docs/manual/stopping.xml

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<manualpage metafile="stopping.xml.meta">

  <title>Stopping and Restarting Apache HTTP Server</title>

<summary>
    <p>This document covers stopping and restarting Apache HTTP Server on
    Unix-like systems. Windows NT, 2000 and XP users should see
    <a href="platform/windows.html#winsvc">Running httpd as a
    Service</a> and Windows 9x and ME users should see <a
    href="platform/windows.html#wincons">Running httpd as a
    Console Application</a> for information on how to control
    httpd on those platforms.</p>
</summary>

<seealso><program>httpd</program></seealso>
<seealso><program>apachectl</program></seealso>
<seealso><a href="invoking.html">Starting</a></seealso>

<section id="introduction"><title>Introduction</title>

    <p>In order to stop or restart the Apache HTTP Server, you must send a signal to
    the running <program>httpd</program> processes.  There are two ways to
    send the signals.  First, you can use the unix <code>kill</code>
    command to directly send signals to the processes. You will
    notice many <program>httpd</program> executables running on your system,
    but you should not send signals to any of them except the parent,
    whose pid is in the <directive
    module="mpm_common">PidFile</directive>. That is to say you
    shouldn't ever need to send signals to any process except the
    parent. There are four signals that you can send the parent:
    <code><a href="#term">TERM</a></code>,
    <code><a href="#graceful">USR1</a></code>,
    <code><a href="#hup">HUP</a></code>, and
    <code><a href="#gracefulstop">WINCH</a></code>, which
    will be described in a moment.</p>

    <p>To send a signal to the parent you should issue a command
    such as:</p>

<example>kill -TERM `cat /usr/local/apache2/logs/httpd.pid`</example>

    <p>The second method of signaling the <program>httpd</program> processes
    is to use the <code>-k</code> command line options: <code>stop</code>,
    <code>restart</code>, <code>graceful</code> and <code>graceful-stop</code>,
    as described below.  These are arguments to the <program>
    httpd</program> binary, but we recommend that
    you send them using the <program>apachectl</program> control script, which
    will pass them through to <program>httpd</program>.</p>

    <p>After you have signaled <program>httpd</program>, you can read about
    its progress by issuing:</p>

<example>tail -f /usr/local/apache2/logs/error_log</example>

    <p>Modify those examples to match your <directive
    module="core">ServerRoot</directive> and <directive
    module="mpm_common">PidFile</directive> settings.</p>
</section>

<section id="term"><title>Stop Now</title>

<dl><dt>Signal: TERM</dt>
<dd><code>apachectl -k stop</code></dd>
</dl>

    <p>Sending the <code>TERM</code> or <code>stop</code> signal to
    the parent causes it to immediately attempt to kill off all of its
    children. It may take it several seconds to complete killing off
    its children.  Then the parent itself exits. Any requests in
    progress are terminated, and no further requests are served.</p>
</section>

<section id="graceful"><title>Graceful Restart</title>

<dl><dt>Signal: USR1</dt>
<dd><code>apachectl -k graceful</code></dd>
</dl>

    <p>The <code>USR1</code> or <code>graceful</code> signal causes
    the parent process to <em>advise</em> the children to exit after
    their current request (or to exit immediately if they're not
    serving anything). The parent re-reads its configuration files and
    re-opens its log files. As each child dies off the parent replaces
    it with a child from the new <em>generation</em> of the
    configuration, which begins serving new requests immediately.</p>

    <p>This code is designed to always respect the process control
    directive of the MPMs, so the number of processes and threads
    available to serve clients will be maintained at the appropriate
    values throughout the restart process.  Furthermore, it respects
    <directive module="mpm_common">StartServers</directive> in the
    following manner: if after one second at least <directive
    module="mpm_common">StartServers</directive> new children have not
    been created, then create enough to pick up the slack. Hence the
    code tries to maintain both the number of children appropriate for
    the current load on the server, and respect your wishes with the
    <directive module="mpm_common">StartServers</directive>
    parameter.</p>

    <p>Users of <module>mod_status</module>
    will notice that the server statistics are <strong>not</strong>
    set to zero when a <code>USR1</code> is sent. The code was
    written to both minimize the time in which the server is unable
    to serve new requests (they will be queued up by the operating
    system, so they're not lost in any event) and to respect your
    tuning parameters. In order to do this it has to keep the
    <em>scoreboard</em> used to keep track of all children across
    generations.</p>

    <p>The status module will also use a <code>G</code> to indicate
    those children which are still serving requests started before
    the graceful restart was given.</p>

    <p>At present there is no way for a log rotation script using
    <code>USR1</code> to know for certain that all children writing
    the pre-restart log have finished. We suggest that you use a
    suitable delay after sending the <code>USR1</code> signal
    before you do anything with the old log. For example if most of
    your hits take less than 10 minutes to complete for users on
    low bandwidth links then you could wait 15 minutes before doing
    anything with the old log.</p>

    <note>
    <p>When you issue a restart, a syntax check is first run, to
    ensure that there are no errors in the configuration files.
    If your configuration file has errors in it, you will get an
    error message about that syntax error, and the server will refuse to
    restart. This avoids the situation where the server halts and then
    cannot restart, leaving you with a non-functioning server.</p>

    <p>This still will not
    guarantee that the server will restart correctly. To check the
    semantics of the configuration files as well as the syntax, you
    can try starting <program>httpd</program> as a non-root user. If there
    are no errors it will attempt to open its sockets and logs and fail
    because it's not root (or because the currently running
    <program>httpd</program> already has those ports bound). If it fails
    for any other reason then it's probably a config file error and the error
    should be fixed before issuing the graceful restart.</p></note>
</section>

<section id="hup"><title>Restart Now</title>

<dl><dt>Signal: HUP</dt>
<dd><code>apachectl -k restart</code></dd>
</dl>

    <p>Sending the <code>HUP</code> or <code>restart</code> signal to
    the parent causes it to kill off its children like in
    <code>TERM</code>, but the parent doesn't exit. It re-reads its
    configuration files, and re-opens any log files. Then it spawns a
    new set of children and continues serving hits.</p>

    <p>Users of <module>mod_status</module>
    will notice that the server statistics are set to zero when a
    <code>HUP</code> is sent.</p>

<note>As with a graceful restart, a syntax check is run before the
restart is attempted. If your configuration file has errors in it, the
restart will not be attempted, and you will receive notification of the
syntax error(s).</note>
</section>

<section id="gracefulstop"><title>Graceful Stop</title>

<dl><dt>Signal: WINCH</dt>
<dd><code>apachectl -k graceful-stop</code></dd>
</dl>

    <p>The <code>WINCH</code> or <code>graceful-stop</code> signal causes
    the parent process to <em>advise</em> the children to exit after
    their current request (or to exit immediately if they're not
    serving anything). The parent will then remove its <directive
    module="mpm_common">PidFile</directive> and cease listening on
    all ports. The parent will continue to run, and monitor children
    which are handling requests. Once all children have finalized
    and exited or the timeout specified by the <directive
    module="mpm_common">GracefulShutdownTimeout</directive> has been
    reached, the parent will also exit.  If the timeout is reached,
    any remaining children will be sent the <code>TERM</code> signal
    to force them to exit.</p>

    <p>A <code>TERM</code> signal will immediately terminate the
    parent process and all children when in the "graceful" state. However
    as the <directive module="mpm_common">PidFile</directive> will
    have been removed, you will not be able to use
    <code>apachectl</code> or <code>httpd</code> to send this signal.</p>

    <note><p>The <code>graceful-stop</code> signal allows you to run multiple
    identically configured instances of <program>httpd</program> at the
    same time. This is a powerful feature when performing graceful
    upgrades of httpd, however it can also cause deadlocks and race
    conditions with some configurations.</p>

    <p>Care has been taken to ensure that on-disk files such as lock files
    (<directive module="core">Mutex</directive>) and Unix socket files
    (<directive module="mod_cgid">ScriptSock</directive>) contain the server
    PID, and should coexist without problem. However, if a configuration
    directive, third-party module or persistent CGI utilizes any other on-disk
    lock or  state files, care should be taken to ensure that multiple running
    instances of <program>httpd</program> do not clobber each other's files.</p>

    <p>You should also be wary of other potential race conditions, such as
    using <program>rotatelogs</program> style piped logging. Multiple running
    instances of <program>rotatelogs</program> attempting to rotate the same
    logfiles at the same time may destroy each other's logfiles.</p></note>
</section>

</manualpage>