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Merge pull request #17869 from akka/wip-17447-split-docs-patriknw

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=cls #17447 Split Cluster Sharding and Tools docs into java/scala
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.. _cluster-client-java:
Cluster Client
==============
An actor system that is not part of the cluster can communicate with actors
somewhere in the cluster via this ``ClusterClient``. The client can of course be part of
another cluster. It only needs to know the location of one (or more) nodes to use as initial
contact points. It will establish a connection to a ``ClusterReceptionist`` somewhere in
the cluster. It will monitor the connection to the receptionist and establish a new
connection if the link goes down. When looking for a new receptionist it uses fresh
contact points retrieved from previous establishment, or periodically refreshed contacts,
i.e. not necessarily the initial contact points.
.. note::
``ClusterClient`` should not be used when sending messages to actors that run
within the same cluster. Similar functionality as the ``ClusterClient`` is
provided in a more efficient way by :ref:`distributed-pub-sub-java` for actors that
belong to the same cluster.
Also, note it's necessary to change ``akka.actor.provider`` from ``akka.actor.LocalActorRefProvider``
to ``akka.remote.RemoteActorRefProvider`` or ``akka.cluster.ClusterActorRefProvider`` when using
the cluster client.
The receptionist is supposed to be started on all nodes, or all nodes with specified role,
in the cluster. The receptionist can be started with the ``ClusterClientReceptionist`` extension
or as an ordinary actor.
You can send messages via the ``ClusterClient`` to any actor in the cluster that is registered
in the ``DistributedPubSubMediator`` used by the ``ClusterReceptionist``.
The ``ClusterClientReceptionist`` provides methods for registration of actors that
should be reachable from the client. Messages are wrapped in ``ClusterClient.Send``,
``ClusterClient.SendToAll`` or ``ClusterClient.Publish``.
**1. ClusterClient.Send**
The message will be delivered to one recipient with a matching path, if any such
exists. If several entries match the path the message will be delivered
to one random destination. The sender() of the message can specify that local
affinity is preferred, i.e. the message is sent to an actor in the same local actor
system as the used receptionist actor, if any such exists, otherwise random to any other
matching entry.
**2. ClusterClient.SendToAll**
The message will be delivered to all recipients with a matching path.
**3. ClusterClient.Publish**
The message will be delivered to all recipients Actors that have been registered as subscribers
to the named topic.
Response messages from the destination actor are tunneled via the receptionist
to avoid inbound connections from other cluster nodes to the client, i.e.
the ``sender()``, as seen by the destination actor, is not the client itself.
The ``sender()`` of the response messages, as seen by the client, is preserved
as the original sender(), so the client can choose to send subsequent messages
directly to the actor in the cluster.
While establishing a connection to a receptionist the ``ClusterClient`` will buffer
messages and send them when the connection is established. If the buffer is full
the ``ClusterClient`` will drop old messages when new messages are sent via the client.
The size of the buffer is configurable and it can be disabled by using a buffer size of 0.
It's worth noting that messages can always be lost because of the distributed nature
of these actors. As always, additional logic should be implemented in the destination
(acknowledgement) and in the client (retry) actors to ensure at-least-once message delivery.
An Example
----------
On the cluster nodes first start the receptionist. Note, it is recommended to load the extension
when the actor system is started by defining it in the ``akka.extensions`` configuration property::
akka.extensions = ["akka.cluster.client.ClusterClientReceptionist"]
Next, register the actors that should be available for the client.
.. includecode:: ../../../akka-cluster-tools/src/test/java/akka/cluster/client/ClusterClientTest.java#server
On the client you create the ``ClusterClient`` actor and use it as a gateway for sending
messages to the actors identified by their path (without address information) somewhere
in the cluster.
.. includecode:: ../../../akka-cluster-tools/src/test/java/akka/cluster/client/ClusterClientTest.java#client
The ``initialContacts`` parameter is a ``Set<ActorPath>``, which can be created like this:
.. includecode:: ../../../akka-cluster-tools/src/test/java/akka/cluster/client/ClusterClientTest.java#initialContacts
You will probably define the address information of the initial contact points in configuration or system property.
See also :ref:`cluster-client-config-java`.
A more comprehensive sample is available in the `Typesafe Activator <http://www.typesafe.com/platform/getstarted>`_
tutorial named `Distributed workers with Akka and Java! <http://www.typesafe.com/activator/template/akka-distributed-workers-java>`_.
ClusterClientReceptionist Extension
-----------------------------------
In the example above the receptionist is started and accessed with the ``akka.cluster.client.ClusterClientReceptionist`` extension.
That is convenient and perfectly fine in most cases, but it can be good to know that it is possible to
start the ``akka.cluster.client.ClusterReceptionist`` actor as an ordinary actor and you can have several
different receptionists at the same time, serving different types of clients.
Note that the ``ClusterClientReceptionist`` uses the ``DistributedPubSub`` extension, which is described
in :ref:`distributed-pub-sub-java`.
It is recommended to load the extension when the actor system is started by defining it in the
``akka.extensions`` configuration property::
akka.extensions = ["akka.cluster.client.ClusterClientReceptionist"]
Dependencies
------------
To use the Cluster Client you must add the following dependency in your project.
sbt::
"com.typesafe.akka" %% "akka-cluster-tools" % "@version@" @crossString@
maven::
<dependency>
<groupId>com.typesafe.akka</groupId>
<artifactId>akka-cluster-tools_@binVersion@</artifactId>
<version>@version@</version>
</dependency>
.. _cluster-client-config-java:
Configuration
-------------
The ``ClusterClientReceptionist`` extension (or ``ClusterReceptionistSettings``) can be configured
with the following properties:
.. includecode:: ../../../akka-cluster-tools/src/main/resources/reference.conf#receptionist-ext-config
The following configuration properties are read by the ``ClusterClientSettings``
when created with a ``ActorSystem`` parameter. It is also possible to amend the ``ClusterClientSettings``
or create it from another config section with the same layout as below. ``ClusterClientSettings`` is
a parameter to the ``ClusterClient.props`` factory method, i.e. each client can be configured
with different settings if needed.
.. includecode:: ../../../akka-cluster-tools/src/main/resources/reference.conf#cluster-client-config

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akka-docs/rst/java/cluster-metrics.rst
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@ -159,4 +159,12 @@ You can plug-in your own metrics collector instead of built-in
Look at those two implementations for inspiration.
Custom metrics collector implementation class must be specified in the :ref:`cluster_metrics_configuration_java`.
Custom metrics collector implementation class must be specified in the
``akka.cluster.metrics.collector.provider`` configuration property.
Configuration
-------------
The Cluster metrics extension can be configured with the following properties:
.. includecode:: ../../../akka-cluster-metrics/src/main/resources/reference.conf

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.. _cluster_sharding_java:
Cluster Sharding
================
Cluster sharding is useful when you need to distribute actors across several nodes in the cluster and want to
be able to interact with them using their logical identifier, but without having to care about
their physical location in the cluster, which might also change over time.
It could for example be actors representing Aggregate Roots in Domain-Driven Design terminology.
Here we call these actors "entities". These actors typically have persistent (durable) state,
but this feature is not limited to actors with persistent state.
Cluster sharding is typically used when you have many stateful actors that together consume
more resources (e.g. memory) than fit on one machine. If you only have a few stateful actors
it might be easier to run them on a :ref:`cluster-singleton-java` node.
In this context sharding means that actors with an identifier, so called entities,
can be automatically distributed across multiple nodes in the cluster. Each entity
actor runs only at one place, and messages can be sent to the entity without requiring
the sender to know the location of the destination actor. This is achieved by sending
the messages via a ``ShardRegion`` actor provided by this extension, which knows how
to route the message with the entity id to the final destination.
An Example
----------
This is how an entity actor may look like:
.. includecode:: ../../../akka-cluster-sharding/src/test/java/akka/cluster/sharding/ClusterShardingTest.java#counter-actor
The above actor uses event sourcing and the support provided in ``UntypedPersistentActor`` to store its state.
It does not have to be a persistent actor, but in case of failure or migration of entities between nodes it must be able to recover
its state if it is valuable.
Note how the ``persistenceId`` is defined. The name of the actor is the entity entity identifier (utf-8 URL-encoded).
You may define it another way, but it must be unique.
When using the sharding extension you are first, typically at system startup on each node
in the cluster, supposed to register the supported entity types with the ``ClusterSharding.start``
method. ``ClusterSharding.start`` gives you the reference which you can pass along.
.. includecode:: ../../../akka-cluster-sharding/src/test/java/akka/cluster/sharding/ClusterShardingTest.java#counter-start
The ``messageExtractor`` defines application specific methods to extract the entity
identifier and the shard identifier from incoming messages.
.. includecode:: ../../../akka-cluster-sharding/src/test/java/akka/cluster/sharding/ClusterShardingTest.java#counter-extractor
This example illustrates two different ways to define the entity identifier in the messages:
* The ``Get`` message includes the identifier itself.
* The ``EntityEnvelope`` holds the identifier, and the actual message that is
sent to the entity actor is wrapped in the envelope.
Note how these two messages types are handled in the ``entityId`` and ``entityMessage`` methods shown above.
The message sent to the entity actor is what ``entityMessage`` returns and that makes it possible to unwrap envelopes
if needed.
A shard is a group of entities that will be managed together. The grouping is defined by the
``extractShardId`` function shown above. For a specific entity identifier the shard identifier must always
be the same. Otherwise the entity actor might accidentally be started in several places at the same time.
Creating a good sharding algorithm is an interesting challenge in itself. Try to produce a uniform distribution,
i.e. same amount of entities in each shard. As a rule of thumb, the number of shards should be a factor ten greater
than the planned maximum number of cluster nodes. Less shards than number of nodes will result in that some nodes
will not host any shards. Too many shards will result in less efficient management of the shards, e.g. rebalancing
overhead, and increased latency because the coordinator is involved in the routing of the first message for each
shard. The sharding algorithm must be the same on all nodes in a running cluster. It can be changed after stopping
all nodes in the cluster.
A simple sharding algorithm that works fine in most cases is to take the absolute value of the ``hashCode`` of
the entity identifier modulo number of shards. As a convenience this is provided by the
``ShardRegion.HashCodeMessageExtractor``.
Messages to the entities are always sent via the local ``ShardRegion``. The ``ShardRegion`` actor reference for a
named entity type is returned by ``ClusterSharding.start`` and it can also be retrieved with ``ClusterSharding.shardRegion``.
The ``ShardRegion`` will lookup the location of the shard for the entity if it does not already know its location. It will
delegate the message to the right node and it will create the entity actor on demand, i.e. when the
first message for a specific entity is delivered.
.. includecode:: ../../../akka-cluster-sharding/src/test/java/akka/cluster/sharding/ClusterShardingTest.java#counter-usage
How it works
------------
The ``ShardRegion`` actor is started on each node in the cluster, or group of nodes
tagged with a specific role. The ``ShardRegion`` is created with two application specific
functions to extract the entity identifier and the shard identifier from incoming messages.
A shard is a group of entities that will be managed together. For the first message in a
specific shard the ``ShardRegion`` request the location of the shard from a central coordinator,
the ``ShardCoordinator``.
The ``ShardCoordinator`` decides which ``ShardRegion`` shall own the ``Shard`` and informs
that ``ShardRegion``. The region will confirm this request and create the ``Shard`` supervisor
as a child actor. The individual ``Entities`` will then be created when needed by the ``Shard``
actor. Incoming messages thus travel via the ``ShardRegion`` and the ``Shard`` to the target
``Entity``.
If the shard home is another ``ShardRegion`` instance messages will be forwarded
to that ``ShardRegion`` instance instead. While resolving the location of a
shard incoming messages for that shard are buffered and later delivered when the
shard home is known. Subsequent messages to the resolved shard can be delivered
to the target destination immediately without involving the ``ShardCoordinator``.
Scenario 1:
#. Incoming message M1 to ``ShardRegion`` instance R1.
#. M1 is mapped to shard S1. R1 doesn't know about S1, so it asks the coordinator C for the location of S1.
#. C answers that the home of S1 is R1.
#. R1 creates child actor for the entity E1 and sends buffered messages for S1 to E1 child
#. All incoming messages for S1 which arrive at R1 can be handled by R1 without C. It creates entity children as needed, and forwards messages to them.
Scenario 2:
#. Incoming message M2 to R1.
#. M2 is mapped to S2. R1 doesn't know about S2, so it asks C for the location of S2.
#. C answers that the home of S2 is R2.
#. R1 sends buffered messages for S2 to R2
#. All incoming messages for S2 which arrive at R1 can be handled by R1 without C. It forwards messages to R2.
#. R2 receives message for S2, ask C, which answers that the home of S2 is R2, and we are in Scenario 1 (but for R2).
To make sure that at most one instance of a specific entity actor is running somewhere
in the cluster it is important that all nodes have the same view of where the shards
are located. Therefore the shard allocation decisions are taken by the central
``ShardCoordinator``, which is running as a cluster singleton, i.e. one instance on
the oldest member among all cluster nodes or a group of nodes tagged with a specific
role.
The logic that decides where a shard is to be located is defined in a pluggable shard
allocation strategy. The default implementation ``ShardCoordinator.LeastShardAllocationStrategy``
allocates new shards to the ``ShardRegion`` with least number of previously allocated shards.
This strategy can be replaced by an application specific implementation.
To be able to use newly added members in the cluster the coordinator facilitates rebalancing
of shards, i.e. migrate entities from one node to another. In the rebalance process the
coordinator first notifies all ``ShardRegion`` actors that a handoff for a shard has started.
That means they will start buffering incoming messages for that shard, in the same way as if the
shard location is unknown. During the rebalance process the coordinator will not answer any
requests for the location of shards that are being rebalanced, i.e. local buffering will
continue until the handoff is completed. The ``ShardRegion`` responsible for the rebalanced shard
will stop all entities in that shard by sending the specified ``handOffStopMessage``
(default ``PoisonPill``) to them. When all entities have been terminated the ``ShardRegion``
owning the entities will acknowledge the handoff as completed to the coordinator.
Thereafter the coordinator will reply to requests for the location of
the shard and thereby allocate a new home for the shard and then buffered messages in the
``ShardRegion`` actors are delivered to the new location. This means that the state of the entities
are not transferred or migrated. If the state of the entities are of importance it should be
persistent (durable), e.g. with :ref:`persistence-java`, so that it can be recovered at the new
location.
The logic that decides which shards to rebalance is defined in a pluggable shard
allocation strategy. The default implementation ``ShardCoordinator.LeastShardAllocationStrategy``
picks shards for handoff from the ``ShardRegion`` with most number of previously allocated shards.
They will then be allocated to the ``ShardRegion`` with least number of previously allocated shards,
i.e. new members in the cluster. There is a configurable threshold of how large the difference
must be to begin the rebalancing. This strategy can be replaced by an application specific
implementation.
The state of shard locations in the ``ShardCoordinator`` is persistent (durable) with
:ref:`persistence-java` to survive failures. Since it is running in a cluster :ref:`persistence-java`
must be configured with a distributed journal. When a crashed or unreachable coordinator
node has been removed (via down) from the cluster a new ``ShardCoordinator`` singleton
actor will take over and the state is recovered. During such a failure period shards
with known location are still available, while messages for new (unknown) shards
are buffered until the new ``ShardCoordinator`` becomes available.
As long as a sender uses the same ``ShardRegion`` actor to deliver messages to an entity
actor the order of the messages is preserved. As long as the buffer limit is not reached
messages are delivered on a best effort basis, with at-most once delivery semantics,
in the same way as ordinary message sending. Reliable end-to-end messaging, with
at-least-once semantics can be added by using ``AtLeastOnceDelivery`` in :ref:`persistence-java`.
Some additional latency is introduced for messages targeted to new or previously
unused shards due to the round-trip to the coordinator. Rebalancing of shards may
also add latency. This should be considered when designing the application specific
shard resolution, e.g. to avoid too fine grained shards.
Proxy Only Mode
---------------
The ``ShardRegion`` actor can also be started in proxy only mode, i.e. it will not
host any entities itself, but knows how to delegate messages to the right location.
A ``ShardRegion`` is started in proxy only mode with the method ``ClusterSharding.startProxy``
method.
Passivation
-----------
If the state of the entities are persistent you may stop entities that are not used to
reduce memory consumption. This is done by the application specific implementation of
the entity actors for example by defining receive timeout (``context.setReceiveTimeout``).
If a message is already enqueued to the entity when it stops itself the enqueued message
in the mailbox will be dropped. To support graceful passivation without loosing such
messages the entity actor can send ``ShardRegion.Passivate`` to its parent ``Shard``.
The specified wrapped message in ``Passivate`` will be sent back to the entity, which is
then supposed to stop itself. Incoming messages will be buffered by the ``Shard``
between reception of ``Passivate`` and termination of the entity. Such buffered messages
are thereafter delivered to a new incarnation of the entity.
Remembering Entities
--------------------
The list of entities in each ``Shard`` can be made persistent (durable) by setting
the ``rememberEntities`` flag to true in ``ClusterShardingSettings`` when calling
``ClusterSharding.start``. When configured to remember entities, whenever a ``Shard``
is rebalanced onto another node or recovers after a crash it will recreate all the
entities which were previously running in that ``Shard``. To permanently stop entities,
a ``Passivate`` message must be sent to the parent of the entity actor, otherwise the
entity will be automatically restarted after the entity restart backoff specified in
the configuration.
When ``rememberEntities`` is set to false, a ``Shard`` will not automatically restart any entities
after a rebalance or recovering from a crash. Entities will only be started once the first message
for that entity has been received in the ``Shard``. Entities will not be restarted if they stop without
using a ``Passivate``.
Note that the state of the entities themselves will not be restored unless they have been made persistent,
e.g. with :ref:`persistence-java`.
Graceful Shutdown
-----------------
You can send the message ``ClusterSharding.GracefulShutdown`` message (``ClusterSharding.gracefulShutdownInstance
in Java) to the ``ShardRegion`` actor to handoff all shards that are hosted by that ``ShardRegion`` and then the
``ShardRegion`` actor will be stopped. You can ``watch`` the ``ShardRegion`` actor to know when it is completed.
During this period other regions will buffer messages for those shards in the same way as when a rebalance is
triggered by the coordinator. When the shards have been stopped the coordinator will allocate these shards elsewhere.
When the ``ShardRegion`` has terminated you probably want to ``leave`` the cluster, and shut down the ``ActorSystem``.
This is how to do that:
.. includecode:: ../../../akka-cluster-sharding/src/test/java/akka/cluster/sharding/ClusterShardingTest.java#graceful-shutdown
Dependencies
------------
To use the Cluster Sharding you must add the following dependency in your project.
sbt::
"com.typesafe.akka" %% "akka-cluster-sharding" % "@version@" @crossString@
maven::
<dependency>
<groupId>com.typesafe.akka</groupId>
<artifactId>akka-cluster-sharding_@binVersion@</artifactId>
<version>@version@</version>
</dependency>
Configuration
-------------
The ``ClusterSharding`` extension can be configured with the following properties. These configuration
properties are read by the ``ClusterShardingSettings`` when created with a ``ActorSystem`` parameter.
It is also possible to amend the ``ClusterShardingSettings`` or create it from another config section
with the same layout as below. ``ClusterShardingSettings`` is a parameter to the ``start`` method of
the ``ClusterSharding`` extension, i.e. each each entity type can be configured with different settings
if needed.
.. includecode:: ../../../akka-cluster-sharding/src/main/resources/reference.conf#sharding-ext-config
Custom shard allocation strategy can be defined in an optional parameter to
``ClusterSharding.start``. See the API documentation of ``AbstractShardAllocationStrategy`` for details
of how to implement a custom shard allocation strategy.

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.. _cluster-singleton-java:
Cluster Singleton
=================
For some use cases it is convenient and sometimes also mandatory to ensure that
you have exactly one actor of a certain type running somewhere in the cluster.
Some examples:
* single point of responsibility for certain cluster-wide consistent decisions, or
coordination of actions across the cluster system
* single entry point to an external system
* single master, many workers
* centralized naming service, or routing logic
Using a singleton should not be the first design choice. It has several drawbacks,
such as single-point of bottleneck. Single-point of failure is also a relevant concern,
but for some cases this feature takes care of that by making sure that another singleton
instance will eventually be started.
The cluster singleton pattern is implemented by ``akka.cluster.singleton.ClusterSingletonManager``.
It manages one singleton actor instance among all cluster nodes or a group of nodes tagged with
a specific role. ``ClusterSingletonManager`` is an actor that is supposed to be started on
all nodes, or all nodes with specified role, in the cluster. The actual singleton actor is
started by the ``ClusterSingletonManager`` on the oldest node by creating a child actor from
supplied ``Props``. ``ClusterSingletonManager`` makes sure that at most one singleton instance
is running at any point in time.
The singleton actor is always running on the oldest member with specified role.
The oldest member is determined by [[akka.cluster.Member#isOlderThan]].
This can change when removing that member from the cluster. Be aware that there is a short time
period when there is no active singleton during the hand-over process.
The cluster failure detector will notice when oldest node becomes unreachable due to
things like JVM crash, hard shut down, or network failure. Then a new oldest node will
take over and a new singleton actor is created. For these failure scenarios there will
not be a graceful hand-over, but more than one active singletons is prevented by all
reasonable means. Some corner cases are eventually resolved by configurable timeouts.
You can access the singleton actor by using the provided ``akka.cluster.singleton.ClusterSingletonProxy``,
which will route all messages to the current instance of the singleton. The proxy will keep track of
the oldest node in the cluster and resolve the singleton's ``ActorRef`` by explicitly sending the
singleton's ``actorSelection`` the ``akka.actor.Identify`` message and waiting for it to reply.
This is performed periodically if the singleton doesn't reply within a certain (configurable) time.
Given the implementation, there might be periods of time during which the ``ActorRef`` is unavailable,
e.g., when a node leaves the cluster. In these cases, the proxy will buffer the messages sent to the
singleton and then deliver them when the singleton is finally available. If the buffer is full
the ``ClusterSingletonProxy`` will drop old messages when new messages are sent via the proxy.
The size of the buffer is configurable and it can be disabled by using a buffer size of 0.
It's worth noting that messages can always be lost because of the distributed nature of these actors.
As always, additional logic should be implemented in the singleton (acknowledgement) and in the
client (retry) actors to ensure at-least-once message delivery.
Potential problems to be aware of
---------------------------------
This pattern may seem to be very tempting to use at first, but it has several drawbacks, some of them are listed below:
* the cluster singleton may quickly become a *performance bottleneck*,
* you can not rely on the cluster singleton to be *non-stop* available — e.g. when the node on which the singleton has
been running dies, it will take a few seconds for this to be noticed and the singleton be migrated to another node,
* in the case of a *network partition* appearing in a Cluster that is using Automatic Downing (see docs for
:ref:`automatic-vs-manual-downing-java`),
it may happen that the isolated clusters each decide to spin up their own singleton, meaning that there might be multiple
singletons running in the system, yet the Clusters have no way of finding out about them (because of the partition).
Especially the last point is something you should be aware of — in general when using the Cluster Singleton pattern
you should take care of downing nodes yourself and not rely on the timing based auto-down feature.
.. warning::
**Be very careful when using Cluster Singleton together with Automatic Downing**,
since it allows the cluster to split up into two separate clusters, which in turn will result
in *multiple Singletons* being started, one in each separate cluster!
An Example
----------
Assume that we need one single entry point to an external system. An actor that
receives messages from a JMS queue with the strict requirement that only one
JMS consumer must exist to be make sure that the messages are processed in order.
That is perhaps not how one would like to design things, but a typical real-world
scenario when integrating with external systems.
On each node in the cluster you need to start the ``ClusterSingletonManager`` and
supply the ``Props`` of the singleton actor, in this case the JMS queue consumer.
.. includecode:: ../../../akka-cluster-tools/src/test/java/akka/cluster/singleton/ClusterSingletonManagerTest.java#create-singleton-manager
Here we limit the singleton to nodes tagged with the ``"worker"`` role, but all nodes, independent of
role, can be used by not specifying ``withRole``.
Here we use an application specific ``terminationMessage`` to be able to close the
resources before actually stopping the singleton actor. Note that ``PoisonPill`` is a
perfectly fine ``terminationMessage`` if you only need to stop the actor.
With the names given above, access to the singleton can be obtained from any cluster node using a properly
configured proxy.
.. includecode:: ../../../akka-cluster-tools/src/test/java/akka/cluster/singleton/ClusterSingletonManagerTest.java#create-singleton-proxy
A more comprehensive sample is available in the `Typesafe Activator <http://www.typesafe.com/platform/getstarted>`_
tutorial named `Distributed workers with Akka and Java! <http://www.typesafe.com/activator/template/akka-distributed-workers-java>`_.
Dependencies
------------
To use the Cluster Singleton you must add the following dependency in your project.
sbt::
"com.typesafe.akka" %% "akka-cluster-tools" % "@version@" @crossString@
maven::
<dependency>
<groupId>com.typesafe.akka</groupId>
<artifactId>akka-cluster-tools_@binVersion@</artifactId>
<version>@version@</version>
</dependency>
Configuration
-------------
The following configuration properties are read by the ``ClusterSingletonManagerSettings``
when created with a ``ActorSystem`` parameter. It is also possible to amend the ``ClusterSingletonManagerSettings``
or create it from another config section with the same layout as below. ``ClusterSingletonManagerSettings`` is
a parameter to the ``ClusterSingletonManager.props`` factory method, i.e. each singleton can be configured
with different settings if needed.
.. includecode:: ../../../akka-cluster-tools/src/main/resources/reference.conf#singleton-config
The following configuration properties are read by the ``ClusterSingletonProxySettings``
when created with a ``ActorSystem`` parameter. It is also possible to amend the ``ClusterSingletonProxySettings``
or create it from another config section with the same layout as below. ``ClusterSingletonProxySettings`` is
a parameter to the ``ClusterSingletonProxy.props`` factory method, i.e. each singleton proxy can be configured
with different settings if needed.
.. includecode:: ../../../akka-cluster-tools/src/main/resources/reference.conf#singleton-proxy-config

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akka-docs/rst/java/cluster-usage.rst
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@ -313,7 +313,7 @@ you have exactly one actor of a certain type running somewhere in the cluster.
This can be implemented by subscribing to member events, but there are several corner
cases to consider. Therefore, this specific use case is made easily accessible by the
:ref:`cluster-singleton` in the contrib module.
:ref:`cluster-singleton-java`.
Cluster Sharding
^^^^^^^^^^^^^^^^
@ -322,7 +322,7 @@ Distributes actors across several nodes in the cluster and supports interaction
with the actors using their logical identifier, but without having to care about
their physical location in the cluster.
See :ref:`cluster-sharding` in the contrib module.
See :ref:`cluster_sharding_java`.
Distributed Publish Subscribe
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
@ -331,7 +331,7 @@ Publish-subscribe messaging between actors in the cluster, and point-to-point me
using the logical path of the actors, i.e. the sender does not have to know on which
node the destination actor is running.
See :ref:`distributed-pub-sub` in the contrib module.
See :ref:`distributed-pub-sub-scala`.
Cluster Client
^^^^^^^^^^^^^^
@ -340,7 +340,15 @@ Communication from an actor system that is not part of the cluster to actors run
somewhere in the cluster. The client does not have to know on which node the destination
actor is running.
See :ref:`cluster-client` in the contrib module.
See :ref:`cluster-client-java`.
Distributed Data
^^^^^^^^^^^^^^^^
*Akka Distributed Data* is useful when you need to share data between nodes in an
Akka Cluster. The data is accessed with an actor providing a key-value store like API.
See :ref:`distributed_data_java`.
Failure Detector
^^^^^^^^^^^^^^^^
@ -535,7 +543,7 @@ Router Example with Pool of Remote Deployed Routees
---------------------------------------------------
Let's take a look at how to use a cluster aware router on single master node that creates
and deploys workers. To keep track of a single master we use the :ref:`cluster-singleton`
and deploys workers. To keep track of a single master we use the :ref:`cluster-singleton-java`
in the contrib module. The ``ClusterSingletonManager`` is started on each node.
.. includecode:: ../../../akka-samples/akka-sample-cluster-java/src/main/java/sample/cluster/stats/StatsSampleOneMasterMain.java#create-singleton-manager

138
akka-docs/rst/java/distributed-pub-sub.rst Normal file
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@ -0,0 +1,138 @@
.. _distributed-pub-sub-java:
Distributed Publish Subscribe in Cluster
========================================
How do I send a message to an actor without knowing which node it is running on?
How do I send messages to all actors in the cluster that have registered interest
in a named topic?
This pattern provides a mediator actor, ``akka.cluster.pubsub.DistributedPubSubMediator``,
that manages a registry of actor references and replicates the entries to peer
actors among all cluster nodes or a group of nodes tagged with a specific role.
The ``DistributedPubSubMediator`` actor is supposed to be started on all nodes,
or all nodes with specified role, in the cluster. The mediator can be
started with the ``DistributedPubSub`` extension or as an ordinary actor.
The registry is eventually consistent, i.e. changes are not immediately visible at
other nodes, but typically they will be fully replicated to all other nodes after
a few seconds. Changes are only performed in the own part of the registry and those
changes are versioned. Deltas are disseminated in a scalable way to other nodes with
a gossip protocol.
You can send messages via the mediator on any node to registered actors on
any other node. There are four modes of message delivery.
**1. DistributedPubSubMediator.Send**
The message will be delivered to one recipient with a matching path, if any such
exists in the registry. If several entries match the path the message will be sent
via the supplied ``RoutingLogic`` (default random) to one destination. The sender() of the
message can specify that local affinity is preferred, i.e. the message is sent to an actor
in the same local actor system as the used mediator actor, if any such exists, otherwise
route to any other matching entry. A typical usage of this mode is private chat to one
other user in an instant messaging application. It can also be used for distributing
tasks to registered workers, like a cluster aware router where the routees dynamically
can register themselves.
**2. DistributedPubSubMediator.SendToAll**
The message will be delivered to all recipients with a matching path. Actors with
the same path, without address information, can be registered on different nodes.
On each node there can only be one such actor, since the path is unique within one
local actor system. Typical usage of this mode is to broadcast messages to all replicas
with the same path, e.g. 3 actors on different nodes that all perform the same actions,
for redundancy. You can also optionally specify a property (``allButSelf``) deciding
if the message should be sent to a matching path on the self node or not.
**3. DistributedPubSubMediator.Publish**
Actors may be registered to a named topic instead of path. This enables many subscribers
on each node. The message will be delivered to all subscribers of the topic. For
efficiency the message is sent over the wire only once per node (that has a matching topic),
and then delivered to all subscribers of the local topic representation. This is the
true pub/sub mode. A typical usage of this mode is a chat room in an instant messaging
application.
**4. DistributedPubSubMediator.Publish with sendOneMessageToEachGroup**
Actors may be subscribed to a named topic with an optional property (``group``).
If subscribing with a group name, each message published to a topic with the
(``sendOneMessageToEachGroup``) flag is delivered via the supplied ``RoutingLogic``
(default random) to one actor within each subscribing group.
If all the subscribed actors have the same group name, then this works just like
``Send`` and all messages are delivered to one subscriber.
If all the subscribed actors have different group names, then this works like
normal ``Publish`` and all messages are broadcast to all subscribers.
You register actors to the local mediator with ``DistributedPubSubMediator.Put`` or
``DistributedPubSubMediator.Subscribe``. ``Put`` is used together with ``Send`` and
``SendToAll`` message delivery modes. The ``ActorRef`` in ``Put`` must belong to the same
local actor system as the mediator. ``Subscribe`` is used together with ``Publish``.
Actors are automatically removed from the registry when they are terminated, or you
can explicitly remove entries with ``DistributedPubSubMediator.Remove`` or
``DistributedPubSubMediator.Unsubscribe``.
Successful ``Subscribe`` and ``Unsubscribe`` is acknowledged with
``DistributedPubSubMediator.SubscribeAck`` and ``DistributedPubSubMediator.UnsubscribeAck``
replies.
A Small Example
---------------
A subscriber actor:
.. includecode:: ../../../akka-cluster-tools/src/test/java/akka/cluster/pubsub/DistributedPubSubMediatorTest.java#subscriber
Subscriber actors can be started on several nodes in the cluster, and all will receive
messages published to the "content" topic.
.. includecode:: ../../../akka-cluster-tools/src/test/java/akka/cluster/pubsub/DistributedPubSubMediatorTest.java#start-subscribers
A simple actor that publishes to this "content" topic:
.. includecode:: ../../../akka-cluster-tools/src/test/java/akka/cluster/pubsub/DistributedPubSubMediatorTest.java#publisher
It can publish messages to the topic from anywhere in the cluster:
.. includecode:: ../../../akka-cluster-tools/src/test/java/akka/cluster/pubsub/DistributedPubSubMediatorTest.java#publish-message
DistributedPubSub Extension
---------------------------
In the example above the mediator is started and accessed with the ``akka.cluster.pubsub.DistributedPubSub`` extension.
That is convenient and perfectly fine in most cases, but it can be good to know that it is possible to
start the mediator actor as an ordinary actor and you can have several different mediators at the same
time to be able to divide a large number of actors/topics to different mediators. For example you might
want to use different cluster roles for different mediators.
The ``DistributedPubSub`` extension can be configured with the following properties:
.. includecode:: ../../../akka-cluster-tools/src/main/resources/reference.conf#pub-sub-ext-config
It is recommended to load the extension when the actor system is started by defining it in
``akka.extensions`` configuration property. Otherwise it will be activated when first used
and then it takes a while for it to be populated.
::
akka.extensions = ["akka.cluster.pubsub.DistributedPubSub"]
Dependencies
------------
To use the Cluster Singleton you must add the following dependency in your project.
sbt::
"com.typesafe.akka" %% "akka-cluster-tools" % "@version@" @crossString@
maven::
<dependency>
<groupId>com.typesafe.akka</groupId>
<artifactId>akka-cluster-tools_@binVersion@</artifactId>
<version>@version@</version>
</dependency>

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@ -152,7 +152,7 @@ Similarly to `Actor Classification`_, :class:`EventStream` will automatically re
.. note::
The event stream is a *local facility*, meaning that it will *not* distribute events to other nodes in a clustered environment (unless you subscribe a Remote Actor to the stream explicitly).
If you need to broadcast events in an Akka cluster, *without* knowing your recipients explicitly (i.e. obtaining their ActorRefs), you may want to look into: :ref:`distributed-pub-sub`.
If you need to broadcast events in an Akka cluster, *without* knowing your recipients explicitly (i.e. obtaining their ActorRefs), you may want to look into: :ref:`distributed-pub-sub-java`.
Default Handlers
----------------

8
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@ -6,10 +6,10 @@ Networking
../common/cluster
cluster-usage
../scala/cluster-singleton
../scala/distributed-pub-sub
../scala/cluster-client
../scala/cluster-sharding
cluster-singleton
distributed-pub-sub
cluster-client
cluster-sharding
cluster-metrics
distributed-data
remoting