进程单向监控-Monitor

进程单向监控-Monitor

link方式可以建立进程之间的双向链接关系,我们可以通过monitor实现单向的监控,这在gen_server代码里面可以看到对应的例子

erlang:monitor(process, Caller), %建立单向监控
被监控进程死掉发送的消息规格: {'DOWN',Mref,process,Pid,Reason}
解除监控:erlang:demonitor(Ref)

gen_server代码片段:

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), %建立单向监控

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,_,_,_} -> %接受监控消息

%% 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.

handle_call({subscribe, PId}, _From, #state{subscribers = Subscribers} = State) ->

NSubscribers = ordsets:add_element(PId, Subscribers),

erlang:monitor(process, PId),

{reply, {ok, ok}, State#state{subscribers = NSubscribers}};

handle_info({'DOWN', _MRef, _, PId, _Reason}, #state{subscribers = Subscribers} = State) ->

NSubscribers = ordsets:del_element(PId, Subscribers),

{noreply, State#state{subscribers = NSubscribers}};