Leiningen

[org.clojure.gaverhae/okku "0.1.3"]

Okku is primarily developped and tested on Clojure 1.4. It has been occasionally tested on 1.2 and 1.3, and it seems to work fine. Do not hesitate to report any incompatibility.

Introduction

Okku is a Clojure wrapper for the Akka library. Akka is an erlang-inspired Scala library implementing the Actor model for concurrency and distribution.

For explanations on the Actor model itself and how and when to use it, see the documentation of either Akka or Erlang.

Usage

Okku strives to be as thin a wrapper as possible; for example, Okku functions yield and manipulate unwrapped Akka objects, and Okku tries to stay conceptually close to the Akka model. This means that users of Okku should be able to refer directly to the Akka documentation for information on how to use Okku. One only has to keep in mind that the actor macro yields a Props object while the spawn macro is basically a wrapper around .actorOf.

Example usage of Okku is given in the two tutorials: pi and remote. The tutorials are versioned in sync with the library; you should only use release (i.e. non-SNAPSHOT) versions of the tutorials.

Very brief introduction to the Akka actor hierarchy

The Akka actor systems enforces a hierarchical structure. This means that every actor is the child of another actor, and every actor knows its own children.

Of course, every actor needing a parent means we have a chicken-and-egg problem, which Akka solves by creating special actors for you, which do not have (user-accessible) parents, and which are called Actor Systems. Basically, an Akka application begins by creating an Actor System, and then telling this Actor System to spawn the required actors for the rest of the computation. Actors from this first generation of manually-created actors are typically thought of as the roots of the actor hierarchy within an application.

Creating an ActorSystem

The creation of an ActorSystem is done through the actor-system function, which in its most basic form simply takes a name as a parameter. See the documentation (docs folder) for details on the possible options.

Creating an actor with Okku

The first step in creating an actor is to define its behaviour. This is done through the actor macro, which yields an akka.actor.Props object (that could then be passed to .actorOf to create an actor from Akka). It is basically a wrapper around proxy. Okku als defines a few convenience macros to use frequently accessed actor functionalities, such as stop. See the marginalia-generated documentation in the docs folder for more details.

The second step is to use the spawn macro, which takes an "actor" (a Props object as yielded by the actor macro), and a few named arguments to create the actor. If no :in argument is passed, the new actor is spawned as a child of the "current" actor (which means that the :in argument is required if called from outside of an actor, though that can only be detected at runtime). spawn is also used to create an actor on a remote system.

With all that said, here is an example code to illustrate the basics, provided the Okku jar is in your classpath:

(use 'okku.core)
(let [as (actor-system "test")
      echo-actor (spawn (actor (onReceive [msg]
                                 (println msg)))
                        :in as)]
  (.tell echo-actor "Testing...")
  (.tell echo-actor ["more" {"complex" "object"}]))

One restriction of the Akka model is that messages between actors have to be immutable objects. This is the default for Clojure values, but it's still important to bear in mind.

Configuration through configuration file

Configuration through application.conf is supported by Akka, and thus by Okku. See the Akka documentation for details.

One note of interest: the Okku system adds the possibility of changing the look-up address for a remote actor through configuration, which is not directly supported by Akka (though it is not hard to do through accessing the configuration object, which is exactly what Okku does).

To avoid polluting the akka "namespace" in the configuration file, Okku adds an okku.lookup namespace for actor look-up. Supported configuration options are:

okku.lookup.<actor path> {
  protocol
  actor-system
  hostname
  port
  path
}

If the path does not begin with a "/", Okku will automaticall add "/user/" in front of it.

License

Copyright (C) 2012 Gary Verhaegen.

Distributed under the Eclipse Public License, the same as Clojure.