Akka/Scala example source code file (extending-akka.rst)

This example Akka source code file (extending-akka.rst) is included in my "Source Code Warehouse" project. The intent of this project is to help you more easily find Akka and Scala source code examples by using tags.

All credit for the original source code belongs to akka.io; I'm just trying to make examples easier to find. (For my Scala work, see my Scala examples and tutorials.)

Akka tags/keywords

a, actorsystem, akka, configuration, extension, extensionid, extensions, loading, the, you

The extending-akka.rst Akka example source code

.. _extending-akka-scala:

#########################
 Akka Extensions
#########################


If you want to add features to Akka, there is a very elegant, but powerful mechanism for doing so.
It's called Akka Extensions and is comprised of 2 basic components: an ``Extension`` and an ``ExtensionId``.

Extensions will only be loaded once per ``ActorSystem``, which will be managed by Akka.
You can choose to have your Extension loaded on-demand or at ``ActorSystem`` creation time through the Akka configuration.
Details on how to make that happens are below, in the "Loading from Configuration" section.

.. warning::

    Since an extension is a way to hook into Akka itself, the implementor of the extension needs to
    ensure the thread safety of his/her extension.

Building an Extension
=====================

So let's create a sample extension that just lets us count the number of times something has happened.

First, we define what our ``Extension`` should do:

.. includecode:: code/docs/extension/ExtensionDocSpec.scala
   :include: extension

Then we need to create an ``ExtensionId`` for our extension so we can grab ahold of it.

.. includecode:: code/docs/extension/ExtensionDocSpec.scala
   :include: extensionid

Wicked! Now all we need to do is to actually use it:

.. includecode:: code/docs/extension/ExtensionDocSpec.scala
   :include: extension-usage

Or from inside of an Akka Actor:

.. includecode:: code/docs/extension/ExtensionDocSpec.scala
   :include: extension-usage-actor

You can also hide extension behind traits:

.. includecode:: code/docs/extension/ExtensionDocSpec.scala
   :include: extension-usage-actor-trait

That's all there is to it!

Loading from Configuration
==========================

To be able to load extensions from your Akka configuration you must add FQCNs of implementations of either ``ExtensionId`` or ``ExtensionIdProvider``
in the ``akka.extensions`` section of the config you provide to your ``ActorSystem``.

.. includecode:: code/docs/extension/ExtensionDocSpec.scala
   :include: config

Applicability
=============

The sky is the limit!
By the way, did you know that Akka's ``Typed Actors``, ``Serialization`` and other features are implemented as Akka Extensions?

.. _extending-akka-scala.settings:

Application specific settings
-----------------------------

The :ref:`configuration` can be used for application specific settings. A good practice is to place those settings in an Extension.

Sample configuration:

.. includecode:: code/docs/extension/SettingsExtensionDocSpec.scala
   :include: config

The ``Extension``:

.. includecode:: code/docs/extension/SettingsExtensionDocSpec.scala
   :include: imports,extension,extensionid


Use it:

.. includecode:: code/docs/extension/SettingsExtensionDocSpec.scala
   :include: extension-usage-actor