![[APACHE DOCUMENTATION]](images/sub.gif)
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/etc/httpd/logs/httpd.pid`
tail -f /usr/local/etc/httpd/logs/error_log
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. At present the only work around is to check the syntax of your files before doing a restart. The easiest way is to just run 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 method to determine if your
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.
