pekko/akka-docs/scala/routing.rst

.. _routing-scala:

Routing (Scala)
===============

.. sidebar:: Contents

   .. contents:: :local:

Akka-core includes some building blocks to build more complex message flow handlers, they are listed and explained below:

Router
------

A Router is an actor that routes incoming messages to outbound actors.
The router routes the messages sent to it to its underlying actors called 'routees'.

Akka comes with some defined routers out of the box, but as you will see in this chapter it
is really easy to create your own. The routers shipped with Akka are:

* ``akka.routing.RoundRobinRouter``
* ``akka.routing.RandomRouter``
* ``akka.routing.SmallestMailboxRouter``
* ``akka.routing.BroadcastRouter``
* ``akka.routing.ScatterGatherFirstCompletedRouter``

Routers In Action
^^^^^^^^^^^^^^^^^

This is an example of how to create a router that is defined in configuration:

.. includecode:: code/akka/docs/routing/RouterViaConfigExample.scala#config

.. includecode:: code/akka/docs/routing/RouterViaConfigExample.scala#configurableRouting

This is an example of how to programatically create a router and set the number of routees it should create:

.. includecode:: code/akka/docs/routing/RouterViaProgramExample.scala#programmaticRoutingNrOfInstances

You can also give the router already created routees as in:

.. includecode:: code/akka/docs/routing/RouterViaProgramExample.scala#programmaticRoutingRoutees

When you create a router programatically you define the number of routees *or* you pass already created routees to it.
If you send both parameters to the router *only* the latter will be used, i.e. ``nrOfInstances`` is disregarded.

*It is also worth pointing out that if you define the ``router`` in the
configuration file then this value will be used instead of any programmatically
sent parameters. The decision whether to create a router at all, on the other
hand, must be taken within the code, i.e. you cannot make something a router by
external configuration alone (see below for details).*

Once you have the router actor it is just to send messages to it as you would to any actor:

.. code-block:: scala

  router ! MyMsg

The router will apply its behavior to the message it receives and forward it to the routees.

How Routing is Designed within Akka
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Routers behave like single actors, but they should also not hinder scalability.
This apparent contradiction is solved by making routers be represented by a
special :class:`RoutedActorRef`, which dispatches incoming messages destined
for the routees without actually invoking the router actor’s behavior (and thus
avoiding its mailbox; the single router actor’s task is to manage all aspects
related to the lifecycle of the routees). This means that the code which decides
which route to take is invoked concurrently from all possible senders and hence
must be thread-safe, it cannot live the simple and happy life of code within an
actor.

There is one part in the above paragraph which warrants some more background
explanation: Why does a router need a “head” which is actual parent to all the
routees? The initial design tried to side-step this issue, but location
transparency as well as mandatory parental supervision required a redesign.
Each of the actors which the router spawns must have its unique identity, which
translates into a unique actor path. Since the router has only one given name
in its parent’s context, another level in the name space is needed, which
according to the addressing semantics implies the existence of an actor with
the router’s name. This is not only necessary for the internal messaging
involved in creating, restarting and terminating actors, it is also needed when
the pooled actors need to converse with other actors and receive replies in a
deterministic fashion. Since each actor knows its own external representation
as well as that of its parent, the routees decide where replies should be sent
when reacting to a message:

.. includecode:: code/akka/docs/actor/ActorDocSpec.scala#reply-with-sender

.. includecode:: code/akka/docs/actor/ActorDocSpec.scala#reply-without-sender

It is apparent now why routing needs to be enabled in code rather than being
possible to “bolt on” later: whether or not an actor is routed means a change
to the actor hierarchy, changing the actor paths of all children of the router.
The routees especially do need to know that they are routed to in order to
choose the sender reference for any messages they dispatch as shown above.

Router usage
^^^^^^^^^^^^

In this section we will describe how to use the different router types.
First we need to create some actors that will be used in the examples:

.. includecode:: code/akka/docs/routing/RouterTypeExample.scala#printlnActor

and

.. includecode:: code/akka/docs/routing/RouterTypeExample.scala#fibonacciActor


RoundRobinRouter
****************
Routes in a `round-robin <http://en.wikipedia.org/wiki/Round-robin>`_ fashion to its routees.
Code example:

.. includecode:: code/akka/docs/routing/RouterTypeExample.scala#roundRobinRouter

When run you should see a similar output to this:

.. code-block:: scala

  Received message '1' in actor $b
  Received message '2' in actor $c
  Received message '3' in actor $d
  Received message '6' in actor $b
  Received message '4' in actor $e
  Received message '8' in actor $d
  Received message '5' in actor $f
  Received message '9' in actor $e
  Received message '10' in actor $f
  Received message '7' in actor $c

If you look closely to the output you can see that each of the routees received two messages which
is exactly what you would expect from a round-robin router to happen.
(The name of an actor is automatically created in the format ``$letter`` unless you specify it -
hence the names printed above.)

RandomRouter
************
As the name implies this router type selects one of its routees randomly and forwards
the message it receives to this routee.
This procedure will happen each time it receives a message.
Code example:

.. includecode:: code/akka/docs/routing/RouterTypeExample.scala#randomRouter

When run you should see a similar output to this:

.. code-block:: scala

  Received message '1' in actor $e
  Received message '2' in actor $c
  Received message '4' in actor $b
  Received message '5' in actor $d
  Received message '3' in actor $e
  Received message '6' in actor $c
  Received message '7' in actor $d
  Received message '8' in actor $e
  Received message '9' in actor $d
  Received message '10' in actor $d

The result from running the random router should be different, or at least random, every time you run it.
Try to run it a couple of times to verify its behavior if you don't trust us.

SmallestMailboxRouter
*********************
A Router that tries to send to the non-suspended routee with fewest messages in mailbox.
The selection is done in this order:

 * pick any idle routee (not processing message) with empty mailbox
 * pick any routee with empty mailbox
 * pick routee with fewest pending messages in mailbox
 * pick any remote routee, remote actors are consider lowest priority,
   since their mailbox size is unknown

Code example:

.. includecode:: code/akka/docs/routing/RouterTypeExample.scala#smallestMailboxRouter

BroadcastRouter
***************
A broadcast router forwards the message it receives to *all* its routees.
Code example:

.. includecode:: code/akka/docs/routing/RouterTypeExample.scala#broadcastRouter

When run you should see a similar output to this:

.. code-block:: scala

  Received message 'this is a broadcast message' in actor $f
  Received message 'this is a broadcast message' in actor $d
  Received message 'this is a broadcast message' in actor $e
  Received message 'this is a broadcast message' in actor $c
  Received message 'this is a broadcast message' in actor $b

As you can see here above each of the routees, five in total, received the broadcast message.

ScatterGatherFirstCompletedRouter
*********************************
The ScatterGatherFirstCompletedRouter will send the message on to all its routees as a future.
It then waits for first result it gets back. This result will be sent back to original sender.
Code example:

.. includecode:: code/akka/docs/routing/RouterTypeExample.scala#scatterGatherFirstCompletedRouter

When run you should see this:

.. code-block:: scala

  The result of calculating Fibonacci for 10 is 55

From the output above you can't really see that all the routees performed the calculation, but they did!
The result you see is from the first routee that returned its calculation to the router.

Broadcast Messages
^^^^^^^^^^^^^^^^^^

There is a special type of message that will be sent to all routees regardless of the router.
This message is called ``Broadcast`` and is used in the following manner:

.. code-block:: scala

  router ! Broadcast("Watch out for Davy Jones' locker")

Only the actual message is forwarded to the routees, i.e. "Watch out for Davy Jones' locker" in the example above.
It is up to the routee implementation whether to handle the broadcast message or not.

Dynamically Resizable Routers
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

All routers can be used with a fixed number of routees or with a resize strategy to adjust the number
of routees dynamically.

This is an example of how to create a resizable router that is defined in configuration:

.. includecode:: code/akka/docs/routing/RouterViaConfigExample.scala#config-resize

.. includecode:: code/akka/docs/routing/RouterViaConfigExample.scala#configurableRoutingWithResizer

Several more configuration options are availble and described in ``akka.actor.deployment.default.resizer``
section of the reference :ref:`configuration`.

This is an example of how to programatically create a resizable router:

.. includecode:: code/akka/docs/routing/RouterViaProgramExample.scala#programmaticRoutingWithResizer

*It is also worth pointing out that if you define the ``router`` in the configuration file then this value
will be used instead of any programmatically sent parameters.*

Custom Router
^^^^^^^^^^^^^

You can also create your own router should you not find any of the ones provided by Akka sufficient for your needs.
In order to roll your own router you have to fulfill certain criteria which are explained in this section.

The router created in this example is a simple vote counter. It will route the votes to specific vote counter actors.
In this case we only have two parties the Republicans and the Democrats. We would like a router that forwards all
democrat related messages to the Democrat actor and all republican related messages to the Republican actor.

We begin with defining the class:

.. includecode:: ../../akka-actor-tests/src/test/scala/akka/routing/RoutingSpec.scala#crRouter
   :exclude: crRoute

The next step is to implement the ``createRoute`` method in the class just defined:

.. includecode:: ../../akka-actor-tests/src/test/scala/akka/routing/RoutingSpec.scala#crRoute

As you can see above we start off by creating the routees and put them in a collection.

Make sure that you don't miss to implement the line below as it is *really* important.
It registers the routees internally and failing to call this method will
cause a ``ActorInitializationException`` to be thrown when the router is used.
Therefore always make sure to do the following in your custom router:

.. includecode:: ../../akka-actor-tests/src/test/scala/akka/routing/RoutingSpec.scala#crRegisterRoutees

The routing logic is where your magic sauce is applied. In our example it inspects the message types
and forwards to the correct routee based on this:

.. includecode:: ../../akka-actor-tests/src/test/scala/akka/routing/RoutingSpec.scala#crRoutingLogic

As you can see above what's returned in the partial function is a ``List`` of ``Destination(sender, routee)``.
The sender is what "parent" the routee should see - changing this could be useful if you for example want
another actor than the original sender to intermediate the result of the routee (if there is a result).
For more information about how to alter the original sender we refer to the source code of
`ScatterGatherFirstCompletedRouter <https://github.com/jboner/akka/blob/master/akka-actor/src/main/scala/akka/routing/Routing.scala#L375>`_

All in all the custom router looks like this:

.. includecode:: ../../akka-actor-tests/src/test/scala/akka/routing/RoutingSpec.scala#CustomRouter

If you are interested in how to use the VoteCountRouter you can have a look at the test class
`RoutingSpec <https://github.com/jboner/akka/blob/master/akka-actor-tests/src/test/scala/akka/routing/RoutingSpec.scala>`_

Configured Custom Router
************************

It is possible to define configuration properties for custom routers. In the ``router`` property of the deployment
configuration you define the fully qualified class name of the router class. The router class must extend
``akka.routing.RouterConfig`` and and have constructor with ``com.typesafe.config.Config`` parameter.
The deployment section of the configuration is passed to the constructor.

Custom Resizer
**************

A router with dynamically resizable number of routees is implemented by providing a ``akka.routing.Resizer``
in ``resizer`` method of the ``RouterConfig``. See ``akka.routing.DefaultResizer`` for inspiration
of how to write your own resize strategy.