[erlang-questions] gen_server cleanup handling

[Mazen Harake]

Spawn your worker processes in a simple_one_for_one supervisor which is 
under your supervisor. This will propagate the exit to the worker 
processes. Use the gen_server under the first Sup to start a worker 
child in the simple_one_for_one Sup.

In other words; Garrett seems to have gotten it right.

This also eliminates point 3 because the crash will not propagate 
through the gen_server.

I'm assuming from your setup that the gen_server doesn't care if the 
worker crashed or not before it finished, otherwise you might as well 
just trap exits and handle all the exit messages anyway without a 
supervisor.

Hope this makes sense; it is morning here and my coffee cup is still 
full (didn't have a chance to drink it yet :))

Good luck

/Mazen

On 11/05/2010 04:58, Bernard Duggan wrote:
> Hi list,
>     This is something of a followup to my previous question about 
> supervision trees.  Basically we have a gen_server that, in the case 
> of a standard app shutdown, needs to do some cleanup.  A standard app 
> shutdown seems to take the form of supervisors sending 'shutdown' exit 
> messages to their children.  Fine so far.  The catch is that in order 
> for the terminate() handler to be invoked on our server, we have to 
> turn on trap_exit in every gen_server that needs to do cleanup.  This 
> seems non-ideal for a few reasons:
>
> * Elsewhere in the docs, we're cautioned against using trap_exit 
> except when unavoidable (though I'm happy to accept it if this is one 
> such case).
>
> * It means that we don't get automatic propagation of crashes from any 
> worker processes we might spawn.  Instead we have to write a 
> handle_info({'EXIT'...) in every gen_server that might ever link to 
> another process to ensure those crashes are propagated properly.  This 
> seems like it could be solved by instead spawning any worker processes 
> as a child of our supervisor (which is what Garrett suggested we 
> should do anyway) - if that's a good reason to set up things that way, 
> I'm likewise happy to accept it and rearrange our code accordingly.
>
> * Most concerning, though, is the possibility of some library call 
> creating a link to a temporary worker process (or any process for that 
> matter) whose crash should propagate through us - in this case we'd 
> still have to have the handle_info({'EXIT'...) setup as a catchall 
> which seems like a fiddly, repetitive bit of code we'd rather avoid if 
> possible.
>
> So what's the thinking about this?  Am I missing something obvious?  
> Should I just turn on trap_exit willy-nilly wherever I need shutdown 
> cleanup?  Should I just suck it up and write the 'EXIT' message 
> handlers in all such gen_servers?
>
> Cheers,
>
> Bernard
>
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