|
Apache HTTP Server
Stopping and Restarting Apache
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:
kill -TERM `cat /usr/local/apache/logs/httpd.pid`
You can read about its progress by issuing:
tail -f /usr/local/apache/logs/error_log
Modify those examples to match your
ServerRoot and
PidFile settings.
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.
TERM Signal: stop now
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.
HUP Signal: restart now
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.
USR1 Signal: graceful restart
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.
Reviews on effectiveness of macular degeneration
aids for treatment. Effectiveness of
mosquito control systems in mosquito control. Review on
mace pepper spray and it's usefulness.
Appendix: signals and race conditions
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.
Apache HTTP Server
|
