[[Singleton_subsystem]]
= Singleton subsystem

WildFly 10 introduced a "singleton" subsystem, which defines a set of
policies that define how an HA singleton should behave. A singleton
policy can be used to instrument singleton deployments or to create
singleton MSC services.

[[singleton-configuration]]
== Configuration

The
https://github.com/wildfly/wildfly/blob/10.0.0.Final/clustering/singleton/extension/src/main/resources/schema/wildfly-singleton_1_0.xsd[default
subsystem configuration] from WildFly's ha and full-ha profile looks
like:

[source,xml,options="nowrap"]
----
<subsystem xmlns="urn:jboss:domain:singleton:1.0">
    <singleton-policies default="default">
        <singleton-policy name="default" cache-container="server">
            <simple-election-policy/>
        </singleton-policy>
    </singleton-policies>
</subsystem>
----

A singleton policy defines:

1. A unique name
2. A cache container and cache with which to register singleton provider candidates
3. An election policy
4. A quorum (optional)

One can add a new singleton policy via the following management
operation:

[source,options="nowrap"]
----
/subsystem=singleton/singleton-policy=foo:add(cache-container=server)
----

[[cache-configuration]]
=== Cache configuration

The cache-container and cache attributes of a singleton policy must
reference a valid cache from the Infinispan subsystem. If no specific
cache is defined, the default cache of the cache container is assumed.
This cache is used as a registry of which nodes can provide a given
service and will typically use a replicated-cache configuration.

[[election-policies]]
=== Election policies

WildFly includes two singleton election policy implementations:

* *simple* +
Elects the provider (a.k.a. primary provider) of a singleton service based on a
specified position in a circular linked list of eligible nodes sorted by
descending age. Position=0, the default value, refers to the oldest
node, 1 is second oldest, etc. ; while position=-1 refers to the
youngest node, -2 to the second youngest, etc. +
e.g.
+
[source,options="nowrap"]
----
/subsystem=singleton/singleton-policy=foo/election-policy=simple:add(position=-1)
----

* *random* +
Elects a random member to be the provider of a singleton service +
e.g.
+
[source,options="nowrap"]
----
/subsystem=singleton/singleton-policy=foo/election-policy=random:add()
----

[[preferences]]
==== Preferences

Additionally, any singleton election policy may indicate a preference
for one or more members of a cluster. Preferences may be defined either
via node name or via outbound socket binding name. Node preferences
always take precedent over the results of an election policy. +
e.g.

[source,options="nowrap"]
----
/subsystem=singleton/singleton-policy=foo/election-policy=simple:list-add(name=name-preferences, value=nodeA)
/subsystem=singleton/singleton-policy=bar/election-policy=random:list-add(name=socket-binding-preferences, value=nodeA)
----

[[quorum]]
=== Quorum

Network partitions are particularly problematic for singleton services,
since they can trigger multiple singleton providers for the same service
to run at the same time. To defend against this scenario, a singleton
policy may define a quorum that requires a minimum number of nodes to be
present before a singleton provider election can take place. A typical
deployment scenario uses a quorum of N/2 + 1, where N is the anticipated
cluster size. This value can be updated at runtime, and will immediately
affect any active singleton services. +
e.g.

[source,options="nowrap"]
----
/subsystem=singleton/singleton-policy=foo:write-attribute(name=quorum, value=3)
----

[[non-ha-environments]]
== Non-HA environments

The singleton subsystem can be used in a non-HA profile, so long as the
cache that it references uses a local-cache configuration. In this
manner, an application leveraging singleton functionality (via the
singleton API or using a singleton deployment descriptor) will continue
function as if the server was a sole member of a cluster. For obvious
reasons, the use of a quorum does not make sense in such a
configuration.
