Erlang 增加健壯性后的完整示例

增加健壯性后的完整示例

讓我們改進(jìn) Messager 程序以增加該程序的健壯性：

%%% Message passing utility.  
%%% User interface:
%%% login(Name)
%%%     One user at a time can log in from each Erlang node in the
%%%     system messenger: and choose a suitable Name. If the Name
%%%     is already logged in at another node or if someone else is
%%%     already logged in at the same node, login will be rejected
%%%     with a suitable error message.
%%% logoff()
%%%     Logs off anybody at that node
%%% message(ToName, Message)
%%%     sends Message to ToName. Error messages if the user of this 
%%%     function is not logged on or if ToName is not logged on at
%%%     any node.
%%%
%%% One node in the network of Erlang nodes runs a server which maintains
%%% data about the logged on users. The server is registered as "messenger"
%%% Each node where there is a user logged on runs a client process registered
%%% as "mess_client" 
%%%
%%% Protocol between the client processes and the server
%%% ----------------------------------------------------
%%% 
%%% To server: {ClientPid, logon, UserName}
%%% Reply {messenger, stop, user_exists_at_other_node} stops the client
%%% Reply {messenger, logged_on} logon was successful
%%%
%%% When the client terminates for some reason
%%% To server: {'EXIT', ClientPid, Reason}
%%%
%%% To server: {ClientPid, message_to, ToName, Message} send a message
%%% Reply: {messenger, stop, you_are_not_logged_on} stops the client
%%% Reply: {messenger, receiver_not_found} no user with this name logged on
%%% Reply: {messenger, sent} Message has been sent (but no guarantee)
%%%
%%% To client: {message_from, Name, Message},
%%%
%%% Protocol between the "commands" and the client
%%% ---------------------------------------------- 
%%%
%%% Started: messenger:client(Server_Node, Name)
%%% To client: logoff
%%% To client: {message_to, ToName, Message}
%%%
%%% Configuration: change the server_node() function to return the
%%% name of the node where the messenger server runs

-module(messenger).
-export([start_server/0, server/0, 
         logon/1, logoff/0, message/2, client/2]).

%%% Change the function below to return the name of the node where the
%%% messenger server runs
server_node() ->
    messenger@super.

%%% This is the server process for the "messenger"
%%% the user list has the format [{ClientPid1, Name1},{ClientPid22, Name2},...]
server() ->
    process_flag(trap_exit, true),
    server([]).

server(User_List) ->
    receive
        {From, logon, Name} ->
            New_User_List = server_logon(From, Name, User_List),
            server(New_User_List);
        {'EXIT', From, _} ->
            New_User_List = server_logoff(From, User_List),
            server(New_User_List);
        {From, message_to, To, Message} ->
            server_transfer(From, To, Message, User_List),
            io:format("list is now: ~p~n", [User_List]),
            server(User_List)
    end.

%%% Start the server
start_server() ->
    register(messenger, spawn(messenger, server, [])).

%%% Server adds a new user to the user list
server_logon(From, Name, User_List) ->
    %% check if logged on anywhere else
    case lists:keymember(Name, 2, User_List) of
        true ->
            From ! {messenger, stop, user_exists_at_other_node},  %reject logon
            User_List;
        false ->
            From ! {messenger, logged_on},
            link(From),
            [{From, Name} | User_List]        %add user to the list
    end.

%%% Server deletes a user from the user list
server_logoff(From, User_List) ->
    lists:keydelete(From, 1, User_List).

%%% Server transfers a message between user
server_transfer(From, To, Message, User_List) ->
    %% check that the user is logged on and who he is
    case lists:keysearch(From, 1, User_List) of
        false ->
            From ! {messenger, stop, you_are_not_logged_on};
        {value, {_, Name}} ->
            server_transfer(From, Name, To, Message, User_List)
    end.

%%% If the user exists, send the message
server_transfer(From, Name, To, Message, User_List) ->
    %% Find the receiver and send the message
    case lists:keysearch(To, 2, User_List) of
        false ->
            From ! {messenger, receiver_not_found};
        {value, {ToPid, To}} ->
            ToPid ! {message_from, Name, Message}, 
            From ! {messenger, sent} 
    end.

%%% User Commands
logon(Name) ->
    case whereis(mess_client) of 
        undefined ->
            register(mess_client, 
                     spawn(messenger, client, [server_node(), Name]));
        _ -> already_logged_on
    end.

logoff() ->
    mess_client ! logoff.

message(ToName, Message) ->
    case whereis(mess_client) of % Test if the client is running
        undefined ->
            not_logged_on;
        _ -> mess_client ! {message_to, ToName, Message},
             ok
end.

%%% The client process which runs on each user node
client(Server_Node, Name) ->
    {messenger, Server_Node} ! {self(), logon, Name},
    await_result(),
    client(Server_Node).

client(Server_Node) ->
    receive
        logoff ->
            exit(normal);
        {message_to, ToName, Message} ->
            {messenger, Server_Node} ! {self(), message_to, ToName, Message},
            await_result();
        {message_from, FromName, Message} ->
            io:format("Message from ~p: ~p~n", [FromName, Message])
    end,
    client(Server_Node).

%%% wait for a response from the server
await_result() ->
    receive
        {messenger, stop, Why} -> % Stop the client 
            io:format("~p~n", [Why]),
            exit(normal);
        {messenger, What} ->  % Normal response
            io:format("~p~n", [What])
    after 5000 ->
            io:format("No response from server~n", []),
            exit(timeout)
    end.

主要有如下幾處改動:

Messager 服務(wù)器捕捉進(jìn)程退出。如果它收到進(jìn)程終止信號，{'EXIT',From,Reason}，則說明客戶端進(jìn)程已經(jīng)終止或者由于下面的原因變得不可達(dá)：

• 用戶主動退出登錄（取消了 “l(fā)ogoff” 消息）。
• 與客戶端連接的網(wǎng)絡(luò)已經(jīng)斷開。
• 客戶進(jìn)程所處的結(jié)點崩潰。
• 客戶進(jìn)程執(zhí)行了某些非法操作。

如果收到上面所述的退出信號，服務(wù)器調(diào)用 server_logoff 函數(shù)將 {From, Name} 元組從 User_Lists 列表中刪除。如果服務(wù)端所在的結(jié)點崩潰了，那么系統(tǒng)將將自動產(chǎn)生進(jìn)程終止信號，并將其發(fā)送給所有的客戶端進(jìn)程：'EXIT',MessengerPID,noconnection}，客戶端進(jìn)程收到該消息后會終止自身。

同樣，在 await_result 函數(shù)中引入了一個 5 秒鐘的定時器。也就是說，如果服務(wù)器 5 秒鐘之類沒有回復(fù)客戶端，則客戶端終止執(zhí)行。這個只是在服務(wù)端與客戶端建立連接前的登錄階段需要。

一個非常有意思的例子是如果客戶端在服務(wù)端建立連接前終止會發(fā)生什么情況呢？需要特別注意，如果一個進(jìn)程與另一個不存在的進(jìn)程建立連接，則會收到一個終止信號 {'EXIT',From, noproc}。這就好像連接建立后進(jìn)程立馬就結(jié)束了一樣。