[erlang-questions] "calling" a gen_fsm

Garry Hodgson <>
Tue Sep 22 16:03:21 CEST 2015


i'm using a gen_fsm to implement the client side of
a binary protocol. clients connect to a controller, do
a login thing, then settle into connected state. so far,
so good.

when in connected state, i need to be able to query
the fsm for information. some of these queries require
sending a message to a controller and waiting for a
response to come back. herein lies the problem, because
the query relies on the sync event to ask the question,
but must wait for an { ssl, Socket, Data } event to get
the answer.

i've got it sort of working, but only by intercepting the
ssl message inline, vs. the normal handle_info() method.
this feels like cheating, and seems fragile, as it relies on
there being no intervening messages.

so i'm wondering, is there a good/standard way of
accomplishing this? or is gen_fsm not the right approach?
any advice would be appreciated.

thanks

code snippets follow:

who_provides( Service ) -> gen_fsm:sync_send_event( ?MODULE, { 
request_services, Service } ).
...
connected( { request_services, Service }, _From, #state{ socket=Socket, 
transport=Transport } = State ) ->
     Transport:send( Socket, service_request( Service ) ),
     case wait_for_services( Socket ) of
         { ok, Services } ->
             { reply, { ok, Services }, connected, State };
         Other ->
             { reply, { error, Other }, connected, State }
     end.

wait_for_services( Socket ) ->
     receive
         { ssl, Socket, Data } ->
             ssl:setopts( Socket, [ { active, once } ] ),
             { ok, parse( Data ) }
     after 30000 ->
         timeout
     end.


-- 
Garry Hodgson
Lead Member of Technical Staff
AT&T Chief Security Office (CSO)

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