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<h1>Stopping and Restarting Apache</h1>

<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/core.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>To send a signal to the parent you should issue a command such as:
<blockquote><pre>
    kill -TERM `cat /usr/local/etc/httpd/logs/httpd.pid`
</pre></blockquote>

You can read about its progress by issuing:

<blockquote><pre>
    tail -f /usr/local/etc/httpd/logs/error_log
</pre></blockquote>

Modify those examples to match your
<a href="mod/core.html#serverroot">ServerRoot</a> and
<a href="mod/core.html#pidfile">PidFile</a> settings.

<h3>TERM Signal: stop now</h3>

<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.

<h3>HUP Signal: restart now</h3>

<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>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.

<h3>USR1 Signal: graceful restart</h3>

<p><b>Note:</b> prior to release 1.2b9 this code is quite unstable and
shouldn't be used at all.

<p>The <code>USR1</code> signal causes the parent process to <i>advise</i>
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 <i>generation</i> of the
configuration, which begins serving new requests immediately.

<p>This code is designed to always respect the
<a href="mod/core.html#maxclients">MaxClients</a>,
<a href="mod/core.html#minspareservers">MinSpareServers</a>,
and <a href="mod/core.html#maxspareservers">MaxSpareServers</a> settings.
Furthermore, it respects <a href="mod/core.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>Users of the
<a href="mod/mod_status.html">status module</a>
will notice that the server statistics
are <b>not</b> 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 <i>scoreboard</i> used to keep track
of all children across generations.

<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>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.

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

<p>Prior to Apache 1.2b9 there were several <i>race conditions</i>
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>Architectures that use an on disk <a
href="mod/core.html#scoreboardfile">ScoreBoardFile</a> have the potential
to lose track of a child during graceful restart (you'll see an <a
href="mod/core.html#errorlog">ErrorLog</a> message saying something about
a <i>long lost child</i>).  The ScoreBoardFile directive explains how
to figure out if your server uses a file, and possibly how to avoid it.
There is also the potential that the scoreboard will be corrupted during
any signalling, but this only has bad effects on graceful restart.

<p><code>NEXT</code> and <code>MACHTEN</code> 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>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.

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