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<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2016</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
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    </legalnotice>

    <title>pool</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>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
      <seealso marker="slave"><c>slave(3)</c></seealso> 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"/>
      <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"/>
      <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"/>
      <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"/>
      <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"/>
      <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"/>
      <name name="start" arity="2"/>
      <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 <seealso marker="#files">Files</seealso>. The
          startup procedure fails if the file is not found.</p>
        <p>The slave nodes are started with
          <seealso marker="slave#start/2"><c>slave:start/2,3</c></seealso>,
          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"/>
      <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
      <seealso marker="kernel:net_adm#host_file/0">
      <c>net_adm:host_file/0</c></seealso>.</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>

