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<title>Stopping and Restarting</title>

<summary>

<p>This document covers stopping and restarting Apache on

Unix-like systems. Windows NT, 2000 and XP users should see

<a href="platform/windows.html#winsvc">Running Apache as a

Service</a> and Windows 9x and ME users should see <a

href="platform/windows.html#wincons">Running Apache as a

Console Application</a> for information on how to control

Apache on those platforms.</p>

</summary>

<seealso><program>httpd</program></seealso>

<seealso><program>apachectl</program></seealso>

<section id="introduction"><title>Introduction</title>

<p>In order to stop or restart Apache, you must send a signal to

the running <program>httpd</program> processes. There are two ways to

send the signals. First, you can use the unix <code>kill</code>

command to directly send signals to the processes. You will

notice many <program>httpd</program> executables running on your system,

but you should not send signals to any of them except the parent,

whose pid is in the <directive

module="mpm_common">PidFile</directive>. 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><a href="#term">TERM</a></code>,

<code><a href="#hup">HUP</a></code>, and

<code><a href="#graceful">USR1</a></code>, which

will be described in a moment.</p>

<p>To send a signal to the parent you should issue a command

such as:</p>

<example>kill -TERM `cat /usr/local/apache2/logs/httpd.pid`</example>

<p>The second method of signaling the <program>httpd</program> processes

is to use the <code>-k</code> command line options: <code>stop</code>,

<code>restart</code>, and <code>graceful</code>,

as described below. These are arguments to the <program>

httpd</program> binary, but we recommend that

you send them using the <program>apachectl</program> control script, which

will pass them through to <program>httpd</program>.</p>

<p>After you have signaled <program>httpd</program>, you can read about

its progress by issuing:</p>

<example>tail -f /usr/local/apache2/logs/error_log</example>

<p>Modify those examples to match your <directive

module="core">ServerRoot</directive> and <directive

module="mpm_common">PidFile</directive> settings.</p>

</section>

<section id="term"><title>Stop Now</title>

<dl><dt>Signal: TERM</dt>

<dd><code>apachectl -k stop</code></dd>

</dl>

<p>Sending the <code>TERM</code> or <code>stop</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.</p>

</section>

<section id="graceful"><title>Graceful Restart</title>

<dl><dt>Signal: USR1</dt>

<dd><code>apachectl -k graceful</code></dd>

</dl>

<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>

<note>On certain platforms that do not allow <code>USR1</code> to

be used for a graceful restart, an alternative signal may be used (such

as <code>WINCH</code>). The command <code>apachectl graceful</code>

will send the right signal for your platform.</note>

<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

<directive module="mpm_common">StartServers</directive> in the

following manner: if after one second at least <directive

module="mpm_common">StartServers</directive> 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

<directive>StartServers</directive> parameter.</p>

<p>Users of the <module>mod_status</module>

will notice that the server statistics are <strong>not</strong>

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

<em>scoreboard</em> used to keep track of all children across

generations.</p>

<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>

<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.</p>

<note>If your configuration file has errors

in it when you issue a restart then your parent will not

restart, it will exit with an error. In the case of graceful

restarts it will also leave children running when it exits.

(These are the children which are "gracefully exiting" by

handling their last request.) This will cause problems if you

attempt to restart the server -- it will not be able to bind to

its listening ports. Before doing a restart, you can check the

syntax of the configuration files with the <code>-t</code>

command line argument (see <program>httpd</program>). This still will not

guarantee that the server will restart correctly. To check the

semantics of the configuration files as well as the syntax, you

can try starting <program>httpd</program> as a non-root user. If there

are no errors it will attempt to open its sockets and logs and fail

because it's not root (or because the currently running

<program>httpd</program> already has those ports bound). If it fails

for any other reason then it's probably a config file error and the error

should be fixed before issuing the graceful restart.</note>

</section>

<section id="hup"><title>Restart Now</title>

<dl><dt>Signal: HUP</dt>

<dd><code>apachectl -k restart</code></dd>

</dl>

<p>Sending the <code>HUP</code> or <code>restart</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>

<p>Users of <module>mod_status</module>

will notice that the server statistics are set to zero when a

<code>HUP</code> is sent.</p>

<note>If your configuration file has errors in it when you issue a

restart then your parent will not restart, it will exit with an

error. See above for a method of avoiding this.</note>

</section>

<section id="race"><title>Appendix: signals and race conditions</title>

<p>Prior to Apache 1.2b9 there were several <em>race

conditions</em> 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>

<p>Architectures that use an on disk <directive

module="mpm_common">ScoreBoardFile</directive> have the potential

to corrupt their scoreboards. 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 might 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 <directive

module="mpm_common">ScoreBoardFile</directive> documentation for a

architecture 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>

</section>

</manualpage>