How to grap the Application Message response?

Elaborating the answer by @Tomas Ruderfelt , here’s what I have done in a similar scenario where I had to process the reply from an external web service in Apps9 without doing any code modification 

In my scenario, I’m getting the currency rates from an external API and update IFS currencies.

1: Create an event on FNDCN_APPLICATION_MESSAGE_TAB based on STATE field

Event Action: remember to add proper conditions just to trigger the action only for your integration, and state = Finished

This calls another event in a background job. Reason for taking this approach is the message reply is not commit at this time so we can’t access it directly from the event action on FNDCN_APPLICATION_MESSAGE_TAB

Code:

declare

  event_enabled_ BOOLEAN := Event_SYS.Event_Enabled('Event','CURRENCY_UPD_EXECUTE');

  parameters_    VARCHAR2(32000);

  event_msg_     VARCHAR2(32000);

  id_            NUMBER;

begin

  IF event_enabled_ THEN

    Fnd_Session_API.Impersonate_Fnd_User(fnd_session_api.get_app_owner);

    event_msg_ := Message_SYS.Construct('CURRENCY_UPD_EXECUTE');

    Message_SYS.Add_Attribute(event_msg_,'APPLICATION_MESSAGE_ID','&NEW:APPLICATION_MESSAGE_ID');

    Client_SYS.Add_To_Attr('EVENT_LU_NAME_', 'Event', parameters_);

    Client_SYS.Add_To_Attr('EVENT_ID_','CURRENCY_UPD_EXECUTE',parameters_);

    Client_SYS.Add_To_Attr('EVENT_DATA_', event_msg_, parameters_);

  

    Transaction_SYS.Deferred_Call(id_,'EVENT_SYS.EVENT_EXECUTE','PARAMETER',parameters_,'Process cureency XML');

    Fnd_Session_API.Reset_Fnd_User;

  END IF;

end;

Script to create the CURRENCY_UPD_EXECUTE event:

BEGIN

   Event_SYS.Enable_Event('Event', 'CURRENCY_UPD_EXECUTE',

                          'Execute Currency XML',

   'APPLICATION_MESSAGE_ID/STRING^');

END;

Now we can create an event action for this event to do the real work.

Reply from the external web service is stored as a blob in  MESSAGE_VALUE column in fndcn_message_body_tab

 select MESSAGE_VALUE from fndcn_message_body_tab

   where application_message_id = '&APPLICATION_MESSAGE_ID'

   and REPLY = 1;

after this point, it’s converting reply to xml (or json) and process via standard plsql logic.

Hope it helps!