Upstart and system startup

I wrote upstart a while ago, and I've got some text taken from emails describing it below. However, I now think it's a bad idea.

Now, I take the more radical position that there should be no dependencies... no service should depend on another. By this, I mean that if one service needs to use another service, it should be able to wait for that service to become "live", and also be able to suspend operation (not crash!) when a service it uses dies. Obviously, it should be possible to interrogate the state of a service (is it waiting for another service to come alive? which ones?) This way, all services will be resilient to temporary failures.

In this system, you can just start all services simultaneously. I guess deadlocks might (in theory) be a problem, but I can't think of any practical examples of deadlocks.

So, to implement this vision, there will need to be some changes:

• /etc/inittab: should be a wrapper for /etc/rc.d/rc1 (or whatever they're called) scripts, and nothing more. There should be a login daemon (logind?) for opening virtual consoles for login, and a [kgx]dm script should be invoked via /etc/rc.d/rc5
• we can get rid of the numbers in /etc/rc?.d. We only need S and K, since we start everything at the same time, and SIGTERM at the same time. However, a process should never terminate while it's being referenced. This might turn out to be a difficult problem, if there are stale locks, or someone tries to DoS the shutdown by maintaining a reference. (These all feel like solvable problems, though painful at times)
• services will need to be able to keep going, even if vital services aren't ready... they should either return failure, or block on requests while a service it needs is down. This might make debugging more difficult (cultural thing), so there should be an easy way to interrogate the state of a service. SIGINFO writing to syslog perhaps?

It's LOTS (order of magnitude) faster, because it parallelizes everything. It's much more elegant, IMHO :-) I'm not pushing it for general use (yet), because it should use XML, have a nice configuration front-end, and be able to replace init(1), so it can daemonize things, but I don't have time to do this right now.

Anyway, the basic idea is:

• /etc/upstart.conf looks somewhat like a Makefile
• you add these lines to /etc/inittab:

  l0:0:wait:/usr/local/sbin/upstart -u 0
  l1:1:wait:/usr/local/sbin/upstart -u 1
  l2:2:wait:/usr/local/sbin/upstart -u 2
  l3:3:wait:/usr/local/sbin/upstart -u 3
  l4:4:wait:/usr/local/sbin/upstart -u 4
  l5:5:wait:/usr/local/sbin/upstart -u 5
  l6:6:wait:/usr/local/sbin/upstart -u 6
  

• you can manually use it:

  satisfactory:~/upstart-0.0.0$ upstart --help
  Usage: upstart [-dhlruv] [--help] [--version] [--down SERVICE]
          [--up SERVICE] [--restart SERVICE] [--status SERVICE]
  
    -d, --down SERVICE            Shut down SERVICE
    -h, --help                    Display a summary of options
    -l, --list                    List all services and their state
    -r, --restart SERVICE         Restart SERVICE
    -s, --status SERVICE          Display the status of SERVICE
    -u, --up SERVICE              Start up SERVICE
    -v, --version                 Display version and licence information
  

  eg:

  # upstart --up network
  

• Sys V doesn't express relationships between services AT ALL. Basically, you do a topological sort yourself, when you give the service the [SK]xx number. This is Wrong. Hence, Upstart's dependency graph setup :-)
• the scripts in Sys V look like a cut&paste job. This is an indication that the philosophy is wrong, somewhere. (Actually, it may indicate a weakness in shell scripts...)

Back to my ideas page