<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>
<p>The <code>USR1</code> or <code>graceful</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>
<p>This code is designed to always respect the process control
directive of the MPMs, so the number of processes and threads
available to serve clients will be maintained at the appropriate
values throughout the restart process. Furthermore, it respects
<code class="directive"><a href="./mod/mpm_common.html#startservers">StartServers</a></code> in the
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
been created, then create enough to pick up the slack. Hence the
code tries to maintain both the number of children appropriate for
the current load on the server, and respect your wishes with the
<p>The <code>WINCH</code> or <code>graceful-stop</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 will then remove its <code class="directive"><a href="./mod/mpm_common.html#pidfile">PidFile</a></code> and cease listening on
all ports. The parent will continue to run, and monitor children
which are handling requests. Once all children have finalised
and exited or the timeout specified by the <code class="directive"><a href="./mod/mpm_common.html#gracefulshutdowntimeout">GracefulShutdownTimeout</a></code> has been
reached, the parent will also exit. If the timeout is reached,
any remaining children will be sent the <code>TERM</code> signal
to force them to exit.</p>
<p>A <code>TERM</code> signal will immediately terminate the
parent process and all children when in the "graceful" state. However
as the <code class="directive"><a href="./mod/mpm_common.html#pidfile">PidFile</a></code> will
have been removed, you will not be able to use
<code>apachectl</code> or <code>httpd</code> to send this signal.</p>
<div class="note"><p>The <code>graceful-stop</code> signal allows you to run multiple
identically configured instances of <code class="program"><a href="./programs/httpd.html">httpd</a></code> at the
same time. This is a powerful feature when performing graceful
upgrades of Apache, however it can also cause deadlocks and race
conditions with some configurations.</p>
<p>Care has been taken to ensure that on-disk files
such as the <code class="directive"><a href="./mod/core.html#lockfile">Lockfile</a></code> and <code class="directive"><a href="./mod/mod_cgid.html#scriptsock">ScriptSock</a></code> files contain the server
PID, and should coexist without problem. However, if a configuration
directive, third-party module or persistent CGI utilises any other on-disk
lock or state files, care should be taken to ensure that multiple running
instances of <code class="program"><a href="./programs/httpd.html">httpd</a></code> do not clobber each others files.</p>
<p>You should also be wary of other potential race conditions, such as
using <code class="program"><a href="./programs/rotatelogs.html">rotatelogs</a></code> style piped logging. Multiple running
instances of <code class="program"><a href="./programs/rotatelogs.html">rotatelogs</a></code> attempting to rotate the same
logfiles at the same time may destroy each other's logfiles.</p></div>
<h2><a name="race" id="race">Appendix: signals and race conditions</a></h2>
<p>Prior to Apache 1.2b9 there were several <em>race
conditions</em> involving the restart and die signals (a simply put,
a race condition is a time-sensitive problem - if something happens
at just the wrong time or things happen in the wrong order,
undesired behaviour will result. If the same thing happens at the right
time, all will be well). For those architectures that have the "right"
feature set we have eliminated as many as we can. But it should
be noted that race conditions do still exist on certain
architectures.</p>
<p>Architectures that use an on-disk <code class="directive"><a href="./mod/mpm_common.html#scoreboardfile">ScoreBoardFile</a></code> can potentially have
their scoreboards corrupted. 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 may 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 <code class="directive"><a href="./mod/mpm_common.html#scoreboardfile">ScoreBoardFile</a></code> documentation for
architecture which uses it.</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. </p>
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