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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>

<seealso><a href="invoking.html">Starting</a></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 four signals that you can send the parent:

<code><a href="#term">TERM</a></code>,

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

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

<code><a href="#gracefulstop">WINCH</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>, <code>graceful</code> and <code>graceful-stop</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>

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

parameter.</p>

<p>Users of <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>

<p>When you issue a restart, a syntax check is first run, to

ensure that there are no errors in the configuration files.

If your configuration file has errors in it, you will get an

error message about that syntax error, and the server will refuse to

restart. This avoids the situation where the server halts and then

cannot restart, leaving you with a non-functioning server.</p>

<p>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.</p></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>As with a graceful restart, a syntax check is run before the

restart is attempted. If your configuration file has errors in it, the

restart will not be attempted, and you will receive notification of the

syntax error(s).</note>

</section>

<section id="gracefulstop"><title>Graceful Stop</title>

<dl><dt>Signal: WINCH</dt>

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

</dl>

<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 <directive

module="mpm_common">PidFile</directive> 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 <directive

module="mpm_common">GracefulShutdownTimeout</directive> 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 <directive module="mpm_common">PidFile</directive> will

have been removed, you will not be able to use

<code>apachectl</code> or <code>httpd</code> to send this signal,</p>

<note><p>The <code>graceful-stop</code> signal allows you to run multiple

identically configured instances of <program>httpd</program> 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 <directive module="core">Lockfile</directive> and <directive

module="mod_cgid">ScriptSock</directive> files contain the server

PID, and should co-exist 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 <program>httpd</program> do not clobber each others files.</p>

<p>You should also be wary of other potential race conditions, such as

using <program>rotatelogs</program> style piped logging. Multiple running

instances of <program>rotatelogs</program> attempting to rotate the same

logfiles at the same time may destroy each other's logfiles.</p></note>

</section>

</manualpage>