=clt #17447 Split Cluster Singleton docs into java/scala

akka-docs/rst/java/cluster-sharding.rst

@@ -13,7 +13,7 @@ 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` node.
+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

akka-docs/rst/java/cluster-singleton.rst

@@ -0,0 +1,141 @@
+.. _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

akka-docs/rst/java/cluster-usage.rst

@@ -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
 ^^^^^^^^^^^^^^^^
@@ -542,7 +542,7 @@ 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

akka-docs/rst/java/index-network.rst

@@ -6,7 +6,7 @@ Networking
 
    ../common/cluster
    cluster-usage
-   ../scala/cluster-singleton
+   cluster-singleton
    ../scala/distributed-pub-sub
    ../scala/cluster-client
    cluster-sharding