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<h1> How do I perform socket activation with s6 ? </h1>
<p>
First, it's important to realize that you don't <em>need</em>
socket activation. It's a marketing word used by systemd
advocates that mixes a couple useful architecture concepts and several
horrible ideas, for a very minor speed benefit. Read
<a href="//skarnet.org/lists/supervision/0422.html">this mail</a> and
<a href="https://forums.gentoo.org/viewtopic-t-994548-postdays-0-postorder-asc-start-25.html#7581522">this
post</a> for details.
</p>
<ul>
<li> s6 <em>will not</em> help you implement super-servers in process 1,
because doing so is bad engineering.
However, it <em>will</em> help you set up super-servers. The
<a href="s6-ipcserver.html">s6-ipcserver</a>
program, for Unix domain sockets, as well as the
<a href="//skarnet.org/software/s6-networking/s6-tcpserver.html">s6-tcpserver</a>
program, for TCP INET domain sockets (available in the
<a href="//skarnet.org/software/s6-networking/">s6-networking</a>
package) are super-servers you can use to
your heart's content. They are even wrappers around simpler programs,
and you can use their components in the way you choose: bind sockets,
drop privileges, accept connections from clients, it's all about what you
write in your command line. Super-servers are a good thing; using process 1
to act as a super-server is not. s6 provides you with the tools to get
the good without the bad. </li>
<li> s6 <em>will not</em> help you run all your services before their
dependencies are met, because doing so is <em>very</em> bad engineering.
However, it <em>will</em> provide you with:
<ul>
<li> a reliable logging infrastructure, that makes sure your services
never lose logs:
<a href="s6-log.html">s6-log</a>, in conjunction with
<a href="s6-supervise.html">s6-supervise</a> and
<a href="s6-svscan.html">s6-svscan</a>. </li>
<li> ways to open your sockets and bind them as early as you want in
your boot process, and make them accept client connections later:
<a href="s6-ipcserver-socketbinder.html">s6-ipcserver-socketbinder</a> and
<a href="//skarnet.org/software/s6-networking/s6-tcpserver-socketbinder.html">s6-tcpserver-socketbinder</a>.
</li>
<li> A supervision infrastructure that can start as many services in parallel
as you want:
<a href="s6-supervise.html">s6-supervise</a> and
<a href="s6-svscan.html">s6-svscan</a>. </li>
</ul> </li>
<li> s6 <em>will not</em> help you centralize all your socket information
in process 1, because doing so is contrary to modularity and independence
of services. However, s6
<em>will</em> provide you with a way to store your open sockets and
retrieve them when you want, which it calls "fd holding":
<a href="s6-fdholder-daemon.html">s6-fdholder-daemon</a>. </li>
</ul>
<h2> So, how do I open all my sockets first, store them, and dispatch them
to daemons later ? </h2>
<p>
Again, it's not necessary to do that: you'll be fine, and quite speedy,
just starting your
daemons in their good time. You <em>will not</em> reap any noticeable
benefit from performing "socket activation". But if you really want to:
</p>
<ol>
<li> Make sure you have an early supervision infrastructure running.
Ideally, you would <a href="s6-svscan-1.html">make s6-svscan your
process 1</a>. </li>
<li> Start an early <a href="s6-fdholder-daemon.html">fd-holding
service</a>. Let's say the fd-holding daemon is listening on socket
<tt>/service/fdholder/s</tt>. </li>
<li> For every Unix domain socket <em>/my/socket</em> you need to open, run
<tt>s6-ipcserver-socketbinder /my/socket s6-fdholder-store /service/fdholder/s
unix:/my/socket</tt>. You can do the same with INET domain sockets. </li>
<li> Proceed to your initialization. </li>
<li> When you want to run a daemon <em>myserverd</em> that accepts clients
connecting to <em>/my/socket</em>, run <tt>s6-fdholder-retrieve
/service/fdholder/s unix:/my/socket myserverd</tt>. <em>myserverd</em>
will be executed with <em>/my/socket</em> as its standard input. </li>
<li> The descriptors remain safely stored in the fd-holding daemon and you
can retrieve them again whenever you want, for instance when your service
crashes and is restarted. </li>
</ol>
<p>
That is all there is to it. You don't have to use specific libraries
or write complex unit files, you just need to understand how a command
line works. This is Unix.
</p>
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