Re: starting service order

[Sebastien ESTIENNE]

Jan-Henrik Haukeland wrote:

> > > 2) a kind of ordering, or at least, the ability to not startup the
> > > subsequent service in the list if the previous did not already  
> > > completed
> > > to startup _without_ using a dependency
> > it could be implemented by keywords like before/after
> > Gentoo linux already provide this functionnality (look here:  
> > http://www.gentoo.org/doc/en/handbook/handbook-x86.xml? 
> > part=2&chap=5#doc_chap4 )
> > you can define order with before after
> > dependencies need/use and also provide
> >
> > for example you can say that qmail "provide mta" and "need  net"
> > and that nagios "need mta"
> >
> > so the system knows that he must start qmail before nagios (if you 
> > had  use the "use" keyword instead of "need" it means that qmail can 
> > fail  it's okà
> >
> > "provide" is also because you can say that many service provide the  
> > same functionnality, eg: mta -> qmail, postfix, exim
>
> (1) Interesting idea. It sounds like something that monit could 
> benefit  from. It means that we need to replace dependency in monit 
> with the  keyword "need" and add the keyword "use" for start order and 
> as a weak  dependency. (If I got this right?)
Yes that's right, i think we can use the gentoo doc and meaning of these 
keyword as a basis, as i said in another mail, gentoo has a really 
finegrained services order and dependency system (it's explain in the 
docs here: http://www.gentoo.org/doc/en/handbook/handbook-x86.xml? 
part=2&chap=5#doc_chap4) but it missed the great features of monit and 
could benefit a lot of it.

> > > 3) some tool to easily rearrange the services (a chkconfig-like 
> > > tool)  4)
> > > the ability tu support different runlevels
> > I wrote this tool, it was easy, i just modified the default tool 
> > from  gentoo (it was just a matter of 5 lines)
>
>
> Care to share the tool with the list?

of course i can share it, i didn't do i yet because it's really gentoo 
specific (i modified the default tool)
If you want i could wrote a lite version of it just for monit.
It's just a matter of managing symlinks and tell monit to reread it's 
config file (plus some sanity checks).

> > and i don't see why monit couldn't do this?
> > for example djb's daemontools where built to manage services (they  
> > manage qmail/djbdns) and monit seems to be a daemontools++. too bad 
> > he  also missed the ability to define services start order...
>
> I think that monit can remplace the default system, because the onlyt  
> things that the default init system has to do, is stating/stopping  
> system in the right order nothing more
>
> If we missed this it doesn't mean that it cannot be changed. I also  
> think that you can use monit with success as a replacement for 
> sysinit  startup scripts, but means to set the start order is needed, 
> (2) In  addition monit should have the ability to start a process 
> without  depending on a pid file. (As stated in the first comment to 
> monit here,  http://freshmeat.net/projects/monit/) I.e. monit needs to 
> listen to the  SIGCHLD signal for processes it start, so if a child 
> dies monit get a  signal and can restart the process. I haven't looked 
> at the init nor  the daemontools code, but I'm pretty sure this is 
> what those programs  do. The reason we don't do this already is that 
> monit was (initially)  designed to monitor processes that was started 
> outside of monit's  control. (To listen for SIGCHLD monit must start 
> the process).

that's maybe why in system using daemontools (runit for example) all 
services are child or the monitoring process (svscan/supervise), and the 
service MUSTN'T fork to be monitored by daeamontools

> I think as a start we should add (1) and (2) to our TODO list. What 
> do  the other commiters think?

Marco suggested in his email some threading to not "block" monit process 
while it is starting another process, and also to be able to "see" if a 
process is in "starting in progress" state.