## The Event Bus

The `event bus` is the nervous system of Vert.x.

There is a single event bus instance for every Vert.x instance and it is obtained using the method `eventBus`.

The event bus allows different parts of your application to communicate with each other irrespective of what language they are written in, and whether they’re in the same Vert.x instance, or in a different Vert.x instance.

It can even be bridged to allow client side JavaScript running in a browser to communicate on the same event bus.

The event bus forms a distributed peer-to-peer messaging system spanning multiple server nodes and multiple browsers.

The event bus supports publish/subscribe, point to point, and request-response messaging.

The event bus API is very simple. It basically involves registering handlers, unregistering handlers and sending and publishing messages.

First some theory:

### The Theory

#### Addressing

Messages are sent on the event bus to an address.

Vert.x doesn’t bother with any fancy addressing schemes. In Vert.x an address is simply a string. Any string is valid. However it is wise to use some kind of scheme, e.g. using periods to demarcate a namespace.

Some examples of valid addresses are europe.news.feed1, acme.games.pacman, sausages, and X.

#### Handlers

Messages are received in handlers. You register a handler at an address.

Many different handlers can be registered at the same address.

A single handler can be registered at many different addresses.

#### Publish / subscribe messaging

The event bus supports publishing messages.

Messages are published to an address. Publishing means delivering the message to all handlers that are registered at that address.

This is the familiar publish/subscribe messaging pattern.

#### Point to point and Request-Response messaging

The event bus also supports point to point messaging.

Messages are sent to an address. Vert.x will then route it to just one of the handlers registered at that address.

If there is more than one handler registered at the address, one will be chosen using a non-strict round-robin algorithm.

With point to point messaging, an optional reply handler can be specified when sending the message.

When a message is received by a recipient, and has been handled, the recipient can optionally decide to reply to the message. If they do so the reply handler will be called.

When the reply is received back at the sender, it too can be replied to. This can be repeated ad-infinitum, and allows a dialog to be set-up between two different verticles.

This is a common messaging pattern called the request-response pattern.

#### Best-effort delivery

Vert.x does it’s best to deliver messages and won’t consciously throw them away. This is called best-effort delivery.

However, in case of failure of all or parts of the event bus, there is a possibility messages will be lost.

If your application cares about lost messages, you should code your handlers to be idempotent, and your senders to retry after recovery.

#### Types of messages

Out of the box Vert.x allows any primitive/simple type, String, or `buffers` to be sent as messages.

However it’s a convention and common practice in Vert.x to send messages as JSON

JSON is very easy to create, read and parse in all the languages that Vert.x supports so it has become a kind of lingua franca for Vert.x.

However you are not forced to use JSON if you don’t want to.

The event bus is very flexible and also supports sending arbitrary objects over the event bus. You do this by defining a `codec` for the objects you want to send.

### The Event Bus API

Let’s jump into the API

#### Getting the event bus

You get a reference to the event bus as follows:

    def eb = vertx.eventBus()

There is a single instance of the event bus per Vert.x instance.

#### Registering Handlers

This simplest way to register a handler is using `consumer`. Here’s an example:

    def eb = vertx.eventBus()

    eb.consumer("news.uk.sport", { message ->
      println("I have received a message: ${message.body()}")
    })

When a message arrives for your handler, your handler will be called, passing in the `message`.

The object returned from call to consumer() is an instance of `MessageConsumer`

This object can subsequently be used to unregister the handler, or use the handler as a stream.

Alternatively you can use `consumer` to to return a MessageConsumer with no handler set, and then set the handler on that. For example:

    def eb = vertx.eventBus()

    def consumer = eb.consumer("news.uk.sport")
    consumer.handler({ message ->
      println("I have received a message: ${message.body()}")
    })

When registering a handler on a clustered event bus, it can take some time for the registration to reach all nodes of the cluster.

If you want to be notified when this has completed, you can register a `completion handler` on the MessageConsumer object.

    consumer.completionHandler({ res ->
      if (res.succeeded()) {
        println("The handler registration has reached all nodes")
      } else {
        println("Registration failed!")
      }
    })

#### Un-registering Handlers

To unregister a handler, call `unregister`.

If you are on a clustered event bus, un-registering can take some time to propagate across the nodes, if you want to be notified when this is complete use `unregister`.

    consumer.unregister({ res ->
      if (res.succeeded()) {
        println("The handler un-registration has reached all nodes")
      } else {
        println("Un-registration failed!")
      }
    })

#### Publishing messages

Publishing a message is simple. Just use `publish` specifying the address to publish it to.

    eventBus.publish("news.uk.sport", "Yay! Someone kicked a ball")

That message will then be delivered to all handlers registered against the address news.uk.sport.

#### Sending messages

Sending a message will result in only one handler registered at the address receiving the message. This is the point to point messaging pattern. The handler is chosen in a non-strict round-robin fashion.

You can send a message with `send`

    eventBus.send("news.uk.sport", "Yay! Someone kicked a ball")

#### Setting headers on messages

Messages sent over the event bus can also contain headers.

This can be specified by setting options when sending or publishing:

    def options = [
      headers: [
       "some-header" : "some-value"
      ]
    ]
    vertx.eventBus().send("news.uk.sport", "Yay! Someone kicked a ball", options);

On the other side, a consumer can retrieve the headers as follows:

    vertx.eventBus().consumer("news.uk.sport",  { e ->
     println e.headers()["some-header"];
    });

#### The Message object

The object you receive in a message handler is a `Message`.

The `body` of the message corresponds to the object that was sent or published.

The headers of the message are available with `headers`.

#### Replying to messages

Sometimes after you send a message you want to receive a reply from the recipient. This is known as the request-response pattern.

To do this you can specify a reply handler when sending the message.

When the receiver receives the message they can reply to it by calling `reply`.

When this happens it causes a reply to be sent back to the sender and the reply handler is invoked with the reply.

An example will make this clear:

The receiver:

    def consumer = eventBus.consumer("news.uk.sport")
    consumer.handler({ message ->
      println("I have received a message: ${message.body()}")
      message.reply("how interesting!")
    })

The sender:

    eventBus.send("news.uk.sport", "Yay! Someone kicked a ball across a patch of grass", { ar ->
      if (ar.succeeded()) {
        println("Received reply: ${ar.result().body()}")
      }
    })

The replies themselves can also be replied to so you can create a dialog between two different parties consisting of multiple rounds.

#### Sending with timeouts

When sending a message with a reply handler you can specify a timeout in the `DeliveryOptions`.

If a reply is not received within that time, the reply handler will be called with a failure.

The default timeout is 30 seconds.

#### Send Failures

Message sends can fail for other reasons, including:

- There are no handlers available to send the message to

- The recipient has explicitly failed the message using `fail`

In all cases the reply handler will be called with the specific failure.

#### Message Codecs

Message codecs are available exclusively with the Java api.

#### Clustered Event Bus

The event bus doesn’t just exist in a single Vert.x instance. By clustering different Vert.x instances together on your network they can form a single, distributed, event bus.

#### Clustering programmatically

If you’re creating your Vert.x instance programmatically you get a clustered event bus by configuring the Vert.x instance as clustered;

    import io.vertx.groovy.core.Vertx
    def options = [:]
    Vertx.clusteredVertx(options, { res ->
      if (res.succeeded()) {
        def vertx = res.result()
        def eventBus = vertx.eventBus()
        println("We now have a clustered event bus: ${eventBus}")
      } else {
        println("Failed: ${res.cause()}")
      }
    })

You should also make sure you have a `ClusterManager` implementation on your classpath, for example the default `HazelcastClusterManager`.

#### Clustering on the command line

You can run Vert.x clustered on the command line with

    vertx run my-verticle.js -cluster

### Automatic clean-up in verticles

If you’re registering event bus handlers from inside verticles, those handlers will be automatically unregistered when the verticle is undeployed.