Supervision Trees - A Beginner Writes

Gordon Guthrie, BTP, BT, SE <>
Thu Jan 15 14:32:12 CET 2004



> -----Original Message-----
> From: Bengt Kleberg [mailto:] 
> Sent: 15 January 2004 12:33
> To: 
> Subject: Re: Supervision Trees - A Beginner Writes
> 
> 
> Gordon Guthrie, BTP, BT, SE wrote:
> > Folks
> > 
> > I am writing my first supervision tree. I have an application which 
> > starts a simple one-for-one supervisor.
> > 
> is this a ''one-for-one'' supervisor that happens to be 
> simple? or is it 
> ''simple-one-for-one'' as per OTP Design Principles, 5.9 
> (http://erlang.org/doc/r9c/doc/design_principles/part_frame.html).

'simple-one-for-one' as per the spec

> 
> 
> > This supervisor starts workers. The intention is that each 
> worker will 
> > eventually handle an http session, so I am starting them as 
> > transients. These processes will die when the user session they 
> > support times out, so each worker spawns a linked timeout process 
> > which periodically messages its worker "if you haven't done 
> anything 
> > since last time then exit".
> > 
> > The problem I have is that when the worker exits (either 
> normal or a 
> > custom timeout message) the supervisor and then the 
> application both 
> > exit.
> 
> according to the manual a transient child process should 
> behave differently it it exits with an abnormnal exit (vs a 
> normal exit). are you getting the same thing?
> 

I tried exiting with exit(normal) and exit("normal timeout") - the second
being (what I think is) an abnormal exit.

> 
> > I can see that there might be circumstances that would kill 
> a worker 
> > in which a supervisor might also want to terminate for a clean 
> > restart.
> 
> it is only if you have more than MaxR number of restarts occur in the 
> last MaxT seconds (as per OTP Design Principles, 5.4), that the 
> supervisor terminates. or if the supervisiors supervisior 
> terminates...

I used the default values of the Emacs skeleton for MaxR and MaxT (it's on
the other machine at the moment, I can't check just now, but I will).

> 
> 
> > So my question is how do I (under OTP) handle the exit message that 
> > results from the spawn_link to the child. In a 'homebrew' 
> supervision 
> > tree (ie last
> > week) I would have just trapped exits...
> 
> imho the supervision behaviour should handle this for you.

That's what I thought... and that the supervisor wouldn't propogate the exit
up to the application

> 
> 
> bengt
> 

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