Akka features for application integration

Akka is a platform for event-driven, scalable and fault-tolerant architectures on the JVM. It is mainly written in Scala. One of its core features is support for the actor model that provides a higher level of abstraction for writing concurrent and distributed systems.

Since version 0.7, Akka offers a new feature that let actors send and receive messages over a great variety of protocols and APIs. In addition to the native Scala actor API, actors can now exchange messages with other systems over large number of protcols and APIs such as HTTP, SOAP, TCP, FTP, SMTP or JMS, to mention a few. At the moment, approximately 80 protocols and APIs are supported. This new feature is provided by Akka's Camel module.

At the core of this new feature is Apache Camel, IMHO the most powerful and feature-rich integration framework currently available for the JVM. For an introduction to Apache Camel you may want to read this article. Camel comes with a large number of components that provide bindings to different protocols and APIs. Usage of Camel's integration components in Akka is essentially a one-liner. Here's an example.

import se.scalablesolutions.akka.actor.Actor
import se.scalablesolutions.akka.actor.Actor._
import se.scalablesolutions.akka.camel.{Message, Consumer}

class MyActor extends Actor with Consumer {
  def endpointUri =
    "mina:tcp://localhost:6200?textline=true"

  def receive = {
    case msg: Message => { /* ... */}
    case _            => { /* ... */}
  }
}
// start and expose actor via tcp
val myActor = actorOf[MyActor].start

The above example exposes an actor over a tcp endpoint on port 6200 via Apache Camel's Mina component. The endpointUri is an abstract method declared in the Consumer trait. 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 must be redefined.

class MyActor extends Actor with Consumer {
  def endpointUri =
    "jetty:http://localhost:8877/example"

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

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

import se.scalablesolutions.akka.actor.Actor
import se.scalablesolutions.akka.camel.Producer

class MyActor extends Actor with Producer {
  def endpointUri = "jms:queue:example"
  protected def receive = produce
}

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

The number of Camel components is constantly increasing. Akka's 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. Akka's 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, for example.

Highly-scalable eHealth integration solutions with Akka

One goal I had in mind when implementing the Akka Camel module was to have a basis for building highly-scalable eHealth integration solutions. For eHealth information systems it is becoming increasingly important to support standard interfaces as specified by IHE. Financial support from governments strongly depends on eHealth standard compliance.

Building blocks for implementing standard-compliant eHealth applications are provided by the Open eHealth Integration Platform (IPF). IPF is a mature open source integration platform, based on Apache Camel. It provides, among others, extensive support for IHE actor interfaces. These interfaces are based on Apache Camel's component technology. Therefore, it's a one-liner to expose an Akka actor through an IHE compliant interface. The following example implements the server-side interface of the IHE XDS Registry Stored Query (XDS-ITI18) transaction.

class RSQService extends Actor with Consumer {
  def endpointUri = "xds-iti18:RSQService"

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

In IHE, a message exchange between two participants is called a transaction. Here, the IPF xds-iti18 component is used to implement the server-side interface of the XDS ITI18 transaction. This allows any XDS-ITI18-comaptible client to communicate with the actor over an IHE standard protocol using ebXML/SOAP/HTTP (as defined in the XDS specification). The implementor of the receive method, however, doesn't need to care about all the low-level protocol details (which are scary if you take a closer look). The body of the received message is a high-level object graph containing XDS-ITI18-specific transaction data.

A high-level programming model for implementing eHealth standards is only one of several reasons why I consider Akka as a powerful technical basis for building scalable and fault-tolerant eHealth information systems and integration solutions. Akka's support for NoSQL datastores could further be used for implementing scalabale persistence layers in eHealth applications. Over the next weeks I'm going to explore this field in more detail and will keep you updated with further blog posts on that topic.