Launchd: Using Sockets key for on demand jobs

Question

Hi,
I would like to start nginx whenever I send an HTTP request to localhost. This is my plist:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN"
"http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Disabled</key>
<false/>
<key>Label</key>
<string>org.sysoev.nginx</string>
<key>ProgramArguments</key>
<array>
<string>/usr/local/nginx/sbin/nginx</string>
</array>
<key>Sockets</key>
<dict>
<key>Listeners</key>
<dict>
<key>SockNodeName</key>
<string>127.0.0.1</string>
<key>SockServiceName</key>
<string>http</string>
</dict>
</dict>
<key>KeepAlive</key>
<dict>
<key>SuccessfulExit</key>
<false/>
</dict>
</dict>
</plist>
Although nginx is started when I request a webpage from localhost, the request never arrives at nginx, so the requested page is not loaded. When I replace the Sockets dict with RunAtLoad: true, all works fine.
Could it be that launchd somehow intercepts the requests sent to an on-demand daemon?
Kind regards,
Jan

Answer 1

Launchd does intercept the requests sent to an on-demand process. That's how launch on demand works.
I'm not familiar with nginx, but I wouldn't expect launch on demand to work with any web server, for the same reason it doesn't work with Apache. Apache has to start up as root in order to bind the usual ports 80 and 443. It then forks an unprivileged process to handle the actual network traffic. Any sort of automatic forking is incompatible with launch on demand. The solution is the one you already found, run at launch and let the process do its own listening on the network.