進程單向監控-Monitor

轉載：http://www.cnblogs.com/me-sa/archive/2011/11/20/erlang0016.html

link方式可以建立進程之間的雙向連結關係,我們可以通過monitor實現單向的監控,這在gen_server代碼裡面可以看到對應的例子

•  erlang:monitor(process, Caller),
  %建立單向監控
• 被監控進程死掉發送的訊息規格:   {'DOWN',Mref,process,Pid,Reason}
  
• 解除監控:erlang:demonitor(Ref)

gen_server程式碼片段:

do_multi_call(Nodes, Name, Req, Timeout) ->
    Tag = make_ref(),
    Caller = self(),
    Receiver =
 spawn(
   fun() ->
    %% Middleman process. Should be unsensitive to regular
    %% exit signals. The sychronization is needed in case
    %% the receiver would exit before the caller started
    %% the monitor.
    process_flag(trap_exit, true),
    Mref = erlang:monitor(process, Caller),
%建立單向監控
    receive
        {Caller,Tag} ->
     Monitors = send_nodes(Nodes, Name, Tag, Req),
     TimerId = erlang:start_timer(Timeout, self(), ok),
     Result = rec_nodes(Tag, Monitors, Name, TimerId),
     exit({self(),Tag,Result});
        {'DOWN',Mref,_,_,_} ->  %接受監控訊息
     %% Caller died before sending us the go-ahead.
     %% Give up silently.
     exit(normal)
    end
   end),
    Mref = erlang:monitor(process, Receiver),
    Receiver ! {self(),Tag},
    receive
 {'DOWN',Mref,_,_,{Receiver,Tag,Result}} ->
     Result;
 {'DOWN',Mref,_,_,Reason} ->
     %% The middleman code failed. Or someone did
     %% exit(_, kill) on the middleman process => Reason==killed
     exit(Reason)
    end.