[launchd-dev] Re: process dependancies

12 Mar 2007

      On Mar 11, 2007, at 10:39 AM, Joël Brogniart wrote:
...
...
A nice feature of SystemStartup is the notion of dependancies, ie  
the ability to spécify that a process is dependant of other  
processes. I don't see anything similar in launchd documentation.  
Do I have missed something in the doc or this feature doesn't  
exist with launchd?
Le 11 mars 07 à 19:45, Kevin Van Vechten a écrit :
...
There's a relatively large conceptual change with launchd that can  
easily be missed.
OK, so the feature was not so nice, but it was simple to understand.
...
…
The key concept to launchd is to use actual operating system  
primitives for dependencies, not just names.  What does that mean?   
Well, in practical terms, a process really doesn't depend on  
another process unless it communicates with it in some way.  Let's  
say you have a process that depends on the PostgreSQL database --  
that essentially boils down to the process assuming that Postgres'  
UNIX domain socket exists (/tmp/.s.PGSQL.4321).
...
The launchd approach means that the Postgres property list  
specifies that UNIX domain socket.  During startup, launchd  
initializes all of the UNIX domain sockets, network sockets, mach  
services, etc.  When your process runs, it can successfully connect  
the Postgres socket.  This connection, in turn, triggers launchd to  
start up Postgres on demand.  The calling process blocks until  
Postgres bring-up
So, if I understood, lets me give an example.

I have a java web application that run with Tomcat. This java app  
respond on port 10001 with protocol AJP 1.3 and on port 10002 with  
protocol http.
I have an Apache server that connect to the web application with  
mod_jk (with protocol AJP).

In order to have Apache to wait until the web application is ready I  
have to include a "Sockets" and a "inetdCompatibility " keys in the  
web application launchd.plist:

	<key>ProgramArguments</key>
	<array>
		<string>/Library/Java/Home/bin/java</string>
		<string>-server</string>
		...
		<string>org.apache.catalina.startup.Bootstrap</string>
		<string>start</string>
	</array>
	<key>RunAtLoad</key>
	<true/>
	...
	<key>Sockets</key>
	<dict>
		<key>Listeners</key>
		<dict>
			<key>SockNodeName</key>
			<string>192.168.128.16</string>
			<key>SockServiceName</key>
			<array>
   				<integer>10001</integer>
   				<integer>10002</integer>
   			</array>
		</dict>
	</dict>
	<key>inetdCompatibility</key>
	<dict>
		<key>Wait</key>
		<false/>
	</dict>
	...

(mod_jk will likely fail with a timeout error due to the duration of  
the web application's initialisation but that's another topic). Is  
that right?

Joël Brogniart