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POHow can Nginx be upgraded without dropping any requests?
 

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  1. POHow can Nginx be upgraded without dropping any requests?
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    <p>According to <a href="http://wiki.nginx.org/CommandLine" rel="nofollow">the Nginx documentation</a>:</p> <blockquote> <p>If you need to replace nginx binary with a new one (when upgrading to a new version or adding/removing server modules), you can do it without any service downtime - no incoming requests will be lost.</p> </blockquote> <p>My coworker and I were trying to figure out: <strong>how does that work?</strong>. We know (we think) that:</p> <ul> <li>Only one process can be listening on port 80 at a time</li> <li>Nginx creates a socket and connects it to port 80</li> <li>A parent process and any of its children can all bind to the same socket, which is how Nginx can have multiple worker children responding to requests</li> </ul> <p>We also did some experiments with Nginx, like this:</p> <ul> <li>Send a <code>kill -USR2</code> to the current master process </li> <li>Repeatedly run <code>ps -ef | grep unicorn</code> to see any unicorn processes, with their own pids and their parent pids</li> <li>Observe that the new master process is, at first, a child of the old master process, but when the old master process is gone, the new master process has a ppid of 1.</li> </ul> <p>So apparently the new master process can listen to the same socket as the old one while they're both running, because at that time, the new master is a child of the old master. But somehow the new master process can then become... um... nobody's child?</p> <p>I assume this is standard Unix stuff, but <strong>my understanding of processes and ports and sockets is pretty darn fuzzy</strong>. Can anybody explain this in better detail? Are any of our assumptions wrong? And is there a book I can read to really grok this stuff?</p>
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    1. USNathan Long
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      1. USEric Martindale
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    1. COIt should be noted that according to the nginx docs, both the old and new master processes are handling connections until you send a WINCH signal to the old master process.
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