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| author | Laurent Bercot <ska-skaware@skarnet.org> | 2015-07-08 16:09:10 +0000 |
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| committer | Laurent Bercot <ska-skaware@skarnet.org> | 2015-07-08 16:09:10 +0000 |
| commit | cd62d4556a794881f9be6d48dc9db6e3a8be842c (patch) | |
| tree | 7b0433d6e1c79b254d03e688128676d26747a388 /doc/faq.html | |
| parent | 65fe38c65df5807a0f3421b3d62c1ad91a6237f5 (diff) | |
| download | s6-rc-cd62d4556a794881f9be6d48dc9db6e3a8be842c.tar.gz s6-rc-cd62d4556a794881f9be6d48dc9db6e3a8be842c.tar.xz s6-rc-cd62d4556a794881f9be6d48dc9db6e3a8be842c.zip | |
More doc!
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diff --git a/doc/faq.html b/doc/faq.html new file mode 100644 index 0000000..4776b00 --- /dev/null +++ b/doc/faq.html @@ -0,0 +1,206 @@ +<html> + <head> + <meta name="viewport" content="width=device-width, initial-scale=1.0" /> + <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> + <meta http-equiv="Content-Language" content="en" /> + <title>s6-rc: FAQ</title> + <meta name="Description" content="s6-rc: FAQ" /> + <meta name="Keywords" content="s6-rc faq frequently asked questions" /> + <!-- <link rel="stylesheet" type="text/css" href="http://skarnet.org/default.css" /> --> + </head> +<body> + +<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. +</p> + +<p> + Use bundles. +</p> + +</body> +</html> |