[erlang-questions] Message Loop in Gen_Server

Lee Sylvester <>
Wed Apr 17 19:46:27 CEST 2013


Thank you.  I've actually seen that done in the Erlang in Action book.  Didn't think of it myself :-)  You're a star!

Regards,
Lee



On 17 Apr 2013, at 18:41, Garrett Smith <> wrote:

> On Wed, Apr 17, 2013 at 12:33 PM, Lee Sylvester <> wrote:
>> Hey guys,
>> 
>> So, I've hit a "best practice" conundrum in OTP; I have a server utilising gen_server for a RabbitMQ consumer.  In the init of that gen_server, I'm setting up a RabbitMQ connection, but I also need to start a loop.  My guess was that I shouldn't call this before init exits, as I was passing the Connection and Channel objects to state for handling elsewhere.  If I handle the loop in init, surely it will never return?
>> 
>> To simplify what I'm saying (as I'm confusing myself here), here's my code:
>> 
>> init([]) ->
>>    {ok, Connection} = amqp_connection:start(#amqp_params_network{ host="localhost" }),
>>    {ok, Channel} = amqp_connection:open_channel(Connection),
>>    amqp_channel:call(Channel, #'exchange.declare'{exchange = <<"user_msgs">>,
>>                                                   type = <<"direct">>}),
>>    #'queue.declare_ok'{queue = Queue} =
>>        amqp_channel:call(Channel, #'queue.declare'{exclusive = true}),
>>    State = {Channel, Connection},
>>    amqp_channel:call(Channel, #'queue.bind'{exchange = <<"user_msgs">>,
>>                                              routing_key = term_to_binary(node(self())),
>>                                              queue = Queue}),
>>    amqp_channel:subscribe(Channel, #'basic.consume'{queue = Queue,
>>        no_ack = true}, self()),
>>    receive
>>        #'basic.consume_ok'{} -> ok
>>    end,
>>    loop(Channel),
>>    {ok, State}.
> 
> Definitely not in init/1 -- that blocks the caller to start_link.
> 
>> Now, if I don't put the loop in my init, then how can I be sure that the loop is called every time the gen_server restarts?  Can someone please suggest the "right" way to call the loop in my gen_server?
> 
> It's awkward, which is the point of e2_task -- see
> http://e2project.org if you're curious -- but this is how:
> 
> init(Args) ->
>  % init here
>  {ok, State, 0}.
> 
> handle_info(timeout, State) ->
>    loop().
> 
> The 0 element in the init result will cause an immediate 'timeout'
> message to be sent to the process, resulting in a handle_info/2
> callback.
> 
> Garrett



More information about the erlang-questions mailing list