[erlang-questions] [Code Review] Is this a good idea? I linking 3 process manually (not using recursive)
Andreas Schultz
andreas.schultz@REDACTED
Mon Mar 11 08:55:28 CET 2019
You do realize that your code is still "racy" ?
Invoking it like
2> procslinked:starter(), procslinked:call_the_third().
has a good chance of not producing the expected result. Most times it will
simply fail with:
** exception error: bad argument
in function procslinked:call_the_third/0 (procslinked.erl, line
54)
The reason is that there is no guarantee that `call_the_third()` will be
executed after any of the spawn has succeeded or the register was invoked.
In order to get ordering guarantees you need to wait for the processes to
be past the init stage. The normal way to do that is to wait for a message
from the init function of you child process.
BTW: this is exactly what proc_lib:init_ack does, but I guess you are
trying to implement that yourself as a learning exercise.
Andreas
I Gusti Ngurah Oka Prinarjaya <okaprinarjaya@REDACTED> schrieb am So., 10.
März 2019 um 23:38 Uhr:
> Hi Bengt,
>
> Thank you very much for all of your hints and suggestions. It's clear now.
>
>
>
> Pada tanggal Sen, 11 Mar 2019 05.20, bengt <cean.ebengt@REDACTED>
> menulis:
>
>> I think your code is about learning erlang. Do that first. OTP is the
>> next step.
>>
>> Yes, splitting a gen_server into callback module and interface module is
>> not the OTP way (as described in documentation), so while good for a
>> beginner it can be considered bad style by experienced programmers.
>>
>> Sorry, but I have not seen any tutorial about how to avoid thinking the
>> same function/module is always in the same process. It is a very
>> fundamental concept (them not being) in Erlang, so just keep it in mind.
>> One way would be to try and make all your three functions the same
>> function, just have different arguments to the loop. Then you must think
>> about it.
>>
>>
>> On 10 Mar 2019, at 11:11, I Gusti Ngurah Oka Prinarjaya <
>> okaprinarjaya@REDACTED> wrote:
>>
>> Hi Bengt,
>>
>> Thank you for your review :)
>>
>> I don't quite understand with some of your thought. Then i hope you not
>> mind to give more explanation.
>>
>> >> Seasoned Erlang programmers will tell you this is not the OTP way, or
>> a waste of modules
>> Did you mean my code here: https://pastebin.com/ECf3jhZ2 wasn't the OTP
>> way?
>> I learn from here: https://learnyousomeerlang.com/errors-and-processes
>> and i do really a beginner at programming with erlang.
>>
>> >> So only do this until you have a better understanding.
>> What is that? Did you mean *doing one module with the interface
>> functions to a gen_server and one module for the gen_server callbacks.?*
>> or what?
>>
>> >> Conflating the function (or even more often, module) and the process
>> is a common mistake in the beginning. When my fellow programmers are
>> experienced, but not used >> to process oriented programming.
>> Any tutorials / reference for me to avoid this mistake ?
>>
>> Thank you :)
>>
>>
>>
>>
>>
>>
>>
>>
>> Pada tanggal Min, 10 Mar 2019 pukul 14.49 bengt <cean.ebengt@REDACTED>
>> menulis:
>>
>>> Conflating the function (or even more often, module) and the process is
>>> a common mistake in the beginning. When my fellow programmers are
>>> experienced, but not used to process oriented programming, I have found
>>> that it helps to put the code that is run in different process in different
>>> modules. Ex: one module with the interface functions to a gen_server and
>>> one module for the gen_server callbacks. Seasoned Erlang programmers will
>>> tell you this is not the OTP way, or a waste of modules. So only do this
>>> until you have a better understanding.
>>>
>>> bengt
>>>
>>> On 10 Mar 2019, at 04:57, I Gusti Ngurah Oka Prinarjaya <
>>> okaprinarjaya@REDACTED> wrote:
>>>
>>> Hi Bengt,
>>>
>>> Thank you for the idea,
>>>
>>> I've implement your idea, from performance side, i think using two
>>> version of function will have better performance because there's no need to
>>> checking using IF statement.
>>>
>>> I worry at first to implement this idea, because i think different
>>> version of function will have different PID, hahaha it turns out exactly
>>> have same PID.
>>> this is my new code https://pastebin.com/ECf3jhZ2 . Please review if
>>> there's any improvement needs.
>>>
>>> Thank you
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> Pada tanggal Min, 10 Mar 2019 pukul 05.23 bengt <cean.ebengt@REDACTED>
>>> menulis:
>>>
>>>> Greetings,
>>>>
>>>> The error happens the second time any of the three processes call
>>>> erlang:register/2. They already registered themselves the first time. If
>>>> you want this structure, but not the error, you have to do the register
>>>> before starting the loop. Eg, split the functions into two. One that do the
>>>> register and then calls the second that does the loop.
>>>>
>>>> bengt
>>>>
>>>> On 9 Mar 2019, at 14:41, I Gusti Ngurah Oka Prinarjaya <
>>>> okaprinarjaya@REDACTED> wrote:
>>>>
>>>> Hi Attila Rajmund Nohl,
>>>>
>>>> After i implement your suggestion, i get error when execute any of
>>>> function call_the_*_p()
>>>> for example:
>>>>
>>>> 3> newbie:starter().
>>>> <0.112.0>
>>>> 4> newbie:call_the_second().
>>>> =ERROR REPORT==== 9-Mar-2019::20:35:15.069388 ===
>>>> Error in process <0.113.0> with exit value:
>>>> {badarg,[{erlang,register,[pidsecondp,<0.113.0>],[]},
>>>> {newbie,the_second_p,0,
>>>>
>>>> [{file,"/Users/okaprinarjaya/Oprek/Erlang-Oprek/oprek-lagi/src/newbie.erl"},
>>>> {line,17}]}]}
>>>>
>>>> {<0.113.0>,"the_second_p()","Halo second p!"}
>>>>
>>>> This is my new modified code https://pastebin.com/iYCxkkuz
>>>>
>>>>
>>>> Pada tanggal Jum, 8 Mar 2019 pukul 21.03 Attila Rajmund Nohl <
>>>> attila.r.nohl@REDACTED> menulis:
>>>>
>>>>> I Gusti Ngurah Oka Prinarjaya <okaprinarjaya@REDACTED> ezt írta
>>>>> (időpont: 2019. márc. 8., P, 12:11):
>>>>> >
>>>>> > Hi Folks,
>>>>> >
>>>>> > I need your help to review my code. I create and linking 3 process
>>>>> manually without using recursive. And inside p1 and p2 i using IF statement
>>>>> to check to make sure spawning process will only once.
>>>>> >
>>>>> > I mean, is part code below is a good idea?
>>>>> >
>>>>> > IsPidExists = whereis(xxx),
>>>>> > if IsPidExists =:= undefined ->
>>>>> > Pid = spawn_link(?MODULE, the_p, []),
>>>>> > register(xxx, Pid);
>>>>> > true -> true
>>>>> > end,
>>>>>
>>>>> Generally this is not a good idea, there's a race condition between
>>>>> checking that the process is registered (the whereis/1 call) and
>>>>> registering the new process. A better idea is to start the process and
>>>>> let the process itself to register. If register fails, it means that
>>>>> there's already a process registered, so the just started process can
>>>>> terminate. So your code could look like something like this:
>>>>>
>>>>> starter() ->
>>>>> spawn(?MODULE, the_first_p, []).
>>>>>
>>>>> the_first_p() ->
>>>>> register(pidfirstp, self()),
>>>>> spawn_link(?MODULE, the_second_p, []),
>>>>> ...
>>>>> the_first_p().
>>>>>
>>>>> Actually if the register call fails, it throws a badarg and the
>>>>> process dies automatically, simplifying the code.
>>>>>
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