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<h1> Dynamic instantiation under s6 </h1>
<p>
A <em>instanced service</em> is a parameterized service that you want to
run several copies of, with only the parameter changing. Each copy of the
service is called an <em>instance</em>.
</p>
<p>
With s6, a <a href="servicedir.html">service directory</a> can only
handle one process at a time. So, if we want instanced services, there
will have to be one service directory per instance, always.
</p>
<p>
<em>Static instantiation</em> means that the set of possible instances
is finite and known in advance. With s6, it means that all the service
directories for all possible instances are created, typically by a
preprocessor, and instances are treated like regular services.
</p>
<p>
<em>Dynamic instantiation</em> means that instances are created
on demand instead of preallocated. Starting with version 2.11.2.0, s6
provides a few tools to help users set up and manage dynamically
instanced services.
</p>
<h2> How to make a dynamically instanced service under s6 </h2>
<ul>
<li> Write a template for a service directory that would run under
<a href="s6-supervise.html">s6-supervise</a>.
The <tt>run</tt> script should take the name of the instance as its
first argument; the <tt>finish</tt> script should take the name of the
instance as its third argument. </li>
<li> Call the <a href="s6-instance-maker.html">s6-instance-maker</a> program
with this template as first argument, and a path <em>dir</em> as second
argument. <a href="s6-instance-maker.html">s6-instance-maker</a> will create
a service directory in <em>dir</em>. This is an offline tool: it does not
interact with any currently active services or supervision trees. </li>
<li> Supervise <em>dir</em> by adding it to your regular
<a href="scandir.html">scan directory</a>. This will be your instanced
service, but it's not running any instances yet. It is, instead, a nested
supervision tree - the instanced service is an
<a href="s6-svscan.html">s6-svscan</a> process that will supervise all the
instances. </li>
<li> Create and delete instances at will with the
<a href="s6-instance-create.html">s6-instance-create</a> and
<a href="s6-instance-delete.html">s6-instance-delete</a> programs. Instances
are regular supervised processes; you can control them with
<a href="s6-instance-control.html">s6-instance-control</a>. These tools are
<em>online</em>: they work with live service directories (i.e. that are
being supervised by <a href="s6-supervise.html">s6-supervise</a>). They
are really syntactic sugar around the
<a href="s6-svlink.html">s6-svlink</a>,
<a href="s6-svunlink.html">s6-svunlink</a> and
<a href="s6-svc.html">s6-svc</a> programs; they provide you with the
same functionality but allow you to address individual instances via the
instanced service name (the service directory running the nested
supervision tree) and the instance name. </li>
</ul>
<h2> Internal workings </h2>
<p>
This section is not normative; users should not rely on it. It is only
here for informational purposes.
</p>
<ul>
<li> The service directory created by <a href="s6-instance-maker.html">s6-instance-maker</a>
has two specifics subdirectories in it: <tt>instance</tt> and <tt>instances</tt>. They
are initially empty, except that the template service directory is stored in
<tt>instances/.template</tt>. </li>
<li> When the service is active, there is an <a href="s6-svscan.html">s6-svscan</a>
process running on <tt>instance</tt>. </li>
<li> <a href="s6-instance-create.html">s6-instance-create</a> makes a copy of
<tt>instances/.template</tt> into <tt>instances/<em>name</em></tt>, and
<a href="s6-svlink.html">s6-svlink</a>s <tt>instances/<em>name</em></tt> to
<tt>instance</tt>. When it returns, there is an <a href="s6-supervise.html">s6-supervise</a>
process running on <tt>instance/<em>name</em></tt>, and the instance may be up
or not depending on the given options. </li>
<li> <a href="s6-instance-control.html">s6-instance-control</a> is syntactic sugar
around <a href="s6-svc.html">s6-svc</a> on <tt>instance/<em>name</em></tt>. </li>
<li> <a href="s6-instance-delete.html">s6-instance-delete</a> is syntactic sugar
around <a href="s6-svunlink.html">s6-svunlink</a> on <tt>instance/<em>name</em></tt>. </li>
</ul>
<h2> Notes </h2>
<ul>
<li> This implementation of dynamic instances may seem expensive: it
creates one <a href="s6-svscan.html">s6-svscan</a> process per
instanced service, and one <a href="s6-supervise.html">s6-supervise</a>
process per instance. However, remember that these processes use very
little private memory, so having additional copies of them is far less
expensive than it looks. It's really a convenient way to implement the
feature by reusing existing code. </li>
</ul>
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