Launchd dependencies

Question

I have an service that needs to run 24/7. The problem is that it requires 2 separate processes, the first a server running some java for the backend, and the other an Adobe AIR app drawing images and encoding them for saving. So the server obviously must be up and running first. Then, if either the AIR app or the server crashes, the whole thing should startup again fresh (i.e. if one part goes down, the other app should also get relaunched).
I can get launchd to launch my server with this plist file:
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>com.sns.sentinel</string>
<key>KeepAlive</key>
<true/>
<key>ProgramArguments</key>
<array>
<string>/Applications/oc4j/j2ee/home/applications/Sentinel/Sentinel.app</string>
</array>
<key>RunAtLoad</key>
<true/>
<key>UserName</key>
<string>root</string>
</dict>
</plist>
But everywhere I read about launchd it says it doesn't support dependencies and that I "should use IPC". But I have no idea what that means. Can't find an explanation for it anywhere, not even a sample. So how can I ensure that my 2 pronged service stays up and gets relaunched whenever either side goes down?

Answer 1

IPC means Inter-Process Communications. basically, Apple would rather you get your processes talking to each other directly than trying to get them to signal each other through launchd. here's a way-too-unixy link on the topic. in this case, though, I think your best bet would be to write a simple script that checks to see if both processes are running, and relaunches them both if one of them isn't. just have launchd run that script every 30 seconds or so.