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14 <p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="./faq/">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p>
15 <p class="apache">Apache HTTP Server Version 2.1</p>
16 <img alt="" src="./images/feather.gif" /></div>
17 <div class="up"><a href="./"><img title="<-" alt="<-" src="./images/left.gif" /></a></div>
19 <a href="http://www.apache.org/">Apache</a> > <a href="http://httpd.apache.org/">HTTP Server</a> > <a href="http://httpd.apache.org/docs-project/">Documentation</a> > <a href="./">Version 2.1</a></div><div id="page-content"><div id="preamble"><h1>Stopping and Restarting</h1>
20 <p>This document covers stopping and restarting Apache on
21 Unix-like systems. Windows NT, 2000 and XP users should see
22 <a href="platform/windows.html#winsvc">Running Apache as a
23 Service</a> and Windows 9x and ME users should see <a href="platform/windows.html#wincons">Running Apache as a
24 Console Application</a> for information on how to control
25 Apache on those platforms.</p>
27 <div id="quickview"><ul id="toc"><li><img alt="" src="./images/down.gif" /> <a href="#introduction">Introduction</a></li>
28 <li><img alt="" src="./images/down.gif" /> <a href="#term">Stop Now</a></li>
29 <li><img alt="" src="./images/down.gif" /> <a href="#graceful">Graceful Restart</a></li>
30 <li><img alt="" src="./images/down.gif" /> <a href="#hup">Restart Now</a></li>
31 <li><img alt="" src="./images/down.gif" /> <a href="#race">Appendix: signals and race conditions</a></li>
32 </ul><h3>See also</h3><ul class="seealso"><li><a href="programs/httpd.html">httpd</a></li><li><a href="programs/apachectl.html">apachectl</a></li></ul></div>
33 <div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
35 <h2><a name="introduction" id="introduction">Introduction</a></h2>
37 <p>In order to stop or restart Apache, you must send a signal to
38 the running <code>httpd</code> processes. There are two ways to
39 send the signals. First, you can use the unix <code>kill</code>
40 command to directly send signals to the processes. You will
41 notice many <code>httpd</code> executables running on your system,
42 but you should not send signals to any of them except the parent,
43 whose pid is in the <code class="directive"><a href="./mod/mpm_common.html#pidfile">PidFile</a></code>. That is to say you
44 shouldn't ever need to send signals to any process except the
45 parent. There are three signals that you can send the parent:
46 <a href="#term"><code>TERM</code></a>,
47 <a href="#hup"><code>HUP</code></a>, and
48 <a href="#graceful"><code>USR1</code></a>, which
49 will be described in a moment.</p>
51 <p>To send a signal to the parent you should issue a command
54 <div class="example"><p><code>kill -TERM `cat /usr/local/apache2/logs/httpd.pid`</code></p></div>
56 <p>The second method of signaling the <code>httpd</code> processes
57 is to use the <code>-k</code> command line options: <code>stop</code>,
58 <code>restart</code>, and <code>graceful</code>,
59 as described below. These are arguments to the <a href="programs/httpd.html">httpd</a> binary, but we recommend that
60 you send them using the <a href="programs/apachectl.html">apachectl</a> control script, which
61 will pass them through to <code>httpd</code>.</p>
63 <p>After you have signaled <code>httpd</code>, you can read about
64 its progress by issuing:</p>
66 <div class="example"><p><code>tail -f /usr/local/apache2/logs/error_log</code></p></div>
68 <p>Modify those examples to match your <code class="directive"><a href="./mod/core.html#serverroot">ServerRoot</a></code> and <code class="directive"><a href="./mod/mpm_common.html#pidfile">PidFile</a></code> settings.</p>
69 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
71 <h2><a name="term" id="term">Stop Now</a></h2>
73 <dl><dt>Signal: TERM</dt>
74 <dd><code>apachectl -k stop</code></dd>
77 <p>Sending the <code>TERM</code> or <code>stop</code> signal to
78 the parent causes it to immediately attempt to kill off all of its
79 children. It may take it several seconds to complete killing off
80 its children. Then the parent itself exits. Any requests in
81 progress are terminated, and no further requests are served.</p>
82 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
84 <h2><a name="graceful" id="graceful">Graceful Restart</a></h2>
86 <dl><dt>Signal: USR1</dt>
87 <dd><code>apachectl -k graceful</code></dd>
90 <p>The <code>USR1</code> or <code>graceful</code> signal causes
91 the parent process to <em>advise</em> the children to exit after
92 their current request (or to exit immediately if they're not
93 serving anything). The parent re-reads its configuration files and
94 re-opens its log files. As each child dies off the parent replaces
95 it with a child from the new <em>generation</em> of the
96 configuration, which begins serving new requests immediately.</p>
98 <div class="note">On certain platforms that do not allow <code>USR1</code> to
99 be used for a graceful restart, an alternative signal may be used (such
100 as <code>WINCH</code>). The command <code>apachectl graceful</code>
101 will send the right signal for your platform.</div>
103 <p>This code is designed to always respect the process control
104 directive of the MPMs, so the number of processes and threads
105 available to serve clients will be maintained at the appropriate
106 values throughout the restart process. Furthermore, it respects
107 <code class="directive"><a href="./mod/mpm_common.html#startservers">StartServers</a></code> in the
108 following manner: if after one second at least <code class="directive"><a href="./mod/mpm_common.html#startservers">StartServers</a></code> new children have not
109 been created, then create enough to pick up the slack. Hence the
110 code tries to maintain both the number of children appropriate for
111 the current load on the server, and respect your wishes with the
112 <code class="directive">StartServers</code> parameter.</p>
114 <p>Users of the <code class="module"><a href="./mod/mod_status.html">mod_status</a></code>
115 will notice that the server statistics are <strong>not</strong>
116 set to zero when a <code>USR1</code> is sent. The code was
117 written to both minimize the time in which the server is unable
118 to serve new requests (they will be queued up by the operating
119 system, so they're not lost in any event) and to respect your
120 tuning parameters. In order to do this it has to keep the
121 <em>scoreboard</em> used to keep track of all children across
124 <p>The status module will also use a <code>G</code> to indicate
125 those children which are still serving requests started before
126 the graceful restart was given.</p>
128 <p>At present there is no way for a log rotation script using
129 <code>USR1</code> to know for certain that all children writing
130 the pre-restart log have finished. We suggest that you use a
131 suitable delay after sending the <code>USR1</code> signal
132 before you do anything with the old log. For example if most of
133 your hits take less than 10 minutes to complete for users on
134 low bandwidth links then you could wait 15 minutes before doing
135 anything with the old log.</p>
137 <div class="note">If your configuration file has errors
138 in it when you issue a restart then your parent will not
139 restart, it will exit with an error. In the case of graceful
140 restarts it will also leave children running when it exits.
141 (These are the children which are "gracefully exiting" by
142 handling their last request.) This will cause problems if you
143 attempt to restart the server -- it will not be able to bind to
144 its listening ports. Before doing a restart, you can check the
145 syntax of the configuration files with the <code>-t</code>
146 command line argument (see <a href="programs/httpd.html">httpd</a>). This still will not
147 guarantee that the server will restart correctly. To check the
148 semantics of the configuration files as well as the syntax, you
149 can try starting <code>httpd</code> as a non-root user. If there are no
150 errors it will attempt to open its sockets and logs and fail
151 because it's not root (or because the currently running <code>httpd</code>
152 already has those ports bound). If it fails for any other
153 reason then it's probably a config file error and the error
154 should be fixed before issuing the graceful restart.</div>
155 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
156 <div class="section">
157 <h2><a name="hup" id="hup">Restart Now</a></h2>
159 <dl><dt>Signal: HUP</dt>
160 <dd><code>apachectl -k restart</code></dd>
163 <p>Sending the <code>HUP</code> or <code>restart</code> signal to
164 the parent causes it to kill off its children like in
165 <code>TERM</code>, but the parent doesn't exit. It re-reads its
166 configuration files, and re-opens any log files. Then it spawns a
167 new set of children and continues serving hits.</p>
169 <p>Users of <code class="module"><a href="./mod/mod_status.html">mod_status</a></code>
170 will notice that the server statistics are set to zero when a
171 <code>HUP</code> is sent.</p>
173 <div class="note">If your configuration file has errors in it when you issue a
174 restart then your parent will not restart, it will exit with an
175 error. See above for a method of avoiding this.</div>
176 </div><div class="top"><a href="#page-header"><img alt="top" src="./images/up.gif" /></a></div>
177 <div class="section">
178 <h2><a name="race" id="race">Appendix: signals and race conditions</a></h2>
180 <p>Prior to Apache 1.2b9 there were several <em>race
181 conditions</em> involving the restart and die signals (a simple
182 description of race condition is: a time-sensitive problem, as
183 in if something happens at just the wrong time it won't behave
184 as expected). For those architectures that have the "right"
185 feature set we have eliminated as many as we can. But it should
186 be noted that there still do exist race conditions on certain
189 <p>Architectures that use an on disk <code class="directive"><a href="./mod/mpm_common.html#scoreboardfile">ScoreBoardFile</a></code> have the potential
190 to corrupt their scoreboards. This can result in the "bind:
191 Address already in use" (after <code>HUP</code>) or "long lost
192 child came home!" (after <code>USR1</code>). The former is a fatal
193 error, while the latter just causes the server to lose a
194 scoreboard slot. So it might be advisable to use graceful
195 restarts, with an occasional hard restart. These problems are very
196 difficult to work around, but fortunately most architectures do
197 not require a scoreboard file. See the <code class="directive"><a href="./mod/mpm_common.html#scoreboardfile">ScoreBoardFile</a></code> documentation for a
198 architecture uses it.</p>
200 <p>All architectures have a small race condition in each child
201 involving the second and subsequent requests on a persistent
202 HTTP connection (KeepAlive). It may exit after reading the
203 request line but before reading any of the request headers.
204 There is a fix that was discovered too late to make 1.2. In
205 theory this isn't an issue because the KeepAlive client has to
206 expect these events because of network latencies and server
207 timeouts. In practice it doesn't seem to affect anything either
208 -- in a test case the server was restarted twenty times per
209 second and clients successfully browsed the site without
210 getting broken images or empty documents. </p>
213 <p class="apache">Maintained by the <a href="http://httpd.apache.org/docs-project/">Apache HTTP Server Documentation Project</a></p>
214 <p class="menu"><a href="./mod/">Modules</a> | <a href="./mod/directives.html">Directives</a> | <a href="./faq/">FAQ</a> | <a href="./glossary.html">Glossary</a> | <a href="./sitemap.html">Sitemap</a></p></div>