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<erlref>
  <header>
    <copyright>
      <year>1996</year><year>2018</year>
      <holder>Ericsson AB. All Rights Reserved.</holder>
    </copyright>
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      Licensed under the Apache License, Version 2.0 (the "License");
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    <title>heart</title>
    <prepared>Magnus Fr&ouml;berg</prepared>
    <docno></docno>
    <date>1998-01-28</date>
    <rev>A</rev>
  </header>
  <module since="">heart</module>
  <modulesummary>Heartbeat monitoring of an Erlang runtime system.</modulesummary>
  <description>
    <p>This modules contains the interface to the <c>heart</c> process.
      <c>heart</c> sends periodic heartbeats to an external port
      program, which is also named <c>heart</c>. The purpose of
      the <c>heart</c> port program is to check that the Erlang runtime system
      it is supervising is still running. If the port program has not
      received any heartbeats within <c>HEART_BEAT_TIMEOUT</c> seconds
      (defaults to 60 seconds), the system can be rebooted.</p>
    <p>An Erlang runtime system to be monitored by a heart program
      is to be started with command-line flag <c>-heart</c> (see
      also <seealso marker="erts:erl"><c>erl(1)</c></seealso>).
      The <c>heart</c> process is then started automatically:</p>
    <pre>
% <input>erl -heart ...</input></pre>
    <p>If the system is to be rebooted because of missing heartbeats,
      or a terminated Erlang runtime system, environment variable
      <c>HEART_COMMAND</c> must be set before the system is started.
      If this variable is not set, a warning text is printed but
      the system does not reboot.</p>
    <p>To reboot on Windows, <c>HEART_COMMAND</c> can be
      set to <c>heart -shutdown</c> (included in the Erlang delivery)
      or to any other suitable program that can activate a reboot.</p>
    <p>The environment variable <c>HEART_BEAT_TIMEOUT</c>
      can be used to configure the heart
      time-outs; it can be set in the operating system shell before Erlang
      is started or be specified at the command line:</p>
    <pre>
% <input>erl -heart -env HEART_BEAT_TIMEOUT 30 ...</input></pre>
    <p>The value (in seconds) must be in the range 10 &lt; X &lt;= 65535.</p>
    <p>When running on OSs lacking support for monotonic time,
      <c>heart</c> is susceptible to system clock adjustments of more than
      <c>HEART_BEAT_TIMEOUT</c> seconds. When  this happens, <c>heart</c>
      times out and tries to reboot the system. This can occur, for
      example, if the system clock is adjusted automatically by use of the
      Network Time Protocol (NTP).</p>
    <p>If a crash occurs, an <c><![CDATA[erl_crash.dump]]></c> is <em>not</em>
      written unless environment variable
        <c><![CDATA[ERL_CRASH_DUMP_SECONDS]]></c> is set:</p>
    <pre>
% <input>erl -heart -env ERL_CRASH_DUMP_SECONDS 10 ...</input></pre>
    <p>If a regular core dump is wanted, let <c>heart</c> know by setting
      the kill signal to abort using environment variable
      <c><![CDATA[HEART_KILL_SIGNAL=SIGABRT]]></c>. If unset, or not set to
      <c><![CDATA[SIGABRT]]></c>, the default behavior is a kill signal using
      <c><![CDATA[SIGKILL]]></c>:</p>
    <pre>
% <input>erl -heart -env HEART_KILL_SIGNAL SIGABRT ...</input></pre>
  <p> If heart should <em>not</em> kill the Erlang runtime system, this can be indicated
      using the environment variable <c><![CDATA[HEART_NO_KILL=TRUE]]></c>.
      This can be useful if the command executed by heart takes care of this,
      for example as part of a specific cleanup sequence. 
      If unset, or not set to <c><![CDATA[TRUE]]></c>, the default behaviour
      will be to kill as described above.
  </p>

    <pre>
% <input>erl -heart -env HEART_NO_KILL 1 ...</input></pre>

    <p>Furthermore, <c><![CDATA[ERL_CRASH_DUMP_SECONDS]]></c> has the
      following behavior on <c>heart</c>:</p>
    <taglist>
      <tag><c><![CDATA[ERL_CRASH_DUMP_SECONDS=0]]></c></tag>
      <item><p>Suppresses the writing of a crash dump file entirely,
        thus rebooting the runtime system immediately.
        This is the same as not setting the environment variable.</p>
      </item>
      <tag><c><![CDATA[ERL_CRASH_DUMP_SECONDS=-1]]></c></tag>
      <item><p>Setting the environment variable to a negative value does not
        reboot the runtime system until the crash dump file is completly
        written.</p>
      </item>
      <tag><c><![CDATA[ERL_CRASH_DUMP_SECONDS=S]]></c></tag>
      <item><p><c>heart</c> waits for <c>S</c> seconds to let the crash dump
        file be written. After <c>S</c> seconds, <c>heart</c> reboots the
        runtime system, whether the crash dump file is written or not.</p>
      </item>
    </taglist>
    <p>In the following descriptions, all functions fail with reason
      <c>badarg</c> if <c>heart</c> is not started.</p>
  </description>

  <datatypes>
      <datatype>
          <name name="heart_option"/>
      </datatype>
  </datatypes>

  <funcs>
    <func>
      <name name="set_cmd" arity="1" since=""/>
      <fsummary>Set a temporary reboot command.</fsummary>
      <desc>
        <p>Sets a temporary reboot command. This command is used if
          a <c>HEART_COMMAND</c> other than the one specified with
          the environment variable is to be used to reboot
          the system. The new Erlang runtime system uses (if it misbehaves)
          environment variable <c>HEART_COMMAND</c> to reboot.</p>
        <p>Limitations: Command string <c><anno>Cmd</anno></c> is sent to the
          <c>heart</c> program as an ISO Latin-1 or UTF-8 encoded binary,
          depending on the filename encoding mode of the emulator (see
          <seealso marker="kernel:file#native_name_encoding/0"><c>file:native_name_encoding/0</c></seealso>).
          The size of the encoded binary must be less than 2047 bytes.</p>
      </desc>
    </func>

    <func>
      <name name="clear_cmd" arity="0" since=""/>
      <fsummary>Clear the temporary boot command.</fsummary>
      <desc>
        <p>Clears the temporary boot command. If the system terminates,
          the normal <c>HEART_COMMAND</c> is used to reboot.</p>
      </desc>
    </func>

    <func>
      <name name="get_cmd" arity="0" since=""/>
      <fsummary>Get the temporary reboot command.</fsummary>
      <desc>
        <p>Gets the temporary reboot command. If the command is cleared,
          the empty string is returned.</p>
      </desc>
    </func>

    <func>
      <name name="set_callback" arity="2" since="OTP 18.3"/>
      <fsummary>Set a validation callback</fsummary>
      <desc>
          <p> This validation callback will be executed before any
          heartbeat is sent to the port program. For the validation to
          succeed it needs to return with the value <c>ok</c>.
        </p>
        <p>An exception within the callback will be treated as a validation failure.</p>
        <p>The callback will be removed if the system reboots.</p>
      </desc>
    </func>
    <func>
      <name name="clear_callback" arity="0" since="OTP 18.3"/>
      <fsummary>Clear the validation callback</fsummary>
      <desc>
          <p>Removes the validation callback call before heartbeats.</p>
      </desc>
    </func>
    <func>
      <name name="get_callback" arity="0" since="OTP 18.3"/>
      <fsummary>Get the validation callback</fsummary>
      <desc>
          <p>Get the validation callback. If the callback is cleared, <c>none</c> will be returned.</p>
      </desc>
    </func>

    <func>
        <name name="set_options" arity="1" since="OTP 18.3"/>
        <fsummary>Set a list of options</fsummary>
        <desc>
            <p> Valid options <c>set_options</c> are: </p>
            <taglist>
                <tag><c>check_schedulers</c></tag>
                <item>
                    <p>If enabled, a signal will be sent to each scheduler to check its
                        responsiveness. The system check occurs before any heartbeat sent
                        to the port program. If any scheduler is not responsive enough the
                        heart program will not receive its heartbeat and thus eventually terminate the node.
                    </p>
                </item>
            </taglist>
            <p> Returns with the value <c>ok</c> if the options are valid.</p>
        </desc>
    </func>
    <func>
      <name name="get_options" arity="0" since="OTP 18.3"/>
      <fsummary>Get the temporary reboot command</fsummary>
      <desc>
          <p>Returns <c>{ok, Options}</c> where <c>Options</c> is a list of current options enabled for heart.
              If the callback is cleared, <c>none</c> will be returned.</p>
      </desc>
    </func>


  </funcs>
</erlref>

