[erlang-questions] msg mailbox and gen_svr shutdown

[Loïc Hoguin]

Right, this is pretty much what I said to Paul in PM. It was in R14B03 
that the behavior changed. I apparently have a @todo wrong past that 
release, and will take a look if I find other things to fix in the docs.

Copy pasting my private reply on this:

Ultimately if we remove the listener I think we want to stop everything.

For "server is overloaded" situations, you can very well use the 
'onrequest' hook which can be set or changed dynamically through 
cowboy:set_protocol_options, in addition to giving it in start_listener. 
Takes a fun that has a single arg as a Req, returns a Req, and if you 
replied from within it it doesn't dispatch the request and stops there 
(you can also force close the connection by setting the Connection 
header to "close").

And adding this:

For graceful shutdowns, well it's highly application dependent. Some 
apps are just short lived connections, so not accepting requests plus a 
short delay ought to do it. Some are long lived, which probably requires 
to send a shutdown message. Some apps may have connections critical 
enough that you don't want to shutdown them. It's up to the application 
implementor to devise the strategy to use for stopping.

On 05/14/2012 02:34 AM, Fred Hebert wrote:
> This is just a guess, but is it possible that this is due to the fact
> Cowboy is using simple_one_for_one supervision? In Pre R15, if I recall
> correctly, the shutdown of sofo supervisors was asynchronous. The
> supervisor would just die and let its children figure out it was gone.
>
> Starting with R15, things started being synchronous and the supervisor
> would wait. A brutal kill that made things work fine before R15
> (excluding the issue of the application master killing everything) could
> start breaking in later versions.
>
> Again, this is just a guess, without looking at the source or anything.
>
> On Sun May 13 14:12:18 2012, Bob Ippolito wrote:
>> Yes, it is documented that request processes continue after you stop
>> the listener. This is incorrect.
>>
>> On Sunday, May 13, 2012, Anthony Ramine wrote:
>>
>> Paweł, from what I gather your supervisor waits 1 ms before
>> killing its children; I think it's pretty normal that some
>> messages are left unprocessed.
>> Bob, cowboy_requests_sup' shutdown strategy is brutal_kill, is it
>> documented somewhere that Cowboy waits before killing requests'
>> processes?
>>
>> Regards,
>>
>> --
>> Anthony Ramine
>>
>>
>>
>>
>> Le 10 mai 2012 à 22:37, Bob Ippolito a écrit :
>>
>>> I had the same problem with Cowboy recently, really ought to file
>>> an issue to at least fix the docs for that.
>>>
>>> For my app I wanted to implement a graceful shutdown that stopped
>>> accepting new connections but allowed existing requests to
>>> finish. The waiting part I implemented by adding a gproc local
>>> property to the workers that I wanted to wait for and then
>>> monitoring them.
>>>
>>> Looks like this:
>>> https://gist.github.com/2655724
>>>
>>> On Thu, May 10, 2012 at 2:53 AM, Paweł Peregud
>>> <paulperegud@REDACTED <javascript:_e({}, 'cvml',
>>> 'paulperegud@REDACTED');>> wrote:
>>>
>>> I was having fun with supervisors yesterday (Cowboy seems to
>>> fail to fulfill the promise of not killing request processes
>>> after listener removal) and I have an example. I've only
>>> investigated the case when supervisor is killed, so YMMV.
>>> Example code is attached. You may modify it to check the
>>> behavior in your case.
>>>
>>> Start supervisor tree with exp_sup_sup:start_link(). Execute
>>> test with exp_sup_sup:test() and exp_sup_sup:test_simple().
>>>
>>> In case of dying supervisor the answer is "no, it does not".
>>>
>>> When supervisor dies, your process is killed as via link
>>> mechanism, so it may leave some unprocessed messages in
>>> inbox. To make sure that every delivered message is served,
>>> you need to add process_flag(trap_exit, true). Messages that
>>> are sent after the moment when supervisor dies are not processed.
>>>
>>> Best regards,
>>>
>>> Paul.
>>>
>>>
>>> On May 9, 2012 11:06 AM, "Andy Richards"
>>> <andy.richards.iit@REDACTED <javascript:_e({}, 'cvml',
>>> 'andy.richards.iit@REDACTED');>> wrote:
>>>
>>> Hi,
>>>
>>> I can't seem to see any confirmation in the documentation
>>> so was wondering if anyone could confirm if messages are
>>> still sent to a supervised gen_svr following a shutdown
>>> message?
>>>
>>> If so how do I cleanly shutdown my gen_svr without
>>> loosing messages? I read in the supervisor child spec
>>> that a shutdown can be set to infinity which i hoped
>>> would allow me to process the msg's in my mailbox but if
>>> I do this will my module continue to receive messages
>>> from other processes? Is my approach flawed and if so
>>> what other ways are there to cleanly shutting down my
>>> gen_svr without loosing messages?
>>>
>>> Many thanks,
>>>
>>> Andy.
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-- 
Loïc Hoguin
Erlang Cowboy
Nine Nines