Re: Multiple processes started when explicit start commands areused on dependant processes.

[Jan-Henrik Haukeland]

On 7. feb. 2006, at 19.43, Philipp Berndt wrote:

> On Thursday 02 February 2006 15:05, Jan-Henrik Haukeland wrote:
> > On 30. jan. 2006, at 20.14, P Holdaway wrote:
> > >   Conceptually, does the code make an effort to block other start/ 
> > > stop
> > > process commands while the start/stop of a dependency tree is
> > > executing?
> >
> > For start it does not wait, for stop it waits. You can examine
> > control.c to see how this is handled.
>
> What sense does it make to run several start scripts concurrently?

Not concurrently but sequential, that is, sequential forks :) But I  
agree, it would be much better for monit to wait on each process it  
starts in a depend tree.

> Note that checking for the process id is *NOT* enough, because the  
> service may
> have to initialize (spawn other processes, claim resources etc.).

This is exactly the problem, waiting for a process or pid file to be  
written is not enough, since this is typically done at the start and  
before other initializing tasks. So what should we wait for? If you  
have a good _general_ suggestion I'm all ears.

If it's a socket based server, such as apache or mysql, we could wait  
for it to pass a connection w/protocol test. That should work since  
we then now it is initialized and the processing machinery works.  
However this is not the case for many servers started by monit and  
hence not general.

> Only after the start script has terminated you can be sure the  
> service is
> ready and may be used by dependant services.

Nope, if the start script calls 'exec' it will never return or if it  
just does a fork ala 'program&' we are still back to the problem  
situation described above.

> Besides, even running non-dependant start-scripts in parallel  
> probably just
> complicates things, muddles logging

For monit's part, logging is synchronized and so is the code. The  
problem is to find a good condition for saying that a server is up  
and running. Preferably a condition that can be expressed in C. The  
"best" we can do is probably to wait for a pid file to be written  
after all, and monit should check for the existence of a process pid  
file before it starts the next process in a dependency graph. Patch  
is welcome :)

--
Jan-Henrik Haukeland
Mobil +47 97141255