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<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE section PUBLIC "-//OASIS//DTD DocBook XML V4.2//EN"
"http://www.oasis-open.org/docbook/xml/4.2/docbookx.dtd">
<section id="shutdown" xmlns:xi="http://www.w3.org/2001/XInclude">
<sectioninfo>
<revhistory>
<revision>
<revnumber>$Revision$</revnumber>
<date>$Date$</date>
</revision>
</revhistory>
</sectioninfo>
<title>The Server Showdown</title>
<para>
The server shutdown can be triggered by sending a signal to the
server. The server will behave differently upon receiving various types
of signals, here is a brief summary:
</para>
<itemizedlist>
<listitem>
<para>
<emphasis>SIGINT, SIGPIPE, SIGTERM, SIGCHLD</emphasis> will terminate the server.
</para>
</listitem>
<listitem>
<para>
<emphasis>SIGUSR1</emphasis> will print statistics and let the server continue.
</para>
</listitem>
<listitem>
<para>
<emphasis>SIGHUP, SIGUSR2</emphasis> will be ignored.
</para>
</listitem>
</itemizedlist>
<para>
There is only one common signal handler for all signals - function
<function>sig_usr</function> in file <filename>main.c</filename>.
</para>
<para>
In normal mode of operation (<varname>dont_fork</varname> variable is
not set), the main server is not processing any requests, it calls
<function>pause</function> function and will be waiting for signals
only. What happens when a signal arrives is shown in the previous
paragraph.
</para>
<para>
When in normal mode (<varname>dont_fork</varname> is not set), the
signal handler of the main process will only store number of the signal
received. All the processing logic will be executed by the main
process outside the signal handler (function
<function>handle_sigs</function>) The function will be called
immediately after the signal handler finish. The main process usually
does some cleanup and running such things outside the signal handler is
much more safe than doing it from the handler itself. Children only
print statistics and exit or ignore the signal completely, that is
quite safe and can be done directly from the signal handler of
children.
</para>
<para>
When <varname>dont_fork</varname> is set, all the cleanup will be done
directly from the signal handler, because there is only one process -
the main process. This is not so safe as the previous case, but this
mode should be used for debugging only and such shortcoming doesn't
harm in that case.
</para>
<para>
Upon receipt of SIGINT, SIGPIPE or SIGTERM
<function>destroy_modules</function> will be called. Each module may
register so-called <function>destroy</function> function if it needs to
do some cleanup when the server is terminating (flush of cache to disk
for example). <function>destroy_modules</function> will call destroy
function of all loaded modules.
</para>
<para>
If you need to terminate the server and all of its children, the best
way how to do it is to send SIGTERM to the main process, the main
process will in turn send the same signal to its children.
</para>
<para>
The main process and its children are in the same process
group. Therefore the main process can kill all its children simply by
sending a signal to pid 0, sending to pid 0 will send the signal to all
processes in the same process group as the sending process. This is how
the main process will terminate all children when it is going to shut
down.
</para>
<para>
If one child exited during normal operation, the whole server will be
shut down. This is better than let the server continue - a dead child
might hold a lock and that could block the whole server, such situation
cannot be avoided easily. Instead of that it is better to shutdown the
whole server and let it restart.
</para>
</section> <!-- server-shutdown -->
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