This document covers stopping and restarting Apache on Unix and Cygwin only. Windows users should see Signalling Apache when running.
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:
You can read about its progress by issuing:kill -TERM `cat /usr/local/apache/logs/httpd.pid`
Modify those examples to match your ServerRoot and PidFile settings.tail -f /usr/local/apache/logs/error_log
As of Apache 1.3 we provide a script called apachectl which can be used to start, stop, and restart Apache. It may need a little customization for your system, see the comments at the top of the script.
Sending the TERM
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.
Sending the HUP
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 the
status module will notice that the server statistics are set to
zero when a HUP
is sent.
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 below for a method of avoiding this.
Note: prior to release 1.2b9 this code is quite unstable and shouldn't be used at all.
The USR1
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 MaxClients, MinSpareServers, and MaxSpareServers settings. 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. This is to say that 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 the
status module 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.
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 -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.
Prior to Apache 1.2b9 there were several race conditions 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.
Architectures that use an on disk
ScoreBoardFile have the potential to corrupt their scoreboards.
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 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
ScoreBoardFile documentation for a architecture uses it.
NEXT
and MACHTEN
(68k only) have small
race conditions which can cause a restart/die signal to be lost,
but should not cause the server to do anything otherwise
problematic.
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.