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    <title>s6: the s6-supervise program</title>
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<p>
<a href="index.html">s6</a><br />
<a href="http://skarnet.org/software/">Software</a><br />
<a href="http://skarnet.org/">skarnet.org</a>
</p>

<h1> The s6-supervise program </h1>

<p>
s6-supervise monitors a long-lived process (or <em>service</em>), making sure it
stays alive, sending notifications to registered processes when it dies, and
providing an interface to control its state. s6-supervise is designed to be the
last non-leaf branch of a <em>supervision tree</em>, the supervised process
being a leaf.
</p>

<h2> Interface </h2>

<pre>
     s6-supervise <em>servicedir</em>
</pre>

<ul>
 <li> s6-supervise switches to the <em>servicedir</em>
<a href="servicedir.html">service directory</a>. </li>
 <li> It exits 100 if another s6-supervise process is already monitoring this service. </li>
 <li> If the <tt>./event</tt> <a href="fifodir.html">fifodir</a> does not exist,
s6-supervise creates it and allows public subscriptions to it.
If it already exists, it uses it as is, without modifying the subscription rights. </li>
 <li> It <a href="libftrigw.html">sends</a> a <tt>'s'</tt> event to <tt>./event</tt>. </li>
 <li> If the default service state is up, s6-supervise spawns <tt>./run</tt>. </li>
 <li> s6-supervise sends a <tt>'u'</tt> event to <tt>./event</tt> whenever it
successfully spawns <tt>./run</tt>. </li>
 <li> When <tt>./run</tt> dies, s6-supervise sends a <tt>'d'</tt> event to <tt>./event</tt>. </li>
 <li> When <tt>./run</tt> dies, s6-supervise spawns <tt>./finish</tt> if it exists. </li>
 <li> <tt>./finish</tt> must exit in less than 5 seconds. If it takes more than that,
s6-supervise kills it. </li>
 <li> When <tt>./finish</tt> dies, s6-supervise restarts <tt>./run</tt> unless it has been
told not to. </li>
 <li> There is a minimum 1-second delay between two <tt>./run</tt> spawns, to avoid busylooping
if <tt>./run</tt> exits too quickly. </li>
 <li> When killed or asked to exit, it waits for the service to go down one last time, then
sends a <tt>'x'</tt> event to <tt>./event</tt> before exiting 0. </li>
</ul>

<h2> Signals </h2>

<p>
 s6-supervise reacts to the following signals:
</p>

<ul>
 <li> SIGTERM: bring down the service and exit, as if a
<a href="s6-svc.html">s6-svc -xd</a> command had been received </li>
 <li> SIGHUP: exit as soon as the service stops, as if a
<a href="s6-svc.html">s6-svc -x</a> command had been received </li>
 <li> SIGQUIT: currently like SIGTERM, but this might change in the future </li>
</ul>

<h2> Usage notes </h2>

<ul>
 <li> s6-supervise is a long-lived process. It normally runs forever, from the system's
boot scripts, until shutdown time; it should not be killed or told to exit. If you have
no use for a service, just turn it off; the s6-supervise process does not hurt. </li>
 <li> Even in boot scripts, s6-supervise should normally not be run directly. It's
better to have a collection of <a href="servicedir.html">service directories</a> in a
single <a href="scandir.html">scan directory</a>, and just run
<a href="s6-svscan.html">s6-svscan</a> on that scan directory. s6-svscan will spawn
the necessary s6-supervise processes, and will also take care of logged services. </li>
 <li> You can use <a href="s6-svc.html">s6-svc</a> to send commands to the s6-supervise
process; mostly to change the service state and send signals to the monitored
process. </li>
 <li> You can use <a href="s6-svok.html">s6-svok</a> to check whether s6-supervise
is successfully running. </li>
 <li> You can use <a href="s6-svstat.html">s6-svstat</a> to check the status of a
service. </li>
 <li> s6-supervise maintains internal information inside the <tt>./supervise</tt>
subdirectory of <em>servicedir</em>. <em>servicedir</em> itself can be read-only,
but both <em>servicedir</em><tt>/supervise</tt> and <em>servicedir</em><tt>/event</tt>
need to be read-write. </li>
 <li> The <tt>./finish</tt> script is not guaranteed to have stdin and
stdout pointing to the same locations as the <tt>./run</tt> script. More
precisely: the stdin and stdout will be preserved for <tt>./finish</tt>
until s6-supervise is asked to exit, but the last <tt>./finish</tt>
execution will have its stdin and stdout redirected to <tt>/dev/null</tt>.
(This is to avoid maintaining open descriptors when a service is down, which
would prevent its logger from exiting cleanly.) </li>
</ul>

<h2> Implementation notes </h2>

<ul>
 <li> s6-supervise tries its best to stay alive and running despite possible
system call failures. It will write to its standard error everytime it encounters a
problem. However, unlike <a href="s6-svscan.html">s6-svscan</a>, it will not go out
of its way to stay alive; if it encounters an unsolvable situation, it will just
die. </li>
 <li> Unlike other "supervise" implementations, s6-supervise is a fully asynchronous
state machine. That means that it can read and process commands at any time, even
when the machine is in trouble (full process table, for instance). </li>
 <li> s6-supervise <em>does not use malloc()</em>. That means it will <em>never leak
memory</em>. <small>However, s6-supervise uses opendir(), and most opendir()
implementations internally use heap memory - so unfortunately, it's impossible to
guarantee that s6-supervise does not use heap memory at all.</small> </li>
 <li> s6-supervise has been carefully designed so every instance maintains as little
data as possible, so it uses a very small
amount of non-sharable memory. It is not a problem to have several
dozens of s6-supervise processes, even on constrained systems: resource consumption
will be negligible. </li>
</ul>

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