path: root/lib/kernel/doc/src/global_group.xml

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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
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      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 Vierum&auml;ki</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>&nbsp;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>&nbsp;Node = node()</v>
        <v>&nbsp;GroupName = atom()</v>
        <v>Names = [Name]</v>
        <v>&nbsp;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>&nbsp;Node = node()</v>
        <v>&nbsp;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>&nbsp;Node = node()</v>
        <v>&nbsp;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>