[erlang-questions] initializing library applications without processes

[karol skocik]

"Hijacking" supervisor this way is pretty nice idea :)
Will do for my case.

Thank you guys for suggestions.
Karol

On Sat, Apr 3, 2010 at 8:41 PM, Ulf Wiger
<ulf.wiger@REDACTED> wrote:
>
> I received exactly this question while at the SF Erlang Factory.
>
> My suggestion is: start a supervisor with no children. A reasonable
> "cheat" is to put all of this in the app file
>
> -module(myapp).
> -behaviour(application).
> -export([start/2, stop/1]).
> -behaviour(supervisor).
> -export([init/1]).
>
> start(_, _) ->
>   supervisor:start_link({local,?MODULE},?MODULE,[]).
>
> stop(_) -> ok.
>
> init([]) ->
>   %% create tables here...
>   {ok, {{one_for_one,3,10},[]}}.
>
> Baking the supervisor into the application callback module is
> in a sense against the OTP design rules, but the most notable
> example of this is surely OTP's 'kernel' application, so you
> won't be tarred and feathered for doing so. :)
>
> BR,
> Ulf W
>
> karol skocik wrote:
>>
>> Hi,
>>  sometimes, I just need a library "application" without processes,
>> which works with some state in ETS. To function properly,
>> they have to make the ETS table and populate it with values
>> from config files. When that step is done, there is nothing left to do
>> for processes, they would just serve as a useless proxy for
>> querying the ETS table, and that's not needed when you can (in some
>> contexts)
>> access the table directly.
>>
>> I like the mechanism of applications and their dependency solving,
>> and I would ideally do application:start(my_app_with_ets_initialised)
>> but that won't do, since {ok, SupPid} is expected as a result.
>>
>> But I don't want to start any processes, I just want to ensure that
>> application_master:init/4 calls my "start" function,
>> and declare application as "running" when 'ok' is returned
>> or fail otherwise (unless it's {ok, SupPid} - as it is now).
>>
>> Currently, when we have library application, application:start(...)
>> does not call anything since 'mod' is not specified in .app file.
>> Could it be changed, and application behaviour would just call
>> start/2 and stop/1 as usual, but start the SupPid optionally when it's
>> supplied?
>> Or - alternatively - is there any way how to ensure that some init
>> function is done
>> fitting nicely with application dependencies as declared in .rel file?
>>
>> Thanks,
>>  Karol
>>
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>
>
> --
> Ulf Wiger
> CTO, Erlang Solutions Ltd, formerly Erlang Training & Consulting Ltd
> http://www.erlang-solutions.com
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