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<header>
<copyright>
<year>1998</year><year>2009</year>
<holder>Ericsson AB. All Rights Reserved.</holder>
</copyright>
<legalnotice>
The contents of this file are subject to the Erlang Public License,
Version 1.1, (the "License"); you may not use this file except in
compliance with the License. You should have received a copy of the
Erlang Public License along with this software. If not, it can be
retrieved online at http://www.erlang.org/.
Software distributed under the License is distributed on an "AS IS"
basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
the License for the specific language governing rights and limitations
under the License.
</legalnotice>
<title>global_group</title>
<prepared>Esko Vierumäki</prepared>
<docno></docno>
<date>1998-12-18</date>
<rev>b</rev>
</header>
<module>global_group</module>
<modulesummary>Grouping Nodes to Global Name Registration Groups</modulesummary>
<description>
<p>The global group function makes it possible to group the nodes
in a system into partitions, each partition having its own global
name space, refer to <c>global(3)</c>. These partitions are
called global groups.</p>
<p>The main advantage of dividing systems to global groups is that
the background load decreases while the number of nodes to be
updated is reduced when manipulating globally registered names.</p>
<p>The Kernel configuration parameter <c>global_groups</c> defines
the global groups (see also
<seealso marker="kernel_app">kernel(6)</seealso>,
<seealso marker="config">config(4)</seealso>:</p>
<code type="none">
{global_groups, [GroupTuple]}</code>
<p>Types:</p>
<list type="bulleted">
<item><c>GroupTuple = {GroupName, [Node]} | {GroupName, PublishType, [Node]}</c></item>
<item><c>GroupName = atom()</c> (naming a global group)</item>
<item><c>PublishType = normal | hidden</c></item>
<item><c>Node = atom()</c> (naming a node)</item>
</list>
<p>A <c>GroupTuple</c> without <c>PublishType</c> is the same as a
<c>GroupTuple</c> with <c>PublishType == normal</c>.</p>
<p>A node started with the command line flag <c>-hidden</c>, see
<seealso marker="erts:erl">erl(1)</seealso>, is said to be a
<em>hidden</em> node. A hidden node will establish hidden
connections to nodes not part of the same global group, but
normal (visible) connections to nodes part of the same global
group.</p>
<p>A global group defined with <c>PublishType == hidden</c>, is
said to be a hidden global group. All nodes in a hidden global
group are hidden nodes, regardless if they are started with
the <c>-hidden</c> command line flag or not.</p>
<p>For the processes and nodes to run smoothly using the global
group functionality, the following criteria must be met:</p>
<list type="bulleted">
<item>
<p>An instance of the global group server, <c>global_group</c>,
must be running on each node. The processes are automatically
started and synchronized when a node is started.</p>
</item>
<item>
<p>All involved nodes must agree on the global group definition,
or the behavior of the system is undefined.</p>
</item>
<item>
<p><em>All</em> nodes in the system should belong to exactly
one global group.</p>
</item>
</list>
<p>In the following description, a <em>group node</em> is a node
belonging to the same global group as the local node.</p>
</description>
<funcs>
<func>
<name>global_groups() -> {GroupName, GroupNames} | undefined</name>
<fsummary>Return the global group names</fsummary>
<type>
<v>GroupName = atom()</v>
<v>GroupNames = [GroupName]</v>
</type>
<desc>
<p>Returns a tuple containing the name of the global group
the local node belongs to, and the list of all other known
group names. Returns <c>undefined</c> if no global groups are
defined.</p>
</desc>
</func>
<func>
<name>info() -> [{Item, Info}]</name>
<fsummary>Information about global groups</fsummary>
<type>
<v>Item, Info -- see below</v>
</type>
<desc>
<p>Returns a list containing information about the global
groups. Each element of the list is a tuple. The order of
the tuples is not defined.</p>
<taglist>
<tag><c>{state, State}</c></tag>
<item>
<p>If the local node is part of a global group,
<c>State == synced</c>. If no global groups are defined,
<c>State == no_conf</c>.</p>
</item>
<tag><c>{own_group_name, GroupName}</c></tag>
<item>
<p>The name (atom) of the group that the local node belongs
to.</p>
</item>
<tag><c>{own_group_nodes, Nodes}</c></tag>
<item>
<p>A list of node names (atoms), the group nodes.</p>
</item>
<tag><c>{synced_nodes, Nodes}</c></tag>
<item>
<p>A list of node names, the group nodes currently
synchronized with the local node.</p>
</item>
<tag><c>{sync_error, Nodes}</c></tag>
<item>
<p>A list of node names, the group nodes with which
the local node has failed to synchronize.</p>
</item>
<tag><c>{no_contact, Nodes}</c></tag>
<item>
<p>A list of node names, the group nodes to which there are
currently no connections.</p>
</item>
<tag><c>{other_groups, Groups}</c></tag>
<item>
<p><c>Groups</c> is a list of tuples
<c>{GroupName, Nodes}</c>, specifying the name and nodes
of the other global groups.</p>
</item>
<tag><c>{monitoring, Pids}</c></tag>
<item>
<p>A list of pids, specifying the processes which have
subscribed to <c>nodeup</c> and <c>nodedown</c> messages.</p>
</item>
</taglist>
</desc>
</func>
<func>
<name>monitor_nodes(Flag) -> ok </name>
<fsummary>Subscribe to node status changes</fsummary>
<type>
<v>Flag = bool()</v>
</type>
<desc>
<p>Depending on <c>Flag</c>, the calling process starts
subscribing (<c>Flag == true</c>) or stops subscribing
(<c>Flag == false</c>) to node status change messages.</p>
<p>A process which has subscribed will receive the messages
<c>{nodeup, Node}</c> and <c>{nodedown, Node}</c> when a
group node connects or disconnects, respectively.</p>
</desc>
</func>
<func>
<name>own_nodes() -> Nodes</name>
<fsummary>Return the group nodes</fsummary>
<type>
<v>Nodes = [Node]</v>
<v> Node = node()</v>
</type>
<desc>
<p>Returns the names of all group nodes, regardless of their
current status.</p>
</desc>
</func>
<func>
<name>registered_names(Where) -> Names</name>
<fsummary>Return globally registered names</fsummary>
<type>
<v>Where = {node, Node} | {group, GroupName}</v>
<v> Node = node()</v>
<v> GroupName = atom()</v>
<v>Names = [Name]</v>
<v> Name = atom()</v>
</type>
<desc>
<p>Returns a list of all names which are globally registered
on the specified node or in the specified global group.</p>
</desc>
</func>
<func>
<name>send(Name, Msg) -> pid() | {badarg, {Name, Msg}}</name>
<name>send(Where, Name, Msg) -> pid() | {badarg, {Name, Msg}}</name>
<fsummary>Send a message to a globally registered pid</fsummary>
<type>
<v>Where = {node, Node} | {group, GroupName}</v>
<v> Node = node()</v>
<v> GroupName = atom()</v>
<v>Name = atom()</v>
<v>Msg = term()</v>
</type>
<desc>
<p>Searches for <c>Name</c>, globally registered on
the specified node or in the specified global group, or --
if the <c>Where</c> argument is not provided -- in any global
group. The global groups are searched in the order in which
they appear in the value of the <c>global_groups</c>
configuration parameter.</p>
<p>If <c>Name</c> is found, the message <c>Msg</c> is sent to
the corresponding pid. The pid is also the return value of
the function. If the name is not found, the function returns
<c>{badarg, {Name, Msg}}</c>.</p>
</desc>
</func>
<func>
<name>sync() -> ok</name>
<fsummary>Synchronize the group nodes</fsummary>
<desc>
<p>Synchronizes the group nodes, that is, the global name
servers on the group nodes. Also check the names globally
registered in the current global group and unregisters them
on any known node not part of the group.</p>
<p>If synchronization is not possible, an error report is sent
to the error logger (see also <c>error_logger(3)</c>).</p>
<p>Failure:
<c>{error, {'invalid global_groups definition', Bad}}</c> if
the <c>global_groups</c> configuration parameter has an
invalid value <c>Bad</c>.</p>
</desc>
</func>
<func>
<name>whereis_name(Name) -> pid() | undefined</name>
<name>whereis_name(Where, Name) -> pid() | undefined</name>
<fsummary>Get the pid with a given globally registered name</fsummary>
<type>
<v>Where = {node, Node} | {group, GroupName}</v>
<v> Node = node()</v>
<v> GroupName = atom()</v>
<v>Name = atom()</v>
</type>
<desc>
<p>Searches for <c>Name</c>, globally registered on
the specified node or in the specified global group, or -- if
the <c>Where</c> argument is not provided -- in any global
group. The global groups are searched in the order in which
they appear in the value of the <c>global_groups</c>
configuration parameter.</p>
<p>If <c>Name</c> is found, the corresponding pid is returned.
If the name is not found, the function returns
<c>undefined</c>.</p>
</desc>
</func>
</funcs>
<section>
<title>NOTE</title>
<p>In the situation where a node has lost its connections to other
nodes in its global group, but has connections to nodes in other
global groups, a request from another global group may produce an
incorrect or misleading result. For example, the isolated node may
not have accurate information about registered names in its
global group.</p>
<p>Note also that the <c>send/2,3</c> function is not secure.</p>
<p>Distribution of applications is highly dependent of the global
group definitions. It is not recommended that an application is
distributed over several global groups of the obvious reason that
the registered names may be moved to another global group at
failover/takeover. There is nothing preventing doing this, but
the application code must in such case handle the situation.</p>
</section>
<section>
<title>SEE ALSO</title>
<p><seealso marker="erts:erl">erl(1)</seealso>,
<seealso marker="global">global(3)</seealso></p>
</section>
</erlref>
