Concurrency 在Erlang中，多个进程如何同时使用一个公共列表？

我知道Erlang是关于并发的，我们使用spawn/spawn_link来创建一个进程，但我不明白的是，所有进程如何能够同时使用一个通用的用户列表？比如说ordict/dict存储

我想做的是； 1.派生的用户进程订阅/侦听已注册的进程 2.注册进程A存储所有联机用户的{Pid，Userid} 3.当某个用户发送消息时，用户的进程会询问该进程收件人是否在线

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在erlang中发送消息是异步的，但是当一个用户被多个用户发送消息时，它也是异步的吗？

您可以将进程设置为a，并将存储在线用户的任何数据结构保持为进程状态。存储新用户或删除一个用户可以用来完成，检查用户是否在线可以用来完成。或者，您可以让gen_服务器创建一个公开可读的，允许任何进程读取它以检查在线用户，但是存储和删除仍然需要对gen_服务器进行强制转换。您甚至可以使表公开可读写，以便任何进程都可以存储、删除或检查用户。但请记住，创建ets表的进程死亡时，ets表在默认情况下会被销毁，因此，即使创建它的gen_服务器死亡，您也需要保留它，您必须安排它被其他进程继承，或者。

您可以将进程设为A，并保留任何存储在线用户的数据结构作为进程状态。存储新用户或删除一个用户可以用来完成，检查用户是否在线可以用来完成。或者，您可以让gen_服务器创建一个公开可读的，允许任何进程读取它以检查在线用户，但是存储和删除仍然需要对gen_服务器进行强制转换。您甚至可以使表公开可读写，以便任何进程都可以存储、删除或检查用户。但请记住，创建ets表的进程终止时，ets表默认会被销毁，因此，如果即使创建ets表的gen_服务器终止，您也需要保留ets表，则必须安排它被其他进程继承，或者。

一个严肃的解决方案应该使用OTP行为gen_服务器、supervisor。。。正如史蒂夫建议的那样。 无论如何，我编写了一个小的示例模块，它实现了服务器和客户端，可以使用命令erl-sname test在一个节点上启动，例如，也可以使用命令erl-sname node1、erl-sname node2在多个节点上启动

它还包括一个shell会话示例，演示了大多数情况，我希望它可以帮助您跟踪进程之间的同步或异步交换

注意：对用户列表的访问不是并发的，如果该列表像本例中那样由服务器进程拥有，则不可能。这就是为什么Steve建议使用ETS来存储信息并进行真正的并发访问。我曾尝试使用接口编写示例，这些接口应该允许使用ETS而不是元组列表进行快速重构

以下是事件查看器的摘录：

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一个严肃的解决方案应该使用OTP行为gen_服务器、主管。。。正如史蒂夫建议的那样。 无论如何，我编写了一个小的示例模块，它实现了服务器和客户端，可以使用命令erl-sname test在一个节点上启动，例如，也可以使用命令erl-sname node1、erl-sname node2在多个节点上启动

它还包括一个shell会话示例，演示了大多数情况，我希望它可以帮助您跟踪进程之间的同步或异步交换

注意：对用户列表的访问不是并发的，如果该列表像本例中那样由服务器进程拥有，则不可能。这就是为什么Steve建议使用ETS来存储信息并进行真正的并发访问。我曾尝试使用接口编写示例，这些接口应该允许使用ETS而不是元组列表进行快速重构

以下是事件查看器的摘录：

---

除了Steve answer，我还提供了一个玩具模块来说明它是如何工作的。除了Steve answer，我还提供了一个玩具模块来说明它是如何工作的。如何获取事件查看器图片？在send_trace函数中，我添加了一个对et:trace_me/5的调用，它只做了预配置，以便使用et_viewer:start进行跟踪[{trace_global，true}，{trace_pattern，{et，max}，{max_actors，20}]我在trace/0函数中调用。请注意，您需要wx_小部件在您的平台上工作。顺便说一句，如果有人知道如何将进程和服务器强制在同一时间线上，我将不胜感激。如何获取事件查看器图片？在send_trace函数中，我添加了对et:trace_me/5的调用，它除了预配置为使用et_查看器跟踪：我在trace/0函数中调用的start[{trace_global，true}，{trace_pattern，{et，max}，{max_actors，20}] 你需要wx_小部件在你的平台上工作。顺便说一下，如果有人知道如何将进程和服务器强制在同一时间线上，我将不胜感激。

-module(example).

-export([server/0,server_stop/1,server_register_name/2,server_get_address/2, server_quit/2, % server process and its interfaces
         client/1,quit/1,register_name/2,get_address/2,send_message/3,print_messages/1, % client process and its interfaces
         trace/0]). % to call the tracer for a nice message view

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Client interface
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

client(Node) ->
    % connect the current node to the servernode given in parameter
    % it will fail if the connection cannot be established
    true = net_kernel:connect_node(Node),
    % spawn a client process
    spawn(fun () -> client([],unregistered,{server,Node}) end).

register_name(ClientPid,Name) ->
    % use a helper to facilitate the trace of everything
    send_trace(ClientPid,{register_name,self(),Name}),
    % wait for an answer, it is then a synchronous call
    receive
        % no work needed, simply return any value
        M -> M 
    after 1000 ->
        % this introduce a timeout, if no answer is received after 1 second, consider it has failed
        no_answer_from_client
    end.

get_address(ClientPid,UserName) ->
    send_trace(ClientPid,{get_address,self(),UserName}),
    % wait for an answer, it is then a synchronous call
    receive
        % in this case, if the answer is tagged with ok, extract the Value (will be a Pid)
        {ok,Value} -> Value;
        M -> M 
    after 1000 ->
        no_answer_from_client
    end.

send_message(ClientPid,To,Message) ->
    % simply send the message, it is asynchronous
    send_trace(ClientPid,{send_message,To,Message}).

print_messages(ClientPid) ->
    send_trace(ClientPid,print_messages).

quit(ClientPid) ->
    send_trace(ClientPid,quit).



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% client local functions
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

client(Messages,Name,Server) ->
    receive
        {register_name,From,UserName} when Name == unregistered ->
            % if not yet registered send the request to the server and
            % backward the answer to the requester
            Answer = server_register_name(Server,UserName),
            send_trace(From,Answer),
            NName = case Answer of
                registered -> UserName;
                _ -> Name
            end,
            client(Messages,NName,Server);
        {register_name,From,_} ->
            % if already registered reject the request
            send_trace(From,{already_registered_as,Name}),
            client(Messages,Name,Server);
        {get_address,From,UserName} when Name =/= unregistered ->
            Answer = server_get_address(Server,UserName),
            send_trace(From,Answer),
            client(Messages,Name,Server);           
        {send_message,To,Message} ->
            % directly send the message to the user, the server is not concerned
            send_trace(To,{new_message,{erlang:date(),erlang:time(),Name,Message}}),
            client(Messages,Name,Server);
        print_messages ->
            % print all mesages and empty the queue
            do_print_messages(Messages),
            client([],Name,Server);
        quit ->
            server_quit(Server,Name);
        {new_message,M} ->
            % append the new message
            client([M|Messages],Name,Server);
        _ ->
            client(Messages,Name,Server)
        end.

do_print_messages(Messages) ->
    lists:foreach(fun({D,T,W,M}) -> io:format("from ~p, at ~p on ~p, received ~p~n",[W,T,D,M]) end,Messages).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Server interface
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

server() ->
    true = register(server,spawn(fun () -> server([]) end)),
    node().

server_stop(Server) ->
    send_trace(Server,stop).

server_register_name(Server,User) ->
    send_trace(Server,{register_name,self(),User}),
    receive
        M -> M
    after 900 ->
        no_answer_from_server
    end.

server_get_address(Server,User) ->
    send_trace(Server,{get_address,self(),User}),
    receive
        M -> M
    after 900 ->
        no_answer_from_server
    end.

server_quit(Server,Name) ->
    send_trace(Server,{quit,Name}).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% server local functions
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

server(Users) ->
    receive
        stop ->
            ok;
        {register_name,From,User} ->
            case lists:keyfind(User,1,Users) of
                false ->
                    send_trace(From,registered),
                    server([{User,From}|Users]);
                _ -> 
                    send_trace(From,{already_exist,User}),
                    server(Users)
                end;
        {get_address,From,User} ->
            case lists:keyfind(User,1,Users) of
                false ->
                    send_trace(From,{does_not_exist,User}),
                    server(Users);
                {User,Pid} ->
                    send_trace(From,{ok,Pid}),
                    server(Users)
                end;
        {quit,Name} ->
            server(lists:keydelete(Name,1,Users))
    end.



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% global
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

trace() -> 
% start a collector, a viewer and trace the "trace_me" ...
    et_viewer:start([{trace_global, true}, {trace_pattern, {et,max}},{max_actors,20}]).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% helpers
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

send_trace(To,Message) ->
    % all messages will be traced by "et"
    et:trace_me(50,self(),To,Message,[]),
    To ! Message.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% shell commands
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% c(example).
% example:trace().
% N = node().
% C1 = example:client(N).
% example:register_name(pid(0,5555,0),"fails").
% example:register_name(C1,"fails_again").
% example:server().
% example:register_name(C1,"Joe").
% C2 = example:client(N).
% example:register_name(C2,"Bob").
% example:print_messages(C1).
% C2 = example:get_address(C1,"Bob").
% example:send_message(C1,C2,"Hi Bob!").
% example:send_message(C1,C2,"Hi Bob! are you there?").
% example:print_messages(C2).
% example:send_message(C2,C1,"Hi Joe! Got your message.").
% example:print_messages(C2).
% example:print_messages(C1).
% example:quit(C1).
% example:get_address(C2,"Joe").
% example:server_stop({server,N}).
% example:get_address(C2,"Joe").
% example:get_address(C1,"Bob").