[erlang-questions] Message Loop in Gen_Server

Garrett Smith <>
Wed Apr 17 19:41:59 CEST 2013


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



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