Re: hello - hanging services

[Laurent Bercot <ska-supervision@skarnet.org>]

>> which is running a command all X seconds and if it gets a response it
>> knows "ah okay, the service is still running" and if it gets no
>> response "oh, the service seems to have died, let's restart it"?
>> 
>> Difficult to implement?
> 
> Yes.

 More precisely, it's not so much "difficult to implement" (I've done
it for a paying customer's project) as "impossible to do without specific
support in the service you're trying to manage".
 In other words, what Jean-Michel wants is a software watchdog; it can
be done, but it's pretty intrusive. It requires having a library, a daemon,
and making library calls in the managed process' source, sending messages
to the daemon by doing so. The daemon is configured with a certain policy
that decides "the service is running fine" or "the service has hung"
depending on the frequency of the messages it receives.

 It's doable, and a watchdog library/daemon may even have its place in
a supervision suite (I'll think about it), but it certainly has nothing
to do with purely external process management tools such as runsvdir/runsv
or svscan/supervise. It's a whole piece of software on its own.

 I'm certain that a lot of open source software watchdogs already exist
out there. I'm also certain that none of them is as lightweight and easy
to use as I'd like, but that's another story.

-- 
 Laurent