Camel

Apache Camel

The akka-camel module is based on Apache Camel, a powerful and light-weight integration framework for the JVM. For an introduction to Apache Camel you may want to read this Apache Camel article. Camel comes with a large number of components that provide bindings to different protocols and APIs. The camel-extra project provides further components.

Consumer

Usage of Camel's integration components in Akka is essentially a one-liner. Here's an example.

import akka.camel.{ CamelMessage, Consumer }

class MyEndpoint extends Consumer {
  def endpointUri = "mina2:tcp://localhost:6200?textline=true"

  def receive = {
    case msg: CamelMessage => { /* ... */ }
    case _                 => { /* ... */ }
  }
}

// start and expose actor via tcp
import akka.actor.{ ActorSystem, Props }

val system = ActorSystem("some-system")
val mina = system.actorOf(Props[MyEndpoint])

The above example exposes an actor over a TCP endpoint via Apache Camel's Mina component. The actor implements the endpointUri method to define an endpoint from which it can receive messages. After starting the actor, TCP clients can immediately send messages to and receive responses from that actor. If the message exchange should go over HTTP (via Camel's Jetty component), only the actor's endpointUri method must be changed.

import akka.camel.{ CamelMessage, Consumer }

class MyEndpoint extends Consumer {
  def endpointUri = "jetty:http://localhost:8877/example"

  def receive = {
    case msg: CamelMessage => { /* ... */ }
    case _                 => { /* ... */ }
  }
}

Producer

Actors can also trigger message exchanges with external systems i.e. produce to Camel endpoints.

import akka.actor.Actor
import akka.camel.{ Producer, Oneway }
import akka.actor.{ ActorSystem, Props }

class Orders extends Actor with Producer with Oneway {
  def endpointUri = "jms:queue:Orders"
}

val sys = ActorSystem("some-system")
val orders = sys.actorOf(Props[Orders])

orders ! <order amount="100" currency="PLN" itemId="12345"/>

In the above example, any message sent to this actor will be sent to the JMS queue orders. Producer actors may choose from the same set of Camel components as Consumer actors do.

CamelMessage

The number of Camel components is constantly increasing. The akka-camel module can support these in a plug-and-play manner. Just add them to your application's classpath, define a component-specific endpoint URI and use it to exchange messages over the component-specific protocols or APIs. This is possible because Camel components bind protocol-specific message formats to a Camel-specific normalized message format. The normalized message format hides protocol-specific details from Akka and makes it therefore very easy to support a large number of protocols through a uniform Camel component interface. The akka-camel module further converts mutable Camel messages into immutable representations which are used by Consumer and Producer actors for pattern matching, transformation, serialization or storage. In the above example of the Orders Producer, the XML message is put in the body of a newly created Camel Message with an empty set of headers. You can also create a CamelMessage yourself with the appropriate body and headers as you see fit.

CamelExtension

The akka-camel module is implemented as an Akka Extension, the CamelExtension object. Extensions will only be loaded once per ActorSystem, which will be managed by Akka. The CamelExtension object provides access to the Camel trait. The Camel trait in turn provides access to two important Apache Camel objects, the CamelContext and the ProducerTemplate. Below you can see how you can get access to these Apache Camel objects.

val system = ActorSystem("some-system")
val camel = CamelExtension(system)
val camelContext = camel.context
val producerTemplate = camel.template

One CamelExtension is only loaded once for every one ActorSystem, which makes it safe to call the CamelExtension at any point in your code to get to the Apache Camel objects associated with it. There is one CamelContext and one ProducerTemplate for every one ActorSystem that uses a CamelExtension. By Default, a new CamelContext is created when the CamelExtension starts. If you want to inject your own context instead, you can extend the ContextProvider trait and add the FQCN of your implementation in the config, as the value of the "akka.camel.context-provider". This interface define a single method getContext used to load the CamelContext.

Below an example on how to add the ActiveMQ component to the CamelContext, which is required when you would like to use the ActiveMQ component.

// import org.apache.activemq.camel.component.ActiveMQComponent
val system = ActorSystem("some-system")
val camel = CamelExtension(system)
val camelContext = camel.context
// camelContext.addComponent("activemq", ActiveMQComponent.activeMQComponent(
//   "vm://localhost?broker.persistent=false"))

The CamelContext joins the lifecycle of the ActorSystem and CamelExtension it is associated with; the CamelContext is started when the CamelExtension is created, and it is shut down when the associated ActorSystem is shut down. The same is true for the ProducerTemplate.

The CamelExtension is used by both Producer and Consumer actors to interact with Apache Camel internally. You can access the CamelExtension inside a Producer or a Consumer using the camel definition, or get straight at the CamelContext using the camelContext definition. Actors are created and started asynchronously. When a Consumer actor is created, the Consumer is published at its Camel endpoint (more precisely, the route is added to the CamelContext from the Endpoint to the actor). When a Producer actor is created, a SendProcessor and Endpoint are created so that the Producer can send messages to it. Publication is done asynchronously; setting up an endpoint may still be in progress after you have requested the actor to be created. Some Camel components can take a while to startup, and in some cases you might want to know when the endpoints are activated and ready to be used. The Camel trait allows you to find out when the endpoint is activated or deactivated.

import akka.camel.{ CamelMessage, Consumer }
import scala.concurrent.duration._

class MyEndpoint extends Consumer {
  def endpointUri = "mina2:tcp://localhost:6200?textline=true"

  def receive = {
    case msg: CamelMessage => { /* ... */ }
    case _                 => { /* ... */ }
  }
}
val system = ActorSystem("some-system")
val camel = CamelExtension(system)
val actorRef = system.actorOf(Props[MyEndpoint])
// get a future reference to the activation of the endpoint of the Consumer Actor
val activationFuture = camel.activationFutureFor(actorRef)(
  timeout = 10 seconds,
  executor = system.dispatcher)

The above code shows that you can get a Future to the activation of the route from the endpoint to the actor, or you can wait in a blocking fashion on the activation of the route. An ActivationTimeoutException is thrown if the endpoint could not be activated within the specified timeout. Deactivation works in a similar fashion:

system.stop(actorRef)
// get a future reference to the deactivation of the endpoint of the Consumer Actor
val deactivationFuture = camel.deactivationFutureFor(actorRef)(
  timeout = 10 seconds,
  executor = system.dispatcher)

Deactivation of a Consumer or a Producer actor happens when the actor is terminated. For a Consumer, the route to the actor is stopped. For a Producer, the SendProcessor is stopped. A DeActivationTimeoutException is thrown if the associated camel objects could not be deactivated within the specified timeout.

Delivery acknowledgements

With in-out message exchanges, clients usually know that a message exchange is done when they receive a reply from a consumer actor. The reply message can be a CamelMessage (or any object which is then internally converted to a CamelMessage) on success, and a Failure message on failure.

With in-only message exchanges, by default, an exchange is done when a message is added to the consumer actor's mailbox. Any failure or exception that occurs during processing of that message by the consumer actor cannot be reported back to the endpoint in this case. To allow consumer actors to positively or negatively acknowledge the receipt of a message from an in-only message exchange, they need to override the autoAck method to return false. In this case, consumer actors must reply either with a special akka.camel.Ack message (positive acknowledgement) or a akka.actor.Status.Failure (negative acknowledgement).

import akka.camel.{ CamelMessage, Consumer }
import akka.camel.Ack
import akka.actor.Status.Failure

class Consumer3 extends Consumer {
  override def autoAck = false

  def endpointUri = "jms:queue:test"

  def receive = {
    case msg: CamelMessage =>
      sender() ! Ack
      // on success
      // ..
      val someException = new Exception("e1")
      // on failure
      sender() ! Failure(someException)
  }
}

Consumer timeout

Camel Exchanges (and their corresponding endpoints) that support two-way communications need to wait for a response from an actor before returning it to the initiating client. For some endpoint types, timeout values can be defined in an endpoint-specific way which is described in the documentation of the individual Camel components. Another option is to configure timeouts on the level of consumer actors.

Two-way communications between a Camel endpoint and an actor are initiated by sending the request message to the actor with the ask pattern and the actor replies to the endpoint when the response is ready. The ask request to the actor can timeout, which will result in the Exchange failing with a TimeoutException set on the failure of the Exchange. The timeout on the consumer actor can be overridden with the replyTimeout, as shown below.

import akka.camel.{ CamelMessage, Consumer }
import scala.concurrent.duration._

class Consumer4 extends Consumer {
  def endpointUri = "jetty:http://localhost:8877/camel/default"
  override def replyTimeout = 500 millis
  def receive = {
    case msg: CamelMessage => sender() ! ("Hello %s" format msg.bodyAs[String])
  }
}

Custom Processing

Instead of replying to the initial sender, producer actors can implement custom response processing by overriding the routeResponse method. In the following example, the response message is forwarded to a target actor instead of being replied to the original sender.

import akka.actor.{ Actor, ActorRef }
import akka.camel.{ Producer, CamelMessage }
import akka.actor.{ Props, ActorSystem }

class ResponseReceiver extends Actor {
  def receive = {
    case msg: CamelMessage =>
    // do something with the forwarded response
  }
}

class Forwarder(uri: String, target: ActorRef) extends Actor with Producer {
  def endpointUri = uri

  override def routeResponse(msg: Any) { target forward msg }
}
val system = ActorSystem("some-system")
val receiver = system.actorOf(Props[ResponseReceiver])
val forwardResponse = system.actorOf(
  Props(classOf[Forwarder], this, "http://localhost:8080/news/akka", receiver))
// the Forwarder sends out a request to the web page and forwards the response to
// the ResponseReceiver
forwardResponse ! "some request"

Before producing messages to endpoints, producer actors can pre-process them by overriding the Producer.transformOutgoingMessage method.

import akka.actor.Actor
import akka.camel.{ Producer, CamelMessage }

class Transformer(uri: String) extends Actor with Producer {
  def endpointUri = uri

  def upperCase(msg: CamelMessage) = msg.mapBody {
    body: String => body.toUpperCase
  }

  override def transformOutgoingMessage(msg: Any) = msg match {
    case msg: CamelMessage => upperCase(msg)
  }
}

Producer configuration options

The interaction of producer actors with Camel endpoints can be configured to be one-way or two-way (by initiating in-only or in-out message exchanges, respectively). By default, the producer initiates an in-out message exchange with the endpoint. For initiating an in-only exchange, producer actors have to override the oneway method to return true.

import akka.actor.{ Actor, Props, ActorSystem }
import akka.camel.Producer

class OnewaySender(uri: String) extends Actor with Producer {
  def endpointUri = uri
  override def oneway: Boolean = true
}

val system = ActorSystem("some-system")
val producer = system.actorOf(Props(classOf[OnewaySender], this, "activemq:FOO.BAR"))
producer ! "Some message"

Message correlation

To correlate request with response messages, applications can set the Message.MessageExchangeId message header.

import akka.camel.{ Producer, CamelMessage }
import akka.actor.Actor
import akka.actor.{ Props, ActorSystem }

class Producer2 extends Actor with Producer {
  def endpointUri = "activemq:FOO.BAR"
}
val system = ActorSystem("some-system")
val producer = system.actorOf(Props[Producer2])

producer ! CamelMessage("bar", Map(CamelMessage.MessageExchangeId -> "123"))

ProducerTemplate

The Producer trait is a very convenient way for actors to produce messages to Camel endpoints. Actors may also use a Camel ProducerTemplate for producing messages to endpoints.

import akka.actor.Actor
class MyActor extends Actor {
  def receive = {
    case msg =>
      val template = CamelExtension(context.system).template
      template.sendBody("direct:news", msg)
  }
}

For initiating a two-way message exchange, one of the ProducerTemplate.request* methods must be used.

import akka.actor.Actor
class MyActor extends Actor {
  def receive = {
    case msg =>
      val template = CamelExtension(context.system).template
      sender() ! template.requestBody("direct:news", msg)
  }
}

Akka Camel components

Akka actors can be accessed from Camel routes using the actor Camel component. This component can be used to access any Akka actor (not only consumer actors) from Camel routes, as described in the following sections.

Access to actors

To access actors from custom Camel routes, the actor Camel component should be used. It fully supports Camel's asynchronous routing engine.

This component accepts the following endpoint URI format:

• [<actor-path>]?<options>

where <actor-path> is the ActorPath to the actor. The <options> are name-value pairs separated by & (i.e. name1=value1&name2=value2&...).

akka://some-system/user/myconsumer?autoAck=false&replyTimeout=100+millis

import akka.actor.{ Props, ActorSystem, Actor, ActorRef }
import akka.camel.{ CamelMessage, CamelExtension }
import org.apache.camel.builder.RouteBuilder
import akka.camel._
class Responder extends Actor {
  def receive = {
    case msg: CamelMessage =>
      sender() ! (msg.mapBody {
        body: String => "received %s" format body
      })
  }
}

class CustomRouteBuilder(system: ActorSystem, responder: ActorRef)
  extends RouteBuilder {
  def configure {
    from("jetty:http://localhost:8877/camel/custom").to(responder)
  }
}
val system = ActorSystem("some-system")
val camel = CamelExtension(system)
val responder = system.actorOf(Props[Responder], name = "TestResponder")
camel.context.addRoutes(new CustomRouteBuilder(system, responder))

Intercepting route construction

The previous section, Akka Camel components, explained how to setup a route to an actor manually. It was the application's responsibility to define the route and add it to the current CamelContext. This section explains a more convenient way to define custom routes: akka-camel is still setting up the routes to consumer actors (and adds these routes to the current CamelContext) but applications can define extensions to these routes. Extensions can be defined with Camel's Java DSL or Scala DSL. For example, an extension could be a custom error handler that redelivers messages from an endpoint to an actor's bounded mailbox when the mailbox was full.

The following examples demonstrate how to extend a route to a consumer actor for handling exceptions thrown by that actor.

import akka.camel.Consumer

import org.apache.camel.builder.Builder
import org.apache.camel.model.RouteDefinition

class ErrorThrowingConsumer(override val endpointUri: String) extends Consumer {
  def receive = {
    case msg: CamelMessage => throw new Exception("error: %s" format msg.body)
  }
  override def onRouteDefinition = (rd) => rd.onException(classOf[Exception]).
    handled(true).transform(Builder.exceptionMessage).end

  final override def preRestart(reason: Throwable, message: Option[Any]) {
    sender() ! Failure(reason)
  }
}

The above ErrorThrowingConsumer sends the Failure back to the sender in preRestart because the Exception that is thrown in the actor would otherwise just crash the actor, by default the actor would be restarted, and the response would never reach the client of the Consumer.

The akka-camel module creates a RouteDefinition instance by calling from(endpointUri) on a Camel RouteBuilder (where endpointUri is the endpoint URI of the consumer actor) and passes that instance as argument to the route definition handler *). The route definition handler then extends the route and returns a ProcessorDefinition (in the above example, the ProcessorDefinition returned by the end method. See the org.apache.camel.model package for details). After executing the route definition handler, akka-camel finally calls a to(targetActorUri) on the returned ProcessorDefinition to complete the route to the consumer actor (where targetActorUri is the actor component URI as described in Access to actors). If the actor cannot be found, a ActorNotRegisteredException is thrown.

*) Before passing the RouteDefinition instance to the route definition handler, akka-camel may make some further modifications to it.