[[JGroups_Subsystem]]
= JGroups Subsystem

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[[jgroups-purpose]]
== Purpose

The JGroups subsystem provides group communication support for HA
services in the form of JGroups channels.

Named channel instances permit application peers in a cluster to
communicate as a group and in such a way that the communication
satisfies defined properties (e.g. reliable, ordered,
failure-sensitive). Communication properties are configurable for each
channel and are defined by the protocol stack used to create the
channel. Protocol stacks consist of a base transport layer (used to
transport messages around the cluster) together with a user-defined,
ordered stack of protocol layers, where each protocol layer supports a
given communication property.

The JGroups subsystem provides the following features:

* allows definition of named protocol stacks
* view run-time metrics associated with channels
* specify a default stack for general use

In the following sections, we describe the JGroups subsystem.

[IMPORTANT]

JGroups channels are created transparently as part of the clustering
functionality (e.g. on clustered application deployment, channels will
be created behind the scenes to support clustered features such as
session replication or transmission of SSO contexts around the cluster).

[[jgroups-configuration-example]]
== Configuration example

What follows is a sample JGroups subsystem configuration showing all of
the possible elements and attributes which may be configured. We shall
use this example to explain the meaning of the various elements and
attributes.

[IMPORTANT]

The schema for the subsystem, describing all valid elements and
attributes, can be found in the WildFly distribution, in the `docs/schema`
directory.

[source,xml,options="nowrap"]
----
<subsystem xmlns="urn:jboss:domain:jgroups:6.0">
    <channels default="ee">
        <channel name="ee" stack="udp" cluster="ejb"/>
    </channels>
    <stacks>
        <stack name="udp">
            <transport type="UDP" socket-binding="jgroups-udp"/>
            <protocol type="PING"/>
            <protocol type="MERGE3"/>
            <protocol type="FD_SOCK"/>
            <protocol type="FD_ALL"/>
            <protocol type="VERIFY_SUSPECT"/>
            <protocol type="pbcast.NAKACK2"/>
            <protocol type="UNICAST3"/>
            <protocol type="pbcast.STABLE"/>
            <protocol type="pbcast.GMS"/>
            <protocol type="UFC"/>
            <protocol type="MFC"/>
            <protocol type="FRAG3"/>
        </stack>
        <stack name="tcp">
            <transport type="TCP" socket-binding="jgroups-tcp"/>
            <socket-protocol type="MPING" socket-binding="jgroups-mping"/>
            <protocol type="MERGE3"/>
            <protocol type="FD_SOCK"/>
            <protocol type="FD_ALL"/>
            <protocol type="VERIFY_SUSPECT"/>
            <protocol type="pbcast.NAKACK2"/>
            <protocol type="UNICAST3"/>
            <protocol type="pbcast.STABLE"/>
            <protocol type="pbcast.GMS"/>
            <protocol type="MFC"/>
            <protocol type="FRAG3"/>
        </stack>
    </stacks>
</subsystem>
----

[[subsystem]]
=== <subsystem>

This element is used to configure the subsystem within a WildFly system
profile.

* `xmlns` This attribute specifies the XML namespace of the JGroups
subsystem and, in particular, its version.

* `default-stack` This attribute is used to specify a default stack for
the JGroups subsystem. This default stack will be used whenever a stack
is required but no stack is specified.

[[stack]]
=== <stack>

This element is used to configure a JGroups protocol stack.

* `name` This attribute is used to specify the name of the stack.

[[jgroups-transport]]
=== <transport>

This element is used to configure the transport layer (required) of the
protocol stack.

* `type` This attribute specifies the transport type (e.g. UDP, TCP,
TCPGOSSIP)
* `socket-binding` This attribute references a defined socket binding in
the server profile. It is used when JGroups needs to create general
sockets internally.
* `diagnostics-socket-binding` This attribute references a defined
socket binding in the server profile. It is used when JGroups needs to
create sockets for use with the diagnostics program. For more about the
use of diagnostics, see the JGroups documentation for probe.sh.
* `default-executor` This attribute references a defined thread pool
executor in the threads subsystem. It governs the allocation and
execution of runnable tasks to handle incoming JGroups messages.
* `oob-executor` This attribute references a defined thread pool
executor in the threads subsystem. It governs the allocation and
execution of runnable tasks to handle incoming JGroups OOB
(out-of-bound) messages.
* `timer-executor` This attribute references a defined thread pool
executor in the threads subsystem. It governs the allocation and
execution of runnable timer-related tasks.
* `shared` This attribute indicates whether or not this transport is
shared amongst several JGroups stacks or not.
* `thread-factory` This attribute references a defined thread factory in
the threads subsystem. It governs the allocation of threads for running
tasks which are not handled by the executors above.
* `site` This attribute defines a site (data centre) id for this node.
* `rack` This attribute defines a rack (server rack) id for this node.
* `machine` This attribute defines a machine (host) is for this node.

[IMPORTANT]

site, rack and machine ids are used by the Infinispan topology-aware
consistent hash function, which when using dist mode, prevents dist mode
replicas from being stored on the same host, rack or site

.

[[property]]
==== <property>

This element is used to configure a transport property.

* `name` This attribute specifies the name of the protocol property. The
value is provided as text for the property element.

[[protocol]]
=== <protocol>

This element is used to configure a (non-transport) protocol layer in
the JGroups stack. Protocol layers are ordered within the stack.

* `type` This attribute specifies the name of the JGroups protocol
implementation (e.g. MPING, pbcast.GMS), with the package prefix
org.jgroups.protocols removed.
* `socket-binding` This attribute references a defined socket binding in
the server profile. It is used when JGroups needs to create general
sockets internally for this protocol instance.

[[relay]]
=== <relay>

This element is used to configure the RELAY protocol for a JGroups
stack. RELAY is a protocol which provides cross-site replication between
defined sites (data centres). In the RELAY protocol, defined sites
specify the names of remote sites (backup sites) to which their data
should be backed up. Channels are defined between sites to permit the
RELAY protocol to transport the data from the current site to a backup
site.

* `site` This attribute specifies the name of the current site. Site
names can be referenced elsewhere (e.g. in the JGroups remote-site
configuration elements, as well as backup configuration elements in the
Infinispan subsystem)

[[remote-site]]
==== <remote-site>

This element is used to configure a remote site for the RELAY protocol.

* `name` This attribute specifies the name of the remote site to which
this configuration applies.
* `stack` This attribute specifies a JGroups protocol stack to use for
communication between this site and the remote site.
* `cluster` This attribute specifies the name of the JGroups channel to
use for communication between this site and the remote site.

[[jgroups-use-cases]]
== Use Cases

In many cases, channels will be configured via XML as in the example
above, so that the channels will be available upon server startup.
However, channels may also be added, removed or have their
configurations changed in a running server by making use of the WildFly
management API command-line interface (CLI). In this section, we present
some key use cases for the JGroups management API.

The key use cases covered are:

* adding a stack
* adding a protocol to an existing stack
* adding a property to a protocol

[IMPORTANT]

The WildFly management API command-line interface (CLI) itself can be
used to provide extensive information on the attributes and commands
available in the JGroups subsystem interface used in these examples.

[[add-a-stack]]
=== Add a stack

[source,options="nowrap"]
----
/subsystem=jgroups/stack=mystack:add(transport={}, protocols={})
----

[[add-a-protocol-to-a-stack]]
=== Add a protocol to a stack

[source,options="nowrap"]
----
/subsystem=jgroups/stack=mystack/transport=TRANSPORT:add(type=<type>, socket-binding=<socketbinding>)
----

[source,options="nowrap"]
----
/subsystem=jgroups/stack=mystack:add-protocol(type=<type>, socket-binding=<socketbinding>)
----

[[add-a-property-to-a-protocol]]
=== Add a property to a protocol

[source,options="nowrap"]
----
/subsystem=jgroups/stack=mystack/transport=TRANSPORT/property=<property>:add(value=<value>)
----
