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<p>
<a href="index.html">s6-rc</a><br />
<a href="http://skarnet.org/software/">Software</a><br />
<a href="http://skarnet.org/">skarnet.org</a>
</p>
<h1> s6-rc: FAQ </h1>
<h2> The s6-rc-compile source format </h2>
<h3> The source format for
<a href="s6-rc-compile.html">s6-rc-compile</a> is not very convenient.
Why not put all the information for a service in a single file? </h3>
<p>
Because parsing sucks. Writing parsers is an annoying, ungrateful task;
and automatic parser generators produce big and inefficient code. For
security, efficiency and maintainability reasons, I prefer to focus my
efforts on code that actually does stuff, not parsing code.
</p>
<p>
Using the filesystem as a key-value store is
a good technique to avoid parsing, and skarnet.org packages do it
everywhere: for instance, look at
<a href="http://skarnet.org/software/s6/s6-envdir.html">s6-envdir</a>.
The s6-rc-compile source format is just another instance of this
technique.
</p>
<p>
This format generally plays well with automated tools, be it for
reading, as s6-rc-compile does, as for writing.
I fully expect the s6-rc-compile source format
to be used as the input (resp. the output) of some automated tools that
would convert
service definitions to (resp. from) another format, such as systemd
unit files, sysv-rc scripts or OpenRC scripts; at least the
s6-rc source format will make it easy on those tools.
</p>
<p>
And if you love configuration files, don't mind writing a parser (which is
indubitably easier to do in other languages than C), and want to write
a program that takes a text file, parses it and outputs a service
definition directory, it should also be rather easy.
</p>
<h3> There are no "Provides:", no virtual services. What do I do
if I have several implementations for a service? </h3>
<p>
Use bundles. Bundles are awesome.
</p>
<p>
Let's say you want to provide a ssh daemon, and have two possible
implementations, <em>opensshd</em> and <em>dropbear</em>, but you
want to provide a virtual service named <em>sshd</em>.
</p>
<p>
Define your two longruns, <em>opensshd</em> and <em>dropbear</em>;
then define a bundle named <em>sshd</em> that only contains your
default implementation, <em>opensshd</em>. Use the name <em>sshd</em>
in your dependencies. When you run
<a href="s6-rc-compile.html">s6-rc-compile</a>, all the dependencies
will resolve to <em>opensshd</em>, and the compiled service database
will consider <em>opensshd</em> to be the "real" service; but users
will still be able to run
<a href="s6-rc.html">s6-rc</a> commands involving <em>sshd</em>.
And if users want to change the default to <em>dropbear</em>, just
change the <em>sshd</em><tt>/contents</tt> file to <tt>dropbear</tt>,
recompile the database, and
run <a href="s6-rc-update.html">s6-rc-update</a>.
</p>
<p>
The advantage of proceeding this way is that online service
dependencies are kept very simple: dependencies are a directed
acyclic graph, which is easy to handle - that is the reason why
the compiled database is small, and why the
<a href="s6-rc.html">s6-rc</a> program is so small and fast.
There are "AND" dependencies, but no "OR" dependencies, which
would introduce great complexity both in data structures and in
the dependency resolution engine. s6-rc handles this complexity
<em>offline</em>.
</p>
<p>
You can use bundles to represent any collection of services, and
write all your dependencies using only bundle names if you want.
Bundles have multiple uses, and virtual services are definitely
one of them.
</p>
<h2> Switching from another service manager </h2>
<h3> I have a collection of init scripts in another format,
but don't want to wait until the whole collection is converted
before switching to s6-rc. Is there a smooth way in? </h3>
<p>
Yes.
</p>
<p>
If you are using a service manager such as sysv-rc or OpenRC,
you have a collection of init scripts that can be called with
at least <tt>start</tt> and <tt>stop</tt> arguments. You also
know dependencies between those scripts, or at least a
reasonable ordering.
</p>
<p>
You can automatically generate a source directory for
<a href="s6-rc-compile.html">s6-rc-compile</a>. For every
init script <tt>/etc/init.d/<em>foo</em></tt> that you have,
create a service definition directory named <em>foo</em>:
</p>
<ul>
<li> <tt><em>foo</em>/type</tt> contains <tt>oneshot</tt> </li>
<li> <tt><em>foo</em>/dependencies</tt> contains the list of
dependencies for <em>foo</em> </li>
<li> <tt><em>foo</em>/up</tt> contains <tt>/etc/init.d/<em>foo</em> start</tt> </li>
<li> <tt><em>foo</em>/down</tt> contains <tt>/etc/init.d/<em>foo</em> stop</tt> </li>
</ul>
<p>
You can now run compile your s6-rc service database, and use the
<a href="s6-rc.html">s6-rc</a> engine as your service manager.
Transitions will use your original init scripts, and the supervision
features of <a href="http://skarnet.org/software/s6/">s6</a> will
not be used, but you will get proper dependency tracking and
easy state changes.
</p>
<p>
Then, you can improve the database by changing services one by one, turning
them into longruns so daemons get supervised when applicable, rewriting them
into bundles calling more atomic services if needed, etc. That can be done
at your own pace, one service at a time, while still getting some benefits
from s6-rc; and if an iteration doesn't work, you can always roll back while
you fix it.
</p>
<h3> There are no runlevels in s6-rc. I like runlevels. </h3>
<p>
You have better than runlevels. <em>You have bundles.</em>
</p>
<p>
When writing your service database in source format, take note of
the common sets of services that you like to run together, what
other init systems sometimes call runlevels. For each of those
sets, define a bundle containing all those services. For instance,
you could define a <tt>runlevel-1</tt> bundle that contains only
a single getty, a <tt>runlevel-2</tt> bundle that contains only
your local services and no network, a <tt>runlevel-3</tt> bundle
that contains <tt>runlevel-2</tt> as well as network services,
and a <tt>runlevel-5</tt> bundle that contains <tt>runlevel-3</tt>
and your desktop. You can even create a <tt>runlevel-0</tt>
bundle that contains nothing at all!
</p>
<p>
In your boot script (<tt>/etc/rc.init</tt>, for instance, if
you're using
<a href="http://skarnet.org/software/s6-linux-init/">s6-linux-init</a>),
after invoking
<a href="s6-rc-init.html">s6-rc-init</a>, just ask
<a href="s6-rc.html">s6-rc</a> to start the set of services you want up
by default: <tt>s6-rc change runlevel-5</tt>.
</p>
<p>
If you later want to change your current set of services, you can then tell
s6-rc to switch, using the <tt>-p</tt> option to make sure to stop services
you don't want up anymore: <tt>s6-rc -p change runlevel-2</tt>.
</p>
<p>
Bundles are easy to use, they're flexible, and they're powerful.
They give you the same level of functionality as runlevels would, and more.
You can even add bundles to compiled service databases - including the
live one - or remove bundles from them without having to recompile them:
that's what the <a href="s6-rc-bundle.html">s6-rc-bundle</a> utility is
for.
</p>
<p>
When in doubt, use bundles.
</p>
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