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<erlref>
  <header>
    <copyright>
      <year>1996</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
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    </legalnotice>

    <title>pool</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module since="">pool</module>
  <modulesummary>Load distribution facility.</modulesummary>
  <description>
    <p>This module can be used to run a set of Erlang nodes as a pool
      of computational processors. It is organized as a master and a
      set of slave nodes and includes the following features:</p>

    <list type="bulleted">
      <item>The slave nodes send regular reports to the master about
        their current load.</item>
      <item>Queries can be sent to the master to determine which node
        will have the least load.</item>
    </list>

    <p>The BIF <c>statistics(run_queue)</c> is used for estimating
      future loads. It returns the length of the queue of ready to run
      processes in the Erlang runtime system.</p>

    <p>The slave nodes are started with the
      <seeerl marker="slave"><c>slave(3)</c></seeerl> module. This
      effects terminal I/O, file I/O, and code loading.</p>
    <p>If the master node fails, the entire pool exits.</p>
  </description>

  <funcs>
    <func>
      <name name="attach" arity="1" since=""/>
      <fsummary>Ensure that a pool master is running.</fsummary>
      <desc>
        <p>Ensures that a pool master is running and includes
          <c><anno>Node</anno></c> in the pool master's pool of nodes.</p>
      </desc>
    </func>

    <func>
      <name name="get_node" arity="0" since=""/>
      <fsummary>Return the node with the expected lowest future load.</fsummary>
      <desc>
        <p>Returns the node with the expected lowest future load.</p>
      </desc>
    </func>

    <func>
      <name name="get_nodes" arity="0" since=""/>
      <fsummary>Return a list of the current member nodes of the pool.
      </fsummary>
      <desc>
        <p>Returns a list of the current member nodes of the pool.</p>
      </desc>
    </func>

    <func>
      <name name="pspawn" arity="3" since=""/>
      <fsummary>Spawn a process on the pool node with expected lowest future
        load.</fsummary>
      <desc>
        <p>Spawns a process on the pool node that is expected to have
          the lowest future load.</p>
      </desc>
    </func>

    <func>
      <name name="pspawn_link" arity="3" since=""/>
      <fsummary>Spawn and link to a process on the pool node with expected
        lowest future load.</fsummary>
      <desc>
        <p>Spawns and links to a process on the pool node that is expected to
          have the lowest future load.</p>
      </desc>
    </func>

    <func>
      <name name="start" arity="1" since=""/>
      <name name="start" arity="2" since=""/>
      <fsummary>>Start a new pool.</fsummary>
      <desc>
        <p>Starts a new pool. The file <c>.hosts.erlang</c> is read to
          find host names where the pool nodes can be started; see
          section <seeerl marker="#files">Files</seeerl>. The
          startup procedure fails if the file is not found.</p>
        <p>The slave nodes are started with
          <seemfa marker="slave#start/2"><c>slave:start/2,3</c></seemfa>,
          passing along <c><anno>Name</anno></c> and, if provided,
          <c><anno>Args</anno></c>. <c><anno>Name</anno></c> is used as the
          first part of the node names, <c><anno>Args</anno></c> is used to
          specify command-line arguments.</p>
        <p>Access rights must be set so that all nodes in the pool have
          the authority to access each other.</p>
        <p>The function is synchronous and all the nodes, and
          all the system servers, are running when it returns a value.</p>
      </desc>
    </func>

    <func>
      <name name="stop" arity="0" since=""/>
      <fsummary>Stop the pool and kill all the slave nodes.</fsummary>
      <desc>
        <p>Stops the pool and kills all the slave nodes.</p>
      </desc>
    </func>
  </funcs>

  <section>
    <marker id="files"></marker>
    <title>Files</title>
    <p><c>.hosts.erlang</c> is used to pick hosts where nodes can
      be started. For information about format and location of this file, see
      <seemfa marker="kernel:net_adm#host_file/0">
      <c>net_adm:host_file/0</c></seemfa>.</p>
    <p><c>$HOME/.erlang.slave.out.HOST</c> is used for all extra I/O
      that can come from the slave nodes on standard I/O. If the startup
      procedure does not work, this file can indicate the reason.</p>
  </section>
</erlref>

