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- <title>Stopping and Restarting the Server</title>
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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/mpm_common.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/mpm_common.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" -->
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