[erlang-questions] Adding simple process to supervisor?

Bengt Kleberg <>
Fri Oct 10 08:03:58 CEST 2008


Greetings,

After suggestions from the list I have have tried the "proc_lib creates
special process" method. It meant using proc_lib:start_link/3, calling
proc_lib:init_ack/2, adding receive of system messages (2 lines) and 2
exported call-back functions (1 line each). That was acceptable
complications too me since I keep all the restarting in the supervisor.
Note that I think using your method would have been fewer lines.

However, the process blocks in gen_tcp:accept/1, thus missing any system
messages during this time. Is there a way around that?


bengt

On Thu, 2008-10-09 at 21:43 +0100, Oscar Hellström wrote:
> Hi Bengt,
> 
> What I usually do is just to spawn a reader process from the gen_server and link to it. Then depending on if the gen_server should die with errors in the reader process or not I sometimes traps exits in the gen_server. Most of the time you want to restart the gen_server to open a new socket if the reader dies anyway.  IMHO, there are too many issues with trying to get a process that does blocking calls to exist under a supervisor and this kind of process is typically very simple (read small room for errors) as well. Another approach is to use proc_lib to create a "special process" which uses {active, once} for the socket, to get away from doing blocking calls.
> 
> ----- Original Message -----
> From: "Bengt Kleberg" <>
> To: "erlang-questions" <>
> Sent: Thursday, 9 October, 2008 12:03:27 AM GMT -08:00 US/Canada Pacific
> Subject: [erlang-questions] Adding simple process to supervisor?
> 
> Greetings,
> 
> I have a gen_server which is supervised according to OTP rules. Now I  
> want to add a simple process that will read from a TCP port and send  
> the incoming data to the gen_server. I could spawn_link this process  
> from the gen_server and restart it if it dies. However, since I have a  
> supervisor it seems more logical to just add the process there, as a  
> rest_for_one.
> 
> 
> There is the alternative of using a supervisor_bridge, but that seems  
> more complex than using the gen_server. It seems as if I just design  
> my process according to the OTP rules I will not need to use a  
> supervisor_bridge. What are those rules? The only suggestions I have
> found are:
> 1 Module:start_link/1 => {ok, Pid}
> 2 Module:terminate/2 => ok
> 
> 
> The child specification returned by the init/1 call-back in a  
> supervisor mentions the value for Modules if we have a gen_server,  
> gen_fsm or gen_event. What could be used for a simple process?
> 
> 
> But how do I add a simple process to the supervisor? It seems overkill
> to masquerade it as a get_fsm or similar. Since it is not called from
> erlang it is not a gen_server. Since it does not have state it is not a
> gen_fsm. No events are handled.
> 
> 
> bengt
> 
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