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diff --git a/.html/devops/self-daemonization-sucks.html b/.html/devops/self-daemonization-sucks.html new file mode 100644 index 0000000..14e2c01 --- /dev/null +++ b/.html/devops/self-daemonization-sucks.html @@ -0,0 +1,162 @@ +<!DOCTYPE html> +<html> +<head> + <title> + The Codex » + Self-daemonizing code is awful + </title> + + <link + rel='stylesheet' + type='text/css' + href='http://fonts.googleapis.com/css?family=Buenard:400,700&subset=latin,latin-ext'> + <link + rel="stylesheet" + type="text/css" + href="../media/css/reset.css"> + <link + rel="stylesheet" + type="text/css" + href="../media/css/grimoire.css"> +</head> +<body> + +<div id="shell"> + + <ol id="breadcrumbs"> + + <li class="crumb-0 not-last"> + + <a href="../">index</a> + + </li> + + <li class="crumb-1 not-last"> + + <a href="./">devops</a> + + </li> + + <li class="crumb-2 last"> + + self-daemonization-sucks + + </li> + + </ol> + + + + <div id="article"> + <h1 id="self-daemonizing-code-is-awful">Self-daemonizing code is awful</h1> +<p>The classical UNIX approach to services is to implement them as “daemons,” +programs that run without a terminal attached and provide some service. The +key feature of a classical daemon is that, when started, it carefully +detaches itself from its initial environment and terminal, then continues +running in the background.</p> +<p>This is awful and I'm glad modern init replacements discourage it.</p> +<h2 id="process-tracking">Process Tracking</h2> +<p>Daemons don't exist in a vacuum. Administrators and owners need to be able to +start and stop daemons reliably, and check their status. The classic +self-daemonization approach makes this impossible.</p> +<p>Traditionally, daemons run as children of <code>init</code> (pid 1), even if they start +out as children of some terminal or startup process. Posix only provides +deterministic APIs for processes to manage their children and their immediate +parents; the classic daemonisation protocol hands the newly-started daemon +process off from its original parent process, which knows how to start and +stop it, to an unsuspecting <code>init</code>, which has no idea how this specific +daemon is special.</p> +<p>The standard workaround has daemons write their own PIDs to a file, but a +file is “dead” data: it's not automatically updated if the daemon dies, and +can linger long enough to contain the PID of some later, unrelated program. +PID file validity checks generally suffer from subtle (or, sometimes, quite +gross) race conditions.</p> +<h2 id="complexity">Complexity</h2> +<p>The actual <em>code</em> to correctly daemonize a process is surprisingly complex, +given the individual interfaces' relative simplicity:</p> +<ul> +<li> +<p>The daemon must start its own process group</p> +</li> +<li> +<p>The daemon must detach from its controlling terminal</p> +</li> +<li> +<p>The daemon should close (and may reopen) file handles inherited from its + parent process (generally, a shell)</p> +</li> +<li> +<p>The daemon should ensure its working directory is predictable and + controllable</p> +</li> +<li> +<p>The daemon should ensure its umask is predictable and controllable</p> +</li> +<li> +<p>If the daemon uses privileged resources (such as low-numbered ports), it + should carefully manage its effective, real, and session UID and GIDs</p> +</li> +<li> +<p>Daemons must ensure that all of the above steps happen in signal-safe ways, + so that a daemon can be shut down sanely even if it's still starting up</p> +</li> +</ul> +<p>See <a href="http://www.freedesktop.org/software/systemd/man/daemon.html">this list</a> +for a longer version. It's worse than you think.</p> +<p>All of this gets even more complicated if the daemon has its own child +processes, a pattern common to network services. Naturally, a lot of daemons +in the real world get some of these steps wrong.</p> +<h2 id="the-future">The Future</h2> +<p><a href="http://supervisord.org">Supervisord</a>, +<a href="http://ddollar.github.io/foreman/">Foreman</a>, +<a href="http://upstart.ubuntu.com">Upstart</a>, +<a href="https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man1/launchctl.1.html">Launchd</a>, +<a href="http://www.freedesktop.org/wiki/Software/systemd/">systemd</a>, and <a href="http://cr.yp.to/daemontools.html">daemontools</a> all +encourage services <em>not</em> to self-daemonize by providing a sane system for +starting the daemon with the right parent process and the right environment +in the first place.</p> +<p>This is a great application of +<a href="http://c2.com/cgi/wiki?DontRepeatYourself">DRY</a>, as the daemon management +code only needs to be written once (in the daemon-managing daemon) rather +than many times over (in each individual daemon). It also makes daemon +execution more predictable, since daemons “in production” behave more like +they do when run attached to a developer's console during debugging or +development.</p> + </div> + + + +<div id="comments"> +<div id="disqus_thread"></div> +<script type="text/javascript"> + /* * * CONFIGURATION VARIABLES: EDIT BEFORE PASTING INTO YOUR WEBPAGE * * */ + var disqus_shortname = 'grimoire'; // required: replace example with your forum shortname + + /* * * DON'T EDIT BELOW THIS LINE * * */ + (function() { + var dsq = document.createElement('script'); dsq.type = 'text/javascript'; dsq.async = true; + dsq.src = 'http://' + disqus_shortname + '.disqus.com/embed.js'; + (document.getElementsByTagName('head')[0] || document.getElementsByTagName('body')[0]).appendChild(dsq); + })(); +</script> +<noscript>Please enable JavaScript to view the <a href="http://disqus.com/?ref_noscript">comments powered by Disqus.</a></noscript> +<a href="http://disqus.com" class="dsq-brlink">comments powered by <span class="logo-disqus">Disqus</span></a> +</div> + + + + <div id="footer"> + <p> + + The Codex — + + Powered by <a href="http://markdoc.org/">Markdoc</a>. + +<a href="https://bitbucket.org/ojacobson/grimoire.ca/src/master/wiki/devops/self-daemonization-sucks.md">See this page on Bitbucket</a> (<a href="https://bitbucket.org/ojacobson/grimoire.ca/history-node/master/wiki/devops/self-daemonization-sucks.md">history</a>). + + </p> + </div> + +</div> +</body> +</html>
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