uri").subscribe(
    ws -> {
      // Use the websocket
    },
    error -> {
      // Could not connect
    }
);
----

The `link:../../apidocs/io/vertx/rxjava/core/http/WebSocket.html[WebSocket]` can then be turned into an `Observable<Buffer>` easily:

[source,java]
----
socketObservable.subscribe(
    socket -> {
      Observable<Buffer> dataObs = socket.toObservable();
      dataObs.subscribe(buffer -> {
        System.out.println("Got message " + buffer.toString("UTF-8"));
      });
    }
);
----

=== Websocket server

The `link:../../apidocs/io/vertx/rxjava/core/http/HttpServer.html#websocketStream--[websocketStream]` provides a callback for each incoming
connection:

[source,java]
----
Observable<ServerWebSocket> socketObservable = server.websocketStream().toObservable();
socketObservable.subscribe(
    socket -> System.out.println("Web socket connect"),
    failure -> System.out.println("Should never be called"),
    () -> {
      System.out.println("Subscription ended or server closed");
    }
);
----

The `link:../../apidocs/io/vertx/core/http/ServerWebSocket.html[ServerWebSocket]` can be turned into an `Observable<Buffer>` easily:

[source,java]
----
socketObservable.subscribe(
    socket -> {
      Observable<Buffer> dataObs = socket.toObservable();
      dataObs.subscribe(buffer -> {
        System.out.println("Got message " + buffer.toString("UTF-8"));
      });
    }
);
----