1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
|
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<meta http-equiv="Content-Language" content="en" />
<title>s6: the s6-supervise program</title>
<meta name="Description" content="s6: the s6-supervise program" />
<meta name="Keywords" content="s6 command s6-supervise servicedir supervision supervise" />
<!-- <link rel="stylesheet" type="text/css" href="http://skarnet.org/default.css" /> -->
</head>
<body>
<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 subscriptions to it from processes having the same
effective group id as the s6-supervise process.
If it already exists, it uses it as is, without modifying the subscription rights. </li>
<li> It <a href="libs6/ftrigw.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>
</body>
</html>
|
