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SME Server:Documentation:Developers Manual:Chapter9 (view source)
Revision as of 15:25, 5 February 2010
, 15:25, 5 February 2010→Adding a supervised service
<div class="CHAPTER">
<div class="CHAPTER">
=Process startup, supervision and shutdown=
==Process startup, supervision and shutdown==
<div class="SECT1">
<div class="SECT1">
==Process startup==
===Process startup===
In typical Linux systems, services (processes) are started at boot time through a mechanism such as <span class="emphasis">''System V init''</span>. When the system administrator needs to change the settings, they modify the configuration files and then restart the service or notify the process that it needs to re-read the configuration.
In typical Linux systems, services (processes) are started at boot time through a mechanism such as <span class="emphasis">''System V init''</span>. When the system administrator needs to change the settings, they modify the configuration files and then restart the service or notify the process that it needs to re-read the configuration.
----
----
==Process supervision: runit (and supervise)==
===Process supervision: runit (and supervise)===
The SME Server addresses these issues by running processes under the '''runit''' process supervision environment, which:
The SME Server addresses these issues by running processes under the '''runit''' process supervision environment, which:
* restarts the process if it fails
* restarts the process if it fails
* provides a consistent mechanism for controlling the underlying process
* provides a consistent mechanism for controlling the underlying process
{{Note box|msg=Gerrit Pape's '''runit''' came from previous work by Dan Bernstein on the '''supervise''' supervision environment. '''runit''' provides additional features, and has been released under a free software license.}}
<div class="SECT2">
----
----
===The runit process tree===
====The runit process tree====
When a Linux system boots, it starts the '''init''' process, which then starts all other processes. When '''init''' enters "run-level 7", it starts '''/etc/runit/2''' from an entry in <tt class="FILENAME">/etc/inittab</tt>.
When a Linux system boots, it starts the '''init''' process, which then starts all other processes. When '''init''' enters "run-level 7", it starts '''/etc/runit/2''' from an entry in <tt class="FILENAME">/etc/inittab</tt>.
This looks like a complex process tree, but is a critical part of the SME Server's design for reliability. Each process is independent, has a consistent management interface, has process limits imposed on it, and will restart if it happens to fail.
This looks like a complex process tree, but is a critical part of the SME Server's design for reliability. Each process is independent, has a consistent management interface, has process limits imposed on it, and will restart if it happens to fail.
{{Note box|msg=For the curious, if init fails, the system reboots.}}
For further documentation on runit, refer to the runit manual page.
For further documentation on runit, refer to the runit manual page.
----
----
===Run-level 7 and the e-smith-service wrapper===
====Run-level 7 and the e-smith-service wrapper====
The SME Server runs in the normally unused run-level 7. This ensures that the only software running on the SME Server is software that we have chosen to run, and it is started and stopped in a consistent way. If we need to replace a standard startup script with one which runs the process under supervise, we can do so <span class="emphasis">''without modifying the original package''</span>.
The SME Server runs in the normally unused run-level 7. This ensures that the only software running on the SME Server is software that we have chosen to run, and it is started and stopped in a consistent way. If we need to replace a standard startup script with one which runs the process under supervise, we can do so <span class="emphasis">''without modifying the original package''</span>.
* If so, it runs the <tt class="FILENAME">/etc/init.d/syslog</tt> script with the argument <var class="LITERAL">start</var>.
* If so, it runs the <tt class="FILENAME">/etc/init.d/syslog</tt> script with the argument <var class="LITERAL">start</var>.
* If the service is not enabled, it exits without starting the service.
* If the service is not enabled, it exits without starting the service.
{{Note box|msg=If a script exists in the <tt class="FILENAME">/etc/init.d/supervise/</tt> directory, <tt class="FILENAME">e-smith-service</tt> will use that in preference to the one in the <tt class="FILENAME">/etc/init.d/</tt> directory. This allows us to install our own supervised startup scripts <span class="emphasis">''without modifying the original package''</span>.}}
===Adding a supervised service===
See http://cr.yp.to/daemontools.html
Check your application has a -d option or similar which means that it stays in the foreground, and logs to standard output rather than syslog. That makes it suitable for running as a supervised service.
Create a /var/service/XXX directory, containing an executable 'run' script something like:
#! /bin/sh
exec 2>&1
exec /var/service/XXX -d
and a /var/service/XXX/log directory, containing an executable 'run' script something like:
#! /bin/sh
exec setuidgid smelog \
/usr/local/bin/multilog t s500000 \
/var/log/XXX
You would then do:
mkdir /var/log/XXX
chown smelog.smelog /var/log/XXX
ln -s /var/service/XXX /service
touch /var/service/XXX/down
</div></div></div>