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<erlref>
  <header>
    <copyright>
      <year>1997</year><year>2020</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
    <legalnotice>
      Licensed under the Apache License, Version 2.0 (the "License");
      you may not use this file except in compliance with the License.
      You may obtain a copy of the License at
 
          http://www.apache.org/licenses/LICENSE-2.0

      Unless required by applicable law or agreed to in writing, software
      distributed under the License is distributed on an "AS IS" BASIS,
      WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
      See the License for the specific language governing permissions and
      limitations under the License.
    
    </legalnotice>

    <title>pg2</title>
    <prepared>mbj@erlang.ericsson.se</prepared>
    <responsible>mbj@erlang.ericsson.se</responsible>
    <docno></docno>
    <approved>Bjarne D&auml;cker</approved>
    <checked>mbj@erlang.ericsson.se</checked>
    <date>1997-08-18</date>
    <rev>A2</rev>
    <file>pg2.xml</file>
  </header>
  <module since="">pg2</module>
  <modulesummary>Distributed named process groups.</modulesummary>
  <description>
    <warning>
      <p>
	The <c>pg2</c> module is deprecated as of OTP 23 and scheduled
	for removal in OTP 24. You are advised to replace the usage of
	<c>pg2</c> with <seeerl marker="kernel:pg">pg</seeerl>.
	<c>pg</c> has a similar API, but with an implementation that
	is more scalable. See the documentation of <c>pg</c> for more
	information about differences.
      </p>
    </warning>
    <p>This module implements process groups. Each message can be sent
      to one, some, or all group members.</p>
    <p>A group of processes can be accessed by a common name. For
      example, if there is a group named <c>foobar</c>, there can be a
      set of processes (which can be located on different nodes) that
      are all members of the group <c>foobar</c>. There are no special
      functions for sending a message to the group. Instead, client
      functions are to be written with the functions
      <seemfa marker="#get_members/1"><c>get_members/1</c></seemfa> and
      <seemfa marker="#get_local_members/1"><c>get_local_members/1</c></seemfa>
      to determine which processes are members of the group.
      Then the message can be sent to one or more group members.</p>
    <p>If a member terminates, it is automatically removed from the group.</p>
    <warning>
      <p>This module is used by module
        <seeerl marker="disk_log"><c>disk_log</c></seeerl> for
        managing distributed disk logs. The disk log names are used as
        group names, which means that some action can be needed
        to avoid name clashes.</p>
    </warning>
  </description>

  <datatypes>
    <datatype>
      <name name="name"/>
      <desc><p>The name of a process group.</p></desc>
    </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="create" arity="1" since=""/>
      <fsummary>Create a new, empty process group.</fsummary>
      <desc>
        <p>Creates a new, empty process group. The group is globally
          visible on all nodes. If the group exists, nothing happens.</p>
      </desc>
    </func>

    <func>
      <name name="delete" arity="1" since=""/>
      <fsummary>Delete a process group.</fsummary>
      <desc>
        <p>Deletes a process group.</p>
      </desc>
    </func>

    <func>
      <name name="get_closest_pid" arity="1" since=""/>
      <fsummary>Common dispatch function.</fsummary>
      <desc>
        <p>A useful dispatch function that can be used from
          client functions. It returns a process on the local node, if
          such a process exists. Otherwise, it selects one randomly.</p>
      </desc>
    </func>

    <func>
      <name name="get_local_members" arity="1" since=""/>
      <fsummary>Return all local processes in a group.</fsummary>
      <desc>
        <p>Returns all processes running on the local node in the
          group <c>Name</c>. This function is to be used from
          within a client function that accesses the group. It is therefore
          optimized for speed.</p>
      </desc>
    </func>

    <func>
      <name name="get_members" arity="1" since=""/>
      <fsummary>Return all processes in a group.</fsummary>
      <desc>
        <p>Returns all processes in the group <c>Name</c>. This
          function is to be used from within a client function that
          accesses the group. It is therefore optimized for speed.</p>
      </desc>
    </func>

    <func>
      <name name="join" arity="2" since=""/>
      <fsummary>Join a process to a group.</fsummary>
      <desc>
        <p>Joins the process <c>Pid</c> to the group <c>Name</c>.
	  A process can join a group many times and must then
	  leave the group the same number of times.</p>
      </desc>
    </func>

    <func>
      <name name="leave" arity="2" since=""/>
      <fsummary>Make a process leave a group.</fsummary>
      <desc>
        <p>Makes the process <c>Pid</c> leave the group <c>Name</c>.
	  If the process is not a member of the group, <c>ok</c> is
	  returned.</p>
      </desc>
    </func>

    <func>
      <name name="start" arity="0" since=""/>
      <name name="start_link" arity="0" since=""/>
      <fsummary>Start the <c>pg2</c> server.</fsummary>
      <desc>
        <p>Starts the <c>pg2</c> server. Normally, the server does not need
          to be started explicitly, as it is started dynamically if it
          is needed. This is useful during development, but in a
          target system the server is to be started explicitly. Use the
          configuration parameters for
          <seeapp marker="kernel_app#start_pg2"><c>kernel(6)</c></seeapp>
	  for this.</p>
      </desc>
    </func>

    <func>
      <name name="which_groups" arity="0" since=""/>
      <fsummary>Return a list of all known groups.</fsummary>
      <desc>
        <p>Returns a list of all known groups.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <title>See Also</title>
    <p><seeapp marker="kernel_app"><c>kernel(6)</c></seeapp></p>
  </section>
</erlref>

