Messages can be sent via a router to efficiently route them to destination actors, known as
its routees. A Router can be used inside or outside of an actor, and you can manage the
routees yourselves or use a self contained router actor with configuration capabilities.
Different routing strategies can be used, according to your application's needs. Akka comes with several useful routing strategies right out of the box. But, as you will see in this chapter, it is also possible to :ref:`create your own <custom-router-java>`.
The following example illustrates how to use a Router and manage the routees from within an actor.
.. includecode:: code/docs/jrouting/RouterDocTest.java#router-in-actor
We create a Router and specify that it should use RoundRobinRoutingLogic when routing the
messages to the routees.
The routing logic shipped with Akka are:
akka.routing.RoundRobinRoutingLogicakka.routing.RandomRoutingLogicakka.routing.SmallestMailboxRoutingLogicakka.routing.BroadcastRoutingLogicakka.routing.ScatterGatherFirstCompletedRoutingLogicakka.routing.TailChoppingRoutingLogicakka.routing.ConsistentHashingRoutingLogic
We create the routees as ordinary child actors wrapped in ActorRefRoutee. We watch
the routees to be able to replace them if they are terminated.
Sending messages via the router is done with the route method, as is done for the Work messages
in the example above.
The Router is immutable and the RoutingLogic is thread safe; meaning that they can also be used
outside of actors.
Note
In general, any message sent to a router will be sent onwards to its routees, but there is one exception. The special :ref:`broadcast-messages-java` will send to all of a router's routees
A router can also be created as a self contained actor that manages the routees itself and loads routing logic and other settings from configuration.
This type of router actor comes in two distinct flavors:
- Pool - The router creates routees as child actors and removes them from the router if they terminate.
- Group - The routee actors are created externally to the router and the router sends messages to the specified path using actor selection, without watching for termination.
The settings for a router actor can be defined in configuration or programmatically. Although router actors can be defined in the configuration file, they must still be created programmatically, i.e. you cannot make a router through external configuration alone. If you define the router actor in the configuration file then these settings will be used instead of any programmatically provided parameters.
You send messages to the routees via the router actor in the same way as for ordinary actors,
i.e. via its ActorRef. The router actor forwards messages onto its routees without changing
the original sender. When a routee replies to a routed message, the reply will be sent to the
original sender, not to the router actor.
Note
In general, any message sent to a router will be sent onwards to its routees, but there are a few exceptions. These are documented in the :ref:`router-special-messages-java` section below.
The following code and configuration snippets show how to create a :ref:`round-robin
<round-robin-router-java>` router that forwards messages to five Worker routees. The
routees will be created as the router's children.
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-round-robin-pool
.. includecode:: code/docs/jrouting/RouterDocTest.java#round-robin-pool-1
Here is the same example, but with the router configuration provided programmatically instead of from configuration.
.. includecode:: code/docs/jrouting/RouterDocTest.java#round-robin-pool-2
In addition to being able to create local actors as routees, you can instruct the router to
deploy its created children on a set of remote hosts. Routees will be deployed in round-robin
fashion. In order to deploy routees remotely, wrap the router configuration in a
RemoteRouterConfig, attaching the remote addresses of the nodes to deploy to. Remote
deployment requires the akka-remote module to be included in the classpath.
.. includecode:: code/docs/jrouting/RouterDocTest.java#remoteRoutees
When a routee sends a message, it can :ref:`set itself as the sender <actors-tell-sender-java>`.
.. includecode:: code/docs/jrouting/RouterDocTest.java#reply-with-self
However, it is often useful for routees to set the router as a sender. For example, you might want to set the router as the sender if you want to hide the details of the routees behind the router. The following code snippet shows how to set the parent router as sender.
.. includecode:: code/docs/jrouting/RouterDocTest.java#reply-with-parent
Routees that are created by a pool router will be created as the router's children. The router is therefore also the children's supervisor.
The supervision strategy of the router actor can be configured with the
supervisorStrategy property of the Pool. If no configuration is provided, routers default
to a strategy of “always escalate”. This means that errors are passed up to the router's supervisor
for handling. The router's supervisor will decide what to do about any errors.
Note the router's supervisor will treat the error as an error with the router itself. Therefore a directive to stop or restart will cause the router itself to stop or restart. The router, in turn, will cause its children to stop and restart.
It should be mentioned that the router's restart behavior has been overridden so that a restart, while still re-creating the children, will still preserve the same number of actors in the pool.
This means that if you have not specified :meth:`supervisorStrategy` of the router or its parent a failure in a routee will escalate to the parent of the router, which will by default restart the router, which will restart all routees (it uses Escalate and does not stop routees during restart). The reason is to make the default behave such that adding :meth:`.withRouter` to a child’s definition does not change the supervision strategy applied to the child. This might be an inefficiency that you can avoid by specifying the strategy when defining the router.
Setting the strategy is easily done:
.. includecode:: code/docs/jrouting/RouterDocTest.java#supervision
Note
If the child of a pool router terminates, the pool router will not automatically spawn a new child. In the event that all children of a pool router have terminated the router will terminate itself unless it is a dynamic router, e.g. using a resizer.
Sometimes, rather than having the router actor create its routees, it is desirable to create routees
separately and provide them to the router for its use. You can do this by passing an
paths of the routees to the router's configuration. Messages will be sent with ActorSelection
to these paths.
The example below shows how to create a router by providing it with the path strings of three routee actors.
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-round-robin-group
.. includecode:: code/docs/jrouting/RouterDocTest.java#round-robin-group-1
Here is the same example, but with the router configuration provided programmatically instead of from configuration.
.. includecode:: code/docs/jrouting/RouterDocTest.java#round-robin-group-2
The routee actors are created externally from the router:
.. includecode:: code/docs/jrouting/RouterDocTest.java#create-workers
.. includecode:: code/docs/jrouting/RouterDocTest.java#create-worker-actors
The paths may contain protocol and address information for actors running on remote hosts.
Remoting requires the akka-remote module to be included in the classpath.
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-remote-round-robin-group
In this section we will describe how to create the different types of router actors.
The router actors in this section are created from within a top level actor named parent.
Note that deployment paths in the configuration starts with /parent/ followed by the name
of the router actor.
.. includecode:: code/docs/jrouting/RouterDocTest.java#create-parent
Routes in a round-robin fashion to its routees.
RoundRobinPool defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-round-robin-pool
.. includecode:: code/docs/jrouting/RouterDocTest.java#round-robin-pool-1
RoundRobinPool defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java#round-robin-pool-2
RoundRobinGroup defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-round-robin-group
.. includecode:: code/docs/jrouting/RouterDocTest.java#round-robin-group-1
RoundRobinGroup defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java :include: paths,round-robin-group-2
This router type selects one of its routees randomly for each message.
RandomPool defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-random-pool
.. includecode:: code/docs/jrouting/RouterDocTest.java#random-pool-1
RandomPool defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java#random-pool-2
RandomGroup defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-random-group
.. includecode:: code/docs/jrouting/RouterDocTest.java#random-group-1
RandomGroup defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java :include: paths,random-group-2
A Router that will try to redistribute work from busy routees to idle routees. All routees share the same mailbox.
BalancingPool defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-balancing-pool
.. includecode:: code/docs/jrouting/RouterDocTest.java#balancing-pool-1
BalancingPool defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java#balancing-pool-2
Addition configuration for the balancing dispatcher, which is used by the pool,
can be configured in the pool-dispatcher section of the router deployment
configuration.
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-balancing-pool2
The BalancingPool automatically uses a special BalancingDispatcher for its
routees - disregarding any dispatcher that is set on the the routee Props object.
This is needed in order to implement the balancing semantics via
sharing the same mailbox by all the routees.
While it is not possible to change the dispatcher used by the routees, it is possible
to fine tune the used executor. By default the fork-join-dispatcher is used and
can be configured as explained in :ref:`dispatchers-java`. In situations where the
routees are expected to perform blocking operations it may be useful to replace it
with a thread-pool-executor hinting the number of allocated threads explicitly:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-balancing-pool3
There is no Group variant of the BalancingPool.
A Router that tries to send to the non-suspended child 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
SmallestMailboxPool defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-smallest-mailbox-pool
.. includecode:: code/docs/jrouting/RouterDocTest.java#smallest-mailbox-pool-1
SmallestMailboxPool defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java#smallest-mailbox-pool-2
There is no Group variant of the SmallestMailboxPool because the size of the mailbox and the internal dispatching state of the actor is not practically available from the paths of the routees.
A broadcast router forwards the message it receives to all its routees.
BroadcastPool defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-broadcast-pool
.. includecode:: code/docs/jrouting/RouterDocTest.java#broadcast-pool-1
BroadcastPool defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java#broadcast-pool-2
BroadcastGroup defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-broadcast-group
.. includecode:: code/docs/jrouting/RouterDocTest.java#broadcast-group-1
BroadcastGroup defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java :include: paths,broadcast-group-2
Note
Broadcast routers always broadcast every message to their routees. If you do not want to broadcast every message, then you can use a non-broadcasting router and use :ref:`broadcast-messages-java` as needed.
The ScatterGatherFirstCompletedRouter will send the message on to all its routees. It then waits for first reply it gets back. This result will be sent back to original sender. Other replies are discarded.
It is expecting at least one reply within a configured duration, otherwise it will reply with
akka.pattern.AskTimeoutException in a akka.actor.Status.Failure.
ScatterGatherFirstCompletedPool defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-scatter-gather-pool
.. includecode:: code/docs/jrouting/RouterDocTest.java#scatter-gather-pool-1
ScatterGatherFirstCompletedPool defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java#scatter-gather-pool-2
ScatterGatherFirstCompletedGroup defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-scatter-gather-group
.. includecode:: code/docs/jrouting/RouterDocTest.java#scatter-gather-group-1
ScatterGatherFirstCompletedGroup defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java :include: paths,scatter-gather-group-2
The TailChoppingRouter will first send the message to one, randomly picked, routee and then after a small delay to to a second routee (picked randomly from the remaining routees) and so on. It waits for first reply it gets back and forwards it back to original sender. Other replies are discarded.
The goal of this router is to decrease latency by performing redundant queries to multiple routees, assuming that one of the other actors may still be faster to respond than the initial one.
This optimisation was described nicely in a blog post by Peter Bailis: Doing redundant work to speed up distributed queries.
TailChoppingPool defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-tail-chopping-pool
.. includecode:: code/docs/jrouting/RouterDocTest.java#tail-chopping-pool-1
TailChoppingPool defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java#tail-chopping-pool-2
TailChoppingGroup defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-tail-chopping-group
.. includecode:: code/docs/jrouting/RouterDocTest.java#tail-chopping-group-1
TailChoppingGroup defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java :include: paths,tail-chopping-group-2
The ConsistentHashingPool uses consistent hashing to select a routee based on the sent message. This article gives good insight into how consistent hashing is implemented.
There is 3 ways to define what data to use for the consistent hash key.
- You can define
withHashMapperof the router to map incoming messages to their consistent hash key. This makes the decision transparent for the sender. - The messages may implement
akka.routing.ConsistentHashingRouter.ConsistentHashable. The key is part of the message and it's convenient to define it together with the message definition. - The messages can be be wrapped in a
akka.routing.ConsistentHashingRouter.ConsistentHashableEnvelopeto define what data to use for the consistent hash key. The sender knows the key to use.
These ways to define the consistent hash key can be use together and at
the same time for one router. The withHashMapper is tried first.
Code example:
.. includecode:: code/docs/jrouting/ConsistentHashingRouterDocTest.java#cache-actor
.. includecode:: code/docs/jrouting/ConsistentHashingRouterDocTest.java#consistent-hashing-router
In the above example you see that the Get message implements ConsistentHashable itself,
while the Entry message is wrapped in a ConsistentHashableEnvelope. The Evict
message is handled by the hashMapping partial function.
ConsistentHashingPool defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-consistent-hashing-pool
.. includecode:: code/docs/jrouting/RouterDocTest.java#consistent-hashing-pool-1
ConsistentHashingPool defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java#consistent-hashing-pool-2
ConsistentHashingGroup defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-consistent-hashing-group
.. includecode:: code/docs/jrouting/RouterDocTest.java#consistent-hashing-group-1
ConsistentHashingGroup defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java :include: paths,consistent-hashing-group-2
virtual-nodes-factor is the number of virtual nodes per routee that is used in the
consistent hash node ring to make the distribution more uniform.
Most messages sent to router actors will be forwarded according to the routers' routing logic. However there are a few types of messages that have special behavior.
Note that these special messages, except for the Broadcast message, are only handled by
self contained router actors and not by the akka.routing.Router component described
in :ref:`simple-router-java`.
A Broadcast message can be used to send a message to all of a router's routees. When a router
receives a Broadcast message, it will broadcast that message's payload to all routees, no
matter how that router would normally route its messages.
The example below shows how you would use a Broadcast message to send a very important message
to every routee of a router.
.. includecode:: code/docs/jrouting/RouterDocTest.java#broadcastDavyJonesWarning
In this example the router receives the Broadcast message, extracts its payload
("Watch out for Davy Jones' locker"), and then sends the payload on to all of the router's
routees. It is up to each each routee actor to handle the received payload message.
A PoisonPill message has special handling for all actors, including for routers. When any actor
receives a PoisonPill message, that actor will be stopped. See the :ref:`poison-pill-java`
documentation for details.
.. includecode:: code/docs/jrouting/RouterDocTest.java#poisonPill
For a router, which normally passes on messages to routees, it is important to realise that
PoisonPill messages are processed by the router only. PoisonPill messages sent to a router
will not be sent on to routees.
However, a PoisonPill message sent to a router may still affect its routees, because it will
stop the router and when the router stops it also stops its children. Stopping children is normal
actor behavior. The router will stop routees that it has created as children. Each child will
process its current message and then stop. This may lead to some messages being unprocessed.
See the documentation on :ref:`stopping-actors-java` for more information.
If you wish to stop a router and its routees, but you would like the routees to first process all
the messages currently in their mailboxes, then you should not send a PoisonPill message to the
router. Instead you should wrap a PoisonPill message inside a Broadcast message so that each
routee will receive the PoisonPill message. Note that this will stop all routees, even if the
routees aren't children of the router, i.e. even routees programmatically provided to the router.
.. includecode:: code/docs/jrouting/RouterDocTest.java#broadcastPoisonPill
With the code shown above, each routee will receive a PoisonPill message. Each routee will
continue to process its messages as normal, eventually processing the PoisonPill. This will
cause the routee to stop. After all routees have stopped the router will itself be :ref:`stopped
automatically <note-router-terminated-children-java>` unless it is a dynamic router, e.g. using
a resizer.
Note
Brendan W McAdams' excellent blog post Distributing Akka Workloads - And Shutting Down Afterwards
discusses in more detail how PoisonPill messages can be used to shut down routers and routees.
Kill messages are another type of message that has special handling. See
:ref:`killing-actors-java` for general information about how actors handle Kill messages.
When a Kill message is sent to a router the router processes the message internally, and does
not send it on to its routees. The router will throw an ActorKilledException and fail. It
will then be either resumed, restarted or terminated, depending how it is supervised.
Routees that are children of the router will also be suspended, and will be affected by the supervision directive that is applied to the router. Routees that are not the routers children, i.e. those that were created externally to the router, will not be affected.
.. includecode:: code/docs/jrouting/RouterDocTest.java#kill
As with the PoisonPill message, there is a distinction between killing a router, which
indirectly kills its children (who happen to be routees), and killing routees directly (some of whom
may not be children.) To kill routees directly the router should be sent a Kill message wrapped
in a Broadcast message.
.. includecode:: code/docs/jrouting/RouterDocTest.java#broadcastKill
- Sending
akka.routing.GetRouteesto a router actor will make it send back its currently used routees in aakka.routing.Routeesmessage. - Sending
akka.routing.AddRouteeto a router actor will add that routee to its collection of routees. - Sending
akka.routing.RemoveRouteeto a router actor will remove that routee to its collection of routees. - Sending
akka.routing.AdjustPoolSizeto a pool router actor will add or remove that number of routees to its collection of routees.
These management messages may be handled after other messages, so if you send AddRoutee immediately followed
an ordinary message you are not guaranteed that the routees have been changed when the ordinary message
is routed. If you need to know when the change has been applied you can send AddRoutee followed by GetRoutees
and when you receive the Routees reply you know that the preceding change has been applied.
All pools can be used with a fixed number of routees or with a resize strategy to adjust the number of routees dynamically.
Pool with resizer defined in configuration:
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-resize-pool
.. includecode:: code/docs/jrouting/RouterDocTest.java#resize-pool-1
Several more configuration options are available and described in akka.actor.deployment.default.resizer
section of the reference :ref:`configuration`.
Pool with resizer defined in code:
.. includecode:: code/docs/jrouting/RouterDocTest.java#resize-pool-2
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.
Note
Resizing is triggered by sending messages to the actor pool, but it is not
completed synchronously; instead a message is sent to the “head”
RouterActor to perform the size change. Thus you cannot rely on resizing
to instantaneously create new workers when all others are busy, because the
message just sent will be queued to the mailbox of a busy actor. To remedy
this, configure the pool to use a balancing dispatcher, see Configuring
Dispatchers for more information.
On the surface routers look like normal actors, but they are actually implemented differently. Routers are designed to be extremely efficient at receiving messages and passing them quickly on to routees.
A normal actor can be used for routing messages, but an actor's single-threaded processing can
become a bottleneck. Routers can achieve much higher throughput with an optimization to the usual
message-processing pipeline that allows concurrent routing. This is achieved by embedding routers'
routing logic directly in their ActorRef rather than in the router actor. Messages sent to
a router's ActorRef can be immediately routed to the routee, bypassing the single-threaded
router actor entirely.
The cost to this is, of course, that the internals of routing code are more complicated than if routers were implemented with normal actors. Fortunately all of this complexity is invisible to consumers of the routing API. However, it is something to be aware of when implementing your own routers.
You can 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.
Before creating your own router you should consider whether a normal actor with router-like behavior might do the job just as well as a full-blown router. As explained :ref:`above <router-design-java>`, the primary benefit of routers over normal actors is their higher performance. But they are somewhat more complicated to write than normal actors. Therefore if lower maximum throughput is acceptable in your application you may wish to stick with traditional actors. This section, however, assumes that you wish to get maximum performance and so demonstrates how you can create your own router.
The router created in this example is replicating each message to a few destinations.
Start with the routing logic:
.. includecode:: code/docs/jrouting/CustomRouterDocTest.java#routing-logic
select will be called for each message and in this example pick a few destinations by round-robin,
by reusing the existing RoundRobinRoutingLogic and wrap the result in a SeveralRoutees
instance. SeveralRoutees will send the message to all of the supplied routes.
The implementation of the routing logic must be thread safe, since it might be used outside of actors.
A unit test of the routing logic:
.. includecode:: code/docs/jrouting/CustomRouterDocTest.java#unit-test-logic
You could stop here and use the RedundancyRoutingLogic with a akka.routing.Router
as described in :ref:`simple-router-java`.
Let us continue and make this into a self contained, configurable, router actor.
Create a class that extends PoolBase, GroupBase or CustomRouterConfig. That class is a factory
for the routing logic and holds the configuration for the router. Here we make it a Group.
.. includecode:: code/docs/jrouting/RedundancyGroup.java#group
This can be used exactly as the router actors provided by Akka.
.. includecode:: code/docs/jrouting/CustomRouterDocTest.java#usage-1
Note that we added a constructor in RedundancyGroup that takes a Config parameter.
That makes it possible to define it in configuration.
.. includecode:: ../scala/code/docs/routing/CustomRouterDocSpec.scala#jconfig
Note the fully qualified class name in the router property. The router class must extend
akka.routing.RouterConfig (Pool, Group or CustomRouterConfig) and have
constructor with one com.typesafe.config.Config parameter.
The deployment section of the configuration is passed to the constructor.
.. includecode:: code/docs/jrouting/CustomRouterDocTest.java#usage-2
The dispatcher for created children of the pool will be taken from
Props as described in :ref:`dispatchers-scala`.
To make it easy to define the dispatcher of the routees of the pool you can define the dispatcher inline in the deployment section of the config.
.. includecode:: ../scala/code/docs/routing/RouterDocSpec.scala#config-pool-dispatcher
That is the only thing you need to do enable a dedicated dispatcher for a pool.
Note
If you use a group of actors and route to their paths, then they will still use the same dispatcher
that was configured for them in their Props, it is not possible to change an actors dispatcher
after it has been created.
The “head” router cannot always run on the same dispatcher, because it
does not process the same type of messages, hence this special actor does
not use the dispatcher configured in Props, but takes the
routerDispatcher from the :class:`RouterConfig` instead, which defaults to
the actor system’s default dispatcher. All standard routers allow setting this
property in their constructor or factory method, custom routers have to
implement the method in a suitable way.
.. includecode:: code/docs/jrouting/RouterDocTest.java#dispatchers
Note
It is not allowed to configure the routerDispatcher to be a
:class:`akka.dispatch.BalancingDispatcherConfigurator` since the messages meant
for the special router actor cannot be processed by any other actor.