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in particular, the non-english files, to make sure I did not screw anything up. They look fine to me. git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@91112 13f79535-47bb-0310-9956-ffa450edef68
210 lines
9.3 KiB
HTML
210 lines
9.3 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta name="generator" content="HTML Tidy, see www.w3.org" />
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<title>Stopping and Restarting the Server</title>
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</head>
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<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
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<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
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vlink="#000080" alink="#FF0000">
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<!--#include virtual="header.html" -->
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<h1 align="CENTER">Stopping and Restarting the Server</h1>
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<p>This document covers stopping and restarting Apache on
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Unix-like systems. Windows users should see <a
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href="platform/windows.html#signal">Signalling Apache when
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running</a>.</p>
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<p>You will notice many <code>httpd</code> executables running
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on your system, but you should not send signals to any of them
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except the parent, whose pid is in the <a
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href="mod/core.html#pidfile">PidFile</a>. That is to say you
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shouldn't ever need to send signals to any process except the
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parent. There are three signals that you can send the parent:
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<code>TERM</code>, <code>HUP</code>, and <code>USR1</code>,
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which will be described in a moment.</p>
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<p>To send a signal to the parent you should issue a command
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such as:</p>
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<blockquote>
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<pre>
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kill -TERM `cat /usr/local/apache/logs/httpd.pid`
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</pre>
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</blockquote>
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You can read about its progress by issuing:
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<blockquote>
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<pre>
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tail -f /usr/local/apache/logs/error_log
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</pre>
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</blockquote>
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Modify those examples to match your <a
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href="mod/core.html#serverroot">ServerRoot</a> and <a
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href="mod/core.html#pidfile">PidFile</a> settings.
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<p>A shell script called <a
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href="programs/apachectl.html">apachectl</a> is provided which
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automates the processing of signalling Apache. For details
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about this script, see the documentation on <a
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href="invoking.html">starting Apache</a>.</p>
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<h3>Stop Now</h3>
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<p><strong>Signal:</strong> TERM<br />
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<code>apachectl stop</code></p>
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<p>Sending the <code>TERM</code> signal to the parent causes it
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to immediately attempt to kill off all of its children. It may
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take it several seconds to complete killing off its children.
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Then the parent itself exits. Any requests in progress are
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terminated, and no further requests are served.</p>
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<h3>Graceful Restart</h3>
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<p><strong>Signal:</strong> USR1<br />
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<code>apachectl graceful</code></p>
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<p>The <code>USR1</code> signal causes the parent process to
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<em>advise</em> the children to exit after their current
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request (or to exit immediately if they're not serving
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anything). The parent re-reads its configuration files and
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re-opens its log files. As each child dies off the parent
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replaces it with a child from the new <em>generation</em> of
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the configuration, which begins serving new requests
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immediately.</p>
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<i>On certain platforms that do not allow USR1 to be used for a
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graceful restart, an alternative signal may be used (such as
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WINCH). apachectl graceful will send the right signal for your
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platform.</i>
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<p>This code is designed to always respect the <a
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href="mod/mpm_common.html#maxclients">MaxClients</a>, <a
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href="mod/prefork.html#minspareservers">MinSpareServers</a>,
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and <a
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href="mod/prefork.html#maxspareservers">MaxSpareServers</a>
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settings. Furthermore, it respects <a
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href="mod/mpm_common.html#startservers">StartServers</a> in the
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following manner: if after one second at least StartServers new
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children have not been created, then create enough to pick up
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the slack. This is to say that the code tries to maintain both
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the number of children appropriate for the current load on the
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server, and respect your wishes with the StartServers
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parameter.</p>
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<p>Users of the <a href="mod/mod_status.html">status module</a>
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will notice that the server statistics are <strong>not</strong>
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set to zero when a <code>USR1</code> is sent. The code was
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written to both minimize the time in which the server is unable
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to serve new requests (they will be queued up by the operating
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system, so they're not lost in any event) and to respect your
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tuning parameters. In order to do this it has to keep the
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<em>scoreboard</em> used to keep track of all children across
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generations.</p>
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<p>The status module will also use a <code>G</code> to indicate
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those children which are still serving requests started before
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the graceful restart was given.</p>
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<p>At present there is no way for a log rotation script using
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<code>USR1</code> to know for certain that all children writing
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the pre-restart log have finished. We suggest that you use a
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suitable delay after sending the <code>USR1</code> signal
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before you do anything with the old log. For example if most of
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your hits take less than 10 minutes to complete for users on
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low bandwidth links then you could wait 15 minutes before doing
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anything with the old log.</p>
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<p><strong>Note:</strong> If your configuration file has errors
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in it when you issue a restart then your parent will not
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restart, it will exit with an error. In the case of graceful
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restarts it will also leave children running when it exits.
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(These are the children which are "gracefully exiting" by
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handling their last request.) This will cause problems if you
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attempt to restart the server -- it will not be able to bind to
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its listening ports. Before doing a restart, you can check the
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syntax of the configuration files with the <code>-t</code>
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command line argument (see <a
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href="programs/httpd.html">httpd</a>). This still will not
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guarantee that the server will restart correctly. To check the
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semantics of the configuration files as well as the syntax, you
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can try starting httpd as a non-root user. If there are no
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errors it will attempt to open its sockets and logs and fail
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because it's not root (or because the currently running httpd
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already has those ports bound). If it fails for any other
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reason then it's probably a config file error and the error
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should be fixed before issuing the graceful restart.</p>
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<h3>Restart Now</h3>
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<p><strong>Signal:</strong> HUP<br />
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<code>apachectl restart</code></p>
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<p>Sending the <code>HUP</code> signal to the parent causes it
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to kill off its children like in <code>TERM</code> but the
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parent doesn't exit. It re-reads its configuration files, and
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re-opens any log files. Then it spawns a new set of children
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and continues serving hits.</p>
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<p>Users of the <a href="mod/mod_status.html">status module</a>
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will notice that the server statistics are set to zero when a
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<code>HUP</code> is sent.</p>
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<p><strong>Note:</strong> If your configuration file has errors
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in it when you issue a restart then your parent will not
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restart, it will exit with an error. See below for a method of
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avoiding this.</p>
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<h3>Appendix: signals and race conditions</h3>
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<p>Prior to Apache 1.2b9 there were several <em>race
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conditions</em> involving the restart and die signals (a simple
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description of race condition is: a time-sensitive problem, as
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in if something happens at just the wrong time it won't behave
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as expected). For those architectures that have the "right"
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feature set we have eliminated as many as we can. But it should
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be noted that there still do exist race conditions on certain
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architectures.</p>
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<p>Architectures that use an on disk <a
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href="mod/core.html#scoreboardfile">ScoreBoardFile</a> have the
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potential to corrupt their scoreboards. This can result in the
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"bind: Address already in use" (after <code>HUP</code>) or
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"long lost child came home!" (after <code>USR1</code>). The
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former is a fatal error, while the latter just causes the
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server to lose a scoreboard slot. So it might be advisable to
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use graceful restarts, with an occasional hard restart. These
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problems are very difficult to work around, but fortunately
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most architectures do not require a scoreboard file. See the <a
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href="mod/core.html#scoreboardfile">ScoreBoardFile</a>
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documentation for a architecture uses it.</p>
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<p><code>NEXT</code> and <code>MACHTEN</code> (68k only) have
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small race conditions which can cause a restart/die signal to
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be lost, but should not cause the server to do anything
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otherwise problematic.
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<!-- they don't have sigaction, or we're not using it -djg -->
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</p>
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<p>All architectures have a small race condition in each child
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involving the second and subsequent requests on a persistent
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HTTP connection (KeepAlive). It may exit after reading the
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request line but before reading any of the request headers.
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There is a fix that was discovered too late to make 1.2. In
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theory this isn't an issue because the KeepAlive client has to
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expect these events because of network latencies and server
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timeouts. In practice it doesn't seem to affect anything either
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-- in a test case the server was restarted twenty times per
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second and clients successfully browsed the site without
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getting broken images or empty documents.
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<!--#include virtual="footer.html" -->
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</p>
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</body>
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</html>
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