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| floob |
 Posted: Fri May 07, 2010 9:25 pm |
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Joined: 07 May 2010
Posts: 5
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If anyone would like some easy points on StackOverflow, I originally posted this question over there (link).
I'm new to erlang, I've been reading through the online documentation, and I'm trying to get a grip on the basics of the OTP framework.
I've found that when testing a gen_server independently, from the command line, I can spawn a process with my gen_server implementation's start_link function and interact with the server just fine.
e.g.
Code:
1> spawn(echo_server, start_link, []).
<0.37.0>
2> echo_server:echo(hi).
hi
This is not the case for my supervisor. If I spawn a process that runs my supervisor's start_link, it appears to start the supervisor and gen_server (SASL turned on, I get a progress report saying they've started), but I cannot interact with my gen_server.
e.g.
Code:
1> spawn(echo_sup, start_link, []).
<0.37.0>
2> echo_server:echo(hi).
** exception exit: {noproc,{gen_server,call,[echo_server,{echo,hi}]}}
in function gen_server:call/2
In both implementations (supervisor and gen_server), their respective start_link methods call the appropriate module's start_link function (supervisor and gen_server, respectively) with Code: ...:start_link({local, ?MODULE}, ?MODULE, [])
Can someone explain the discrepancy? |
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| zajda |
| Posted: Sat May 08, 2010 4:17 pm |
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User
Joined: 22 Aug 2009
Posts: 83
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please, copy-paste the code of your supervisor.
Michał Zajda
------------------
Erlang Solutions Ltd |
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| Michal Ptaszek |
| Posted: Sat May 08, 2010 4:19 pm |
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User
Joined: 01 May 2008
Posts: 35
Location: Krakow
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When you are starting supervisor, you run:
Code:
1> spawn(echo_sup, start_link, []).
<0.37.0>
The new process is spawned, (pid <0.37.0>). Within its context the supervisor:start_link is called which creates new supervisor process for you and links it to <0.37.0>. Since execution of echo_sup:start_link/0 reaches end of the function, <0.37.0> terminates and sends an exit signal to the supervisor causing its termination which in turn is propagated down, to your gen_server.
In order to avoid this situation run echo_sup:start_link/0 directly from shell, without spawning new process.
Ideally you would use the application behaviour and start it via: application:start(my_app). |
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| floob |
| Posted: Sat May 08, 2010 7:49 pm |
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Joined: 07 May 2010
Posts: 5
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@Michal Ptaszek: That would explain why I can't spawn a supervisor, but that doesn't explain why I can spawn a gen_server in the same way.
Also, if the death of the spawned process <0.37.0> propogated death to my supervisor, wouldn't the SASL application have reported the death of the supervisor and gen_server? It only shows a progress report that they've started.
@zajda
The gen_server code can be found here, if you need
echo_sup.erl:
Code:
-module(echo_sup).
-behaviour(supervisor).
-export([start_link/0]).
-export([init/1]).
start_link() ->
supervisor:start_link(echo_sup, []).
init(_Args) ->
{ok, {{one_for_one, 5, 60},
[{echo_server, {echo_server, start_link, []},
permanent, brutal_kill, worker, [echo_server]}]}}.
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