erlang R17带来了新的socket选项{active,N} 。与{active,once}连同应用层提供的流量控制。为什么会这样选择，{active,once}不能够有效地抑制了很多socket消息做？
我们知道，，{active,once}一次设置active选项，才干继续接收erlang的消息通知。实际上。每次设定{active,once}都意味着调用一次epoll_ctl， 假设请求过于频繁，就会有大量的epoll_ctl调用。erlang眼下仅仅有一个线程会收割epoll_wait事件。epoll_wait要轮询已经就绪的ctl队列。假设大量的ctl事件将会堵塞了epoll_wait的操作，造成网络处理能力的下降。
那么。我们能不能设定接收N个的socket消息后再运行一次epoll_ctl。这样能够有效降低epoll_ctl的调用。{active,N}就是这样出现的。
以下来看一下{active,N}的说明
Add the {active,N} socket option for TCP, UDP, and SCTP,where N is an integer in the range -32768..32767, to allow a caller to specify the number of data messages to be delivered to the controlling process. Once the
socket's delivered message count either reaches 0 or is explicitly set to 0 with inet:setopts/2 or by including {active,0} as an option when the socket is created, the socket transitions to passive({active, false}) mode and the socket's controlling process receives
a message to inform it of the transition. TCP sockets receive {tcp_passive,Socket}, UDP sockets receive {udp_passive,Socket} and SCTP sockets receive {sctp_passive,Socket}.
The socket's delivered message counter defaults to 0, but it can be set using {active,N} via any gen_tcp, gen_udp, or gen_sctp function that takes socket options as arguments, or via inet:setopts/2. New N values are added to the socket's current counter value,
and negative numbers can be used to reduce the counter value. Specifying a number that would cause the socket's counter value to go above 32767 causes an einval error. If a negative number is specified such that the counter value would become negative, the
socket's counter value is set to 0 and the socket transitions to passive mode. If the counter value is already 0 and inet:setopts(Socket, [{active,0}]) is specified, the counter value remains at 0 but the appropriate passive mode transition message is generated
for the socket.
设定了{active,N}选项后，进程在接收了N个包后，会收到{tcp_passive, Socket}消息，意味着这个Socket进入被动模式，须要又一次设置active选项。
接下来。在实际的样例中測试这个參数：
-module(server). -export([start/0]).
-export([continue/1]).
-define( PORT, 8888). start() ->
{ok, LSock} = gen_tcp:listen(?PORT, [binary, {packet, 0},{active, false}]),
io:format("socket listen: ~p on ~p ~n",[LSock, ? PORT]),
accept(LSock). accept(LSock) ->
{ok, ASock} = gen_tcp:accept(LSock),
Pid = spawn(fun() -> do_loop(ASock) end),
gen_tcp:controlling_process(ASock, Pid),
inet:setopts(ASock, [{active, 3}]),
accept(LSock). do_loop(ASock) ->
receive
{tcp, Socket, Data} ->
io:format("socket ~p recv: ~p ~n",[Socket, Data]);
{tcp_closed, Socket} ->
io:format("socket ~p close ~n",[Socket]);
{tcp_passive,Socket} ->
io:format("socket ~p is passive, please call continue/1 ~p ~n",[Socket, self()]);
release_passive ->
inet:setopts(ASock, [{active, 3}]);
Err ->
io:format("socket may error: ~p ~n",[Err])
end,
do_loop(ASock). continue(Pid) ->
Pid ! release_passive,
ok.
编译启动这个模块后。我们创建一个client来请求这个服务端：
1> f(S), {ok,S} = gen_tcp:connect({127,0,0,1},8888,[{packet,0}]).
{ok,#Port<0.526>}
2> gen_tcp:send(S,<<"hello">>).
ok
3> gen_tcp:send(S,<<"hello">>).
ok
4> gen_tcp:send(S,<<"hello">>).
ok
5> gen_tcp:send(S,<<"hello">>).
ok
服务端控制台打印了这种信息：
D:\tmp>erl -s server
socket listen: #Port<0.422> on 8888
Eshell V6.0 (abort with ^G)
1> socket #Port<0.479> recv: <<"hello">>
1> socket #Port<0.479> recv: <<"hello">>
1> socket #Port<0.479> recv: <<"hello">>
1> socket #Port<0.479> is passive, please call continue/1 <0.33.0>
1> server:continue(pid(0,33,0)).
socket #Port<0.479> recv: <<"hello">>
ok
2>
在上面的样例中，我们设定了{active,3}的选项，在接收到client3次数据后，socket进入了passive状态，在又一次设置{active,N}后继续接收tcp消息。
那么，怎样在实际项目中运用{active,N}选项？
inet:setopts(Socket, [{active, 300}]),
erlang:send_after(30 * 1000, self(), release_passive);
大概思路是，在30秒内最多接收300个包。超过就不接收，等待这30秒完毕后继续接收。如此重复。
利用这点还能够加多一个计数器，假设超过10次进入passive状态，说明这个Socket存在问题，有攻击的行为。
參考：
http://blog.csdn.net/mycwq/article/details/24814843
http://www.erlang.org/download/otp_src_17.0.readme
http://blog.yufeng.info/archives/2970