Apache HTTP Server Version 2.2
This document covers stopping and restarting Apache on Unix-like systems. Windows NT, 2000 and XP users should see Running Apache as a Service and Windows 9x and ME users should see Running Apache as a Console Application for information on how to control Apache on those platforms.
In order to stop or restart Apache, you must send a signal to
the running httpd
processes. There are two ways to
send the signals. First, you can use the unix kill
command to directly send signals to the processes. You will
notice many httpd
executables running on your system,
but you should not send signals to any of them except the parent,
whose pid is in the PidFile
. 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:
TERM
,
HUP
, and
USR1
, which
will be described in a moment.
To send a signal to the parent you should issue a command such as:
kill -TERM `cat /usr/local/apache2/logs/httpd.pid`
The second method of signaling the httpd
processes
is to use the -k
command line options: stop
,
restart
, graceful
and graceful-stop
,
as described below. These are arguments to the httpd
binary, but we recommend that
you send them using the apachectl
control script, which
will pass them through to httpd
.
After you have signaled httpd
, you can read about
its progress by issuing:
tail -f /usr/local/apache2/logs/error_log
Modify those examples to match your ServerRoot
and PidFile
settings.
apachectl -k stop
Sending the TERM
or stop
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.
apachectl -k graceful
The USR1
or graceful
signal causes
the parent process to advise 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 generation of the
configuration, which begins serving new requests immediately.
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
StartServers
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. 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
StartServers
parameter.
Users of mod_status
will notice that the server statistics are not
set to zero when a USR1
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
scoreboard used to keep track of all children across
generations.
The status module will also use a G
to indicate
those children which are still serving requests started before
the graceful restart was given.
At present there is no way for a log rotation script using
USR1
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 USR1
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.
-t
command line argument (see httpd
). 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 httpd
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
httpd
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.apachectl -k restart
Sending the HUP
or restart
signal to
the parent causes it to kill off its children like in
TERM
, 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.
Users of mod_status
will notice that the server statistics are set to zero when a
HUP
is sent.
apachectl -k graceful-stop
The WINCH
or graceful-stop
signal causes
the parent process to advise the children to exit after
their current request (or to exit immediately if they're not
serving anything). The parent will then remove its PidFile
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 GracefulShutdownTimeout
has been
reached, the parent will also exit. If the timeout is reached,
any remaining children will be sent the TERM
signal
to force them to exit.
A TERM
signal will immediately terminate the
parent process and all children when in the "graceful" state. However
as the PidFile
will
have been removed, you will not be able to use
apachectl
or httpd
to send this signal.
The graceful-stop
signal allows you to run multiple
identically configured instances of httpd
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.
Care has been taken to ensure that on-disk files
such as the Lockfile
and ScriptSock
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 httpd
do not clobber each others files.
You should also be wary of other potential race conditions, such as
using rotatelogs
style piped logging. Multiple running
instances of rotatelogs
attempting to rotate the same
logfiles at the same time may destroy each other's logfiles.
Prior to Apache 1.2b9 there were several race conditions 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.
Architectures that use an on-disk ScoreBoardFile
can potentially have
their scoreboards corrupted. This can result in the "bind:
Address already in use" (after HUP
) or "long lost
child came home!" (after USR1
). 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 ScoreBoardFile
documentation for
architecture which uses it.
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.