path: root/lib/stdlib/doc/src/pool.xml

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  <header>
    <copyright>
      <year>1996</year><year>2013</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
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      Licensed under the Apache License, Version 2.0 (the "License");
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    <title>pool</title>
    <prepared></prepared>
    <docno></docno>
    <date></date>
    <rev></rev>
  </header>
  <module>pool</module>
  <modulesummary>Load Distribution Facility</modulesummary>
  <description>
    <p><c>pool</c> 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 <c>slave</c> module. This
      effects, tty IO, file IO, and code loading.</p>
    <p>If the master node fails, the entire pool will exit.</p>
  </description>
  <funcs>
    <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> below. The
          start-up procedure fails if the file is not found.</p>
        <p>The slave nodes are started with <c>slave:start/2,3</c>,
          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. See
          <seealso marker="slave#start/2">slave(3)</seealso>.</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, as well as
          all the system servers, are running when it returns a value.</p>
      </desc>
    </func>
    <func>
      <name name="attach" arity="1"/>
      <fsummary>Ensure that a pool master is running</fsummary>
      <desc>
        <p>This function 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="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>
    <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 which 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>Spawn links a process on the pool node which is expected to
          have the lowest future load.</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>
  </funcs>

  <section>
    <marker id="files"></marker>
    <title>Files</title>
    <p><c>.hosts.erlang</c> is used to pick hosts where nodes can
      be started. See
      <seealso marker="kernel:net_adm#host_file/0">net_adm(3)</seealso>
      for information about format and location of this file.</p>
    <p><c>$HOME/.erlang.slave.out.HOST</c> is used for all additional IO
      that may come from the slave nodes on standard IO. If the start-up
      procedure does not work, this file may indicate the reason.</p>
  </section>
</erlref>