pekko/akka-contrib/docs/cluster-singleton.rst

.. _cluster-singleton:

Cluster Singleton Pattern
=========================

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.contrib.pattern.ClusterSingletonManager``.
It manages 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, which can be determined by
``Member#isOlderThan``. This can change when removing members. A graceful hand over can normally
be performed when current oldest node is leaving 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 access the singleton actor with ``actorSelection`` using the names you have
specified when creating the ClusterSingletonManager. You can subscribe to
``akka.cluster.ClusterEvent.MemberEvent`` and sort the members by age
(``Member#isOlderThan``) to keep track of oldest member.
Alternatively the singleton actor may broadcast its existence when it is started.

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.

In Scala:

.. includecode:: @contribSrc@/src/multi-jvm/scala/akka/contrib/pattern/ClusterSingletonManagerSpec.scala#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 specifying ``None`` as ``role`` parameter.

The corresponding Java API for the ``singeltonProps`` function is ``akka.contrib.pattern.ClusterSingletonPropsFactory``.
The Java API takes a plain String for the role parameter and ``null`` means that all nodes, independent of
role, are used.

In Java:

.. includecode:: @contribSrc@/src/test/java/akka/contrib/pattern/ClusterSingletonManagerTest.java#create-singleton-manager

.. note::

  The ``singletonProps``/``singletonPropsFactory`` is invoked when creating
  the singleton actor and it must not use members that are not thread safe, e.g.
  mutable state in enclosing actor.

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.

Here is how the singleton actor handles the ``terminationMessage`` in this example.

.. includecode:: @contribSrc@/src/multi-jvm/scala/akka/contrib/pattern/ClusterSingletonManagerSpec.scala#consumer-end

Note that you can send back current state to the ``ClusterSingletonManager`` before terminating.
This message will be sent over to the ``ClusterSingletonManager`` at the new oldest node and it
will be passed to the ``singletonProps`` factory when creating the new singleton instance.

With the names given above the path of singleton actor can be constructed by subscribing to
``MemberEvent`` cluster event and sort the members by age to keep track of oldest member.

In Scala:

.. includecode:: @contribSrc@/src/multi-jvm/scala/akka/contrib/pattern/ClusterSingletonManagerSpec.scala#singleton-proxy

In Java:

.. includecode:: @contribSrc@/src/test/java/akka/contrib/pattern/ClusterSingletonManagerTest.java#singleton-proxy

The checks of ``role`` can be omitted if you don't limit the singleton to the group of members
tagged with a specific role.

Note that the hand-over might still be in progress and the singleton actor might not be started yet
when you receive the member event.

A nice alternative to the above proxy is to use :ref:`distributed-pub-sub`. Let the singleton
actor register itself to the mediator with ``DistributedPubSubMediator.Put`` message when it is
started. Send messages to the singleton actor via the mediator with ``DistributedPubSubMediator.SendToAll``.

.. note:: The singleton pattern will be simplified, perhaps provided out-of-the-box, when the cluster handles automatic actor partitioning.