[erlang-questions] supervisor started from gen_server

Kannan <>
Fri May 20 15:04:33 CEST 2011


You will have assume and concentrate on many other things other than
concentrating on your application logic.


On Fri, May 20, 2011 at 5:53 PM, Mazen Harake <>wrote:

> Correct.
>
> A Bot (in this case) is seen as an extension of the client. Thus if the bot
> (which by implication is the client) is misbehaving or enters a faulty state
> the client crashes. The result of this is that the client is restarted (and
> the corresponding bots are reset).
>
> /M
>
>
> On 20 May 2011 14:19, Ahmed Omar <> wrote:
>
>> Hi Mazen ,
>>
>> One question rises here about this. What happens when the processes under
>> that supervisor reach the maximum number of restarts, and the supervisor is
>> terminated. In a "traditional" supervision tree, the higher level supervisor
>> shall react to that. In your case, it will be the gen_server which i imagine
>> will crash then?
>>
>> On Fri, May 20, 2011 at 2:00 PM, Mazen Harake <>wrote:
>>
>>> Of course you *can* but the point I'm trying to make is that it's not
>>> "wrong" to do it like I did and it doesn't break OTP behaviour and is
>>> actually convenient. Doing it like I did eliminates all the clean up code I
>>> have to make when starting and stopping when the supervisor is "parallel"
>>> rather than "under" my client process.
>>>
>>> As I said, in general you wouldn't want to do it like that but you *can*
>>> and it isn't *wrong* and it isn't a *hack*. The Erlang documentation doesn't
>>> (AFAIK) bring up this scenario but it doesn't make it less useful.
>>>
>>> That is all.
>>>
>>>
>>> On 20 May 2011 13:52, Kannan <> wrote:
>>>
>>>> Mazen,
>>>>
>>>> You can do the same thing with simple_one_for_one supervisors,
>>>> one_for_all supervisors and one_for_one supervisors stacked in different
>>>> levels.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Fri, May 20, 2011 at 2:55 PM, Mazen Harake <>wrote:
>>>>
>>>>> I don't agree with the "never" part. There are corner cases and I
>>>>> would, in general, absolutely agree with what you wrote but here is one
>>>>> example of an exception.
>>>>>
>>>>> In my IRC library (https://github.com/mazenharake/eirc) I have the
>>>>> following scenario:
>>>>>
>>>>> One can create 1..n clients. Each client is a process (running a
>>>>> gen_server in eirc_cl.erl). A client can have many processes associated with
>>>>> it, in my case these are called Bots. Each bot is started and supervised by
>>>>> a client. A bot shall not terminate unless told to do so (gracefully) or if
>>>>> the parent dies (i.e the client). Now if I manually start a supervisor
>>>>> instance inside the gen_server I can simply make use of the supervisor
>>>>> functionality without handling any of that in my gen_server. I can add and
>>>>> remove children (bots) dynamically from my supervisor I.e. I don't have to
>>>>> maintain my own list of them and when I (the client) exits my children (I.e.
>>>>> the supervisor) will exit and thus my bots will exit... all according to OTP
>>>>> principals and I don't need to catch exit signals. This makes it REALLY easy
>>>>> to clean up things since all I have to do is to close the client process,
>>>>> the client process doesn't even need logic in the terminate callback since
>>>>> the children will exit with the same reason. If my supervisor was "on the
>>>>> outside" (like you suggest and which it normally is) I would have to make
>>>>> sure to clean up after myself with the supervisor.
>>>>>
>>>>> So it is much simpler (from an isolation point of view) to (in my case)
>>>>> start a supervisor Inside the gen_server.
>>>>>
>>>>> never say never ;)
>>>>>
>>>>> /M
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On 18 May 2011 14:13, Vance Shipley <> wrote:
>>>>>
>>>>>> On Wed, May 18, 2011 at 11:38:50AM +0200, Roberto Ostinelli wrote:
>>>>>> }  just a quick question: can you think of any case in which starting
>>>>>> a
>>>>>> }  supervisor from a gen_server would actually be a perfectly fit
>>>>>> idea?
>>>>>>
>>>>>> Never.
>>>>>>
>>>>>>                      +---------+
>>>>>>                      | foo_sup |
>>>>>>                      +-+-----+-+
>>>>>>                       /       \
>>>>>>           +----------+--+   +--+---------+
>>>>>>           | bar_sup_sup |   | foo_server |
>>>>>>           +---+---------+   +------------+
>>>>>>               |
>>>>>>          +----+----+
>>>>>>          | bar_sup |
>>>>>>          +---------+
>>>>>>
>>>>>> Start a supervisor for the new dynamic supervisor(s) and use
>>>>>> supervisor:start_child/2 from the gen_server to start it.
>>>>>>
>>>>>> --
>>>>>>         -Vance
>>>>>> _______________________________________________
>>>>>> erlang-questions mailing list
>>>>>> 
>>>>>> http://erlang.org/mailman/listinfo/erlang-questions
>>>>>>
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> erlang-questions mailing list
>>>>> 
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>>>>>
>>>>
>>>
>>> _______________________________________________
>>> erlang-questions mailing list
>>> 
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>>>
>>
>>
>> --
>> Best Regards,
>> - Ahmed Omar
>> http://nl.linkedin.com/in/adiaa
>> Follow me on twitter
>>  @spawn_think <http://twitter.com/#%21/spawn_think>
>>
>>
>
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