docs/manual/stopping.html.en - httpd - Git at Google

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     <h1 align="CENTER">Stopping and Restarting the Server</h1>

     <p>This document covers stopping and restarting Apache on
     Unix-like systems. Windows users should see <a
     href="platform/windows.html#signal">Signalling Apache when
     running</a>.</p>

     <p>You will notice many <code>httpd</code> executables running
     on your system, but you should not send signals to any of them
     except the parent, whose pid is in the <a
     href="mod/mpm_common.html#pidfile">PidFile</a>. That is to say you
     shouldn't ever need to send signals to any process except the
     parent. There are three signals that you can send the parent:
     <code>TERM</code>, <code>HUP</code>, and <code>USR1</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>

     <blockquote>
 <pre>
     kill -TERM `cat /usr/local/apache/logs/httpd.pid`
 </pre>
     </blockquote>
     You can read about its progress by issuing:

     <blockquote>
 <pre>
     tail -f /usr/local/apache/logs/error_log
 </pre>
     </blockquote>
     Modify those examples to match your <a
     href="mod/core.html#serverroot">ServerRoot</a> and <a
     href="mod/mpm_common.html#pidfile">PidFile</a> settings.

     <p>A shell script called <a
     href="programs/apachectl.html">apachectl</a> is provided which
     automates the processing of signalling Apache. For details
     about this script, see the documentation on <a
     href="invoking.html">starting Apache</a>.</p>

     <h3>Stop Now</h3>

     <p><strong>Signal:</strong> TERM<br />
      <code>apachectl stop</code></p>

     <p>Sending the <code>TERM</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>

     <h3>Graceful Restart</h3>

     <p><strong>Signal:</strong> USR1<br />
      <code>apachectl graceful</code></p>

     <p>The <code>USR1</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>
     <i>On certain platforms that do not allow USR1 to be used for a
     graceful restart, an alternative signal may be used (such as
     WINCH). apachectl graceful will send the right signal for your
     platform.</i>

     <p>This code is designed to always respect the <a
     href="mod/mpm_common.html#maxclients">MaxClients</a>, <a
     href="mod/prefork.html#minspareservers">MinSpareServers</a>,
     and <a
     href="mod/prefork.html#maxspareservers">MaxSpareServers</a>
     settings. Furthermore, it respects <a
     href="mod/mpm_common.html#startservers">StartServers</a> in the
     following manner: if after one second at least StartServers new
     children have not been created, then create enough to pick up
     the slack. This is to say that the code tries to maintain both
     the number of children appropriate for the current load on the
     server, and respect your wishes with the StartServers
     parameter.</p>

     <p>Users of the <a href="mod/mod_status.html">status module</a>
     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>

     <p><strong>Note:</strong> If your configuration file has errors
     in it when you issue a restart then your parent will not
     restart, it will exit with an error. In the case of graceful
     restarts it will also leave children running when it exits.
     (These are the children which are "gracefully exiting" by
     handling their last request.) This will cause problems if you
     attempt to restart the server -- it will not be able to bind to
     its listening ports. Before doing a restart, you can check the
     syntax of the configuration files with the <code>-t</code>
     command line argument (see <a
     href="programs/httpd.html">httpd</a>). 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 httpd 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 httpd
     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>

     <h3>Restart Now</h3>

     <p><strong>Signal:</strong> HUP<br />
      <code>apachectl restart</code></p>

     <p>Sending the <code>HUP</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 the <a href="mod/mod_status.html">status module</a>
     will notice that the server statistics are set to zero when a
     <code>HUP</code> is sent.</p>

     <p><strong>Note:</strong> If your configuration file has errors
     in it when you issue a restart then your parent will not
     restart, it will exit with an error. See below for a method of
     avoiding this.</p>

     <h3>Appendix: signals and race conditions</h3>

     <p>Prior to Apache 1.2b9 there were several <em>race
     conditions</em> involving the restart and die signals (a simple
     description of race condition is: a time-sensitive problem, as
     in if something happens at just the wrong time it won't behave
     as expected). For those architectures that have the "right"
     feature set we have eliminated as many as we can. But it should
     be noted that there still do exist race conditions on certain
     architectures.</p>

     <p>Architectures that use an on disk <a
     href="mod/core.html#scoreboardfile">ScoreBoardFile</a> have the
     potential to corrupt their scoreboards. This can result in the
     "bind: Address already in use" (after <code>HUP</code>) or
     "long lost child came home!" (after <code>USR1</code>). The
     former is a fatal error, while the latter just causes the
     server to lose a scoreboard slot. So it might be advisable to
     use graceful restarts, with an occasional hard restart. These
     problems are very difficult to work around, but fortunately
     most architectures do not require a scoreboard file. See the <a
     href="mod/core.html#scoreboardfile">ScoreBoardFile</a>
     documentation for a architecture uses it.</p>

     <p><code>NEXT</code> and <code>MACHTEN</code> (68k only) have
     small race conditions which can cause a restart/die signal to
     be lost, but should not cause the server to do anything
     otherwise problematic.
     <!-- they don't have sigaction, or we're not using it -djg -->
     </p>

     <p>All architectures have a small race condition in each child
     involving the second and subsequent requests on a persistent
     HTTP connection (KeepAlive). It may exit after reading the
     request line but before reading any of the request headers.
     There is a fix that was discovered too late to make 1.2. In
     theory this isn't an issue because the KeepAlive client has to
     expect these events because of network latencies and server
     timeouts. In practice it doesn't seem to affect anything either
     -- in a test case the server was restarted twenty times per
     second and clients successfully browsed the site without
     getting broken images or empty documents.
     <!--#include virtual="footer.html" -->
     </p>
   </body>
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	<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
	"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

	<html xmlns="http://www.w3.org/1999/xhtml">
	<head>
	<meta name="generator" content="HTML Tidy, see www.w3.org" />

	<title>Stopping and Restarting the Server</title>
	</head>
	<!-- Background white, links blue (unvisited), navy (visited), red (active) -->

	<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
	vlink="#000080" alink="#FF0000">
	<!--#include virtual="header.html" -->

	<h1 align="CENTER">Stopping and Restarting the Server</h1>

	<p>This document covers stopping and restarting Apache on
	Unix-like systems. Windows users should see <a
	href="platform/windows.html#signal">Signalling Apache when
	running</a>.</p>

	<p>You will notice many <code>httpd</code> executables running
	on your system, but you should not send signals to any of them
	except the parent, whose pid is in the <a
	href="mod/mpm_common.html#pidfile">PidFile</a>. That is to say you
	shouldn't ever need to send signals to any process except the
	parent. There are three signals that you can send the parent:
	<code>TERM</code>, <code>HUP</code>, and <code>USR1</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>

	<blockquote>
	<pre>
	kill -TERM `cat /usr/local/apache/logs/httpd.pid`
	</pre>
	</blockquote>
	You can read about its progress by issuing:

	<blockquote>
	<pre>
	tail -f /usr/local/apache/logs/error_log
	</pre>
	</blockquote>
	Modify those examples to match your <a
	href="mod/core.html#serverroot">ServerRoot</a> and <a
	href="mod/mpm_common.html#pidfile">PidFile</a> settings.

	<p>A shell script called <a
	href="programs/apachectl.html">apachectl</a> is provided which
	automates the processing of signalling Apache. For details
	about this script, see the documentation on <a
	href="invoking.html">starting Apache</a>.</p>

	<h3>Stop Now</h3>

	<p><strong>Signal:</strong> TERM<br />
	<code>apachectl stop</code></p>

	<p>Sending the <code>TERM</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>

	<h3>Graceful Restart</h3>

	<p><strong>Signal:</strong> USR1<br />
	<code>apachectl graceful</code></p>

	<p>The <code>USR1</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>
	<i>On certain platforms that do not allow USR1 to be used for a
	graceful restart, an alternative signal may be used (such as
	WINCH). apachectl graceful will send the right signal for your
	platform.</i>

	<p>This code is designed to always respect the <a
	href="mod/mpm_common.html#maxclients">MaxClients</a>, <a
	href="mod/prefork.html#minspareservers">MinSpareServers</a>,
	and <a
	href="mod/prefork.html#maxspareservers">MaxSpareServers</a>
	settings. Furthermore, it respects <a
	href="mod/mpm_common.html#startservers">StartServers</a> in the
	following manner: if after one second at least StartServers new
	children have not been created, then create enough to pick up
	the slack. This is to say that the code tries to maintain both
	the number of children appropriate for the current load on the
	server, and respect your wishes with the StartServers
	parameter.</p>

	<p>Users of the <a href="mod/mod_status.html">status module</a>
	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>

	<p><strong>Note:</strong> If your configuration file has errors
	in it when you issue a restart then your parent will not
	restart, it will exit with an error. In the case of graceful
	restarts it will also leave children running when it exits.
	(These are the children which are "gracefully exiting" by
	handling their last request.) This will cause problems if you
	attempt to restart the server -- it will not be able to bind to
	its listening ports. Before doing a restart, you can check the
	syntax of the configuration files with the <code>-t</code>
	command line argument (see <a
	href="programs/httpd.html">httpd</a>). 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 httpd 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 httpd
	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>

	<h3>Restart Now</h3>

	<p><strong>Signal:</strong> HUP<br />
	<code>apachectl restart</code></p>

	<p>Sending the <code>HUP</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 the <a href="mod/mod_status.html">status module</a>
	will notice that the server statistics are set to zero when a
	<code>HUP</code> is sent.</p>

	<p><strong>Note:</strong> If your configuration file has errors
	in it when you issue a restart then your parent will not
	restart, it will exit with an error. See below for a method of
	avoiding this.</p>

	<h3>Appendix: signals and race conditions</h3>

	<p>Prior to Apache 1.2b9 there were several <em>race
	conditions</em> involving the restart and die signals (a simple
	description of race condition is: a time-sensitive problem, as
	in if something happens at just the wrong time it won't behave
	as expected). For those architectures that have the "right"
	feature set we have eliminated as many as we can. But it should
	be noted that there still do exist race conditions on certain
	architectures.</p>

	<p>Architectures that use an on disk <a
	href="mod/core.html#scoreboardfile">ScoreBoardFile</a> have the
	potential to corrupt their scoreboards. This can result in the
	"bind: Address already in use" (after <code>HUP</code>) or
	"long lost child came home!" (after <code>USR1</code>). The
	former is a fatal error, while the latter just causes the
	server to lose a scoreboard slot. So it might be advisable to
	use graceful restarts, with an occasional hard restart. These
	problems are very difficult to work around, but fortunately
	most architectures do not require a scoreboard file. See the <a
	href="mod/core.html#scoreboardfile">ScoreBoardFile</a>
	documentation for a architecture uses it.</p>

	<p><code>NEXT</code> and <code>MACHTEN</code> (68k only) have
	small race conditions which can cause a restart/die signal to
	be lost, but should not cause the server to do anything
	otherwise problematic.
	<!-- they don't have sigaction, or we're not using it -djg -->
	</p>

	<p>All architectures have a small race condition in each child
	involving the second and subsequent requests on a persistent
	HTTP connection (KeepAlive). It may exit after reading the
	request line but before reading any of the request headers.
	There is a fix that was discovered too late to make 1.2. In
	theory this isn't an issue because the KeepAlive client has to
	expect these events because of network latencies and server
	timeouts. In practice it doesn't seem to affect anything either
	-- in a test case the server was restarted twenty times per
	second and clients successfully browsed the site without
	getting broken images or empty documents.
	<!--#include virtual="footer.html" -->
	</p>
	</body>
	</html>