7.5 Erlang并发编程-银行业务示例

这一节我们将展示如何结合BIF monitor_node/2和向远程节点的注册进程发送消息的能力。我们将实现一个非常简单的银行服务，用以处理远程站点的请求，比如ATM机上存款、取款业务。

程序6.1

-module(bank_server).
-export([start/0, server/1]).

start() ->
    register(bank_server, spawn(bank_server, server, [[]])).

server(Data) ->
    receive
        {From, {deposit, Who, Amount}} ->
            From ! {bank_server, ok},
            server(deposit(Who, Amount, Data));
        {From, {ask, Who}} ->
            From ! {bank_server, lookup(Who, Data)},
            server(Data);
        {From, {withdraw, Who, Amount}} ->
            case lookup(Who, Data) of
                undefined ->
                    From ! {bank_server, no},
                    server(Data);
                Balance when Balance > Amount ->
                    From ! {bank_server, ok},
                    server(deposit(Who, -Amount, Data));
                _ ->
                    From ! {bank_server, no},
                    server(Data)
            end
    end.

lookup(Who, [{Who, Value}|_]) -> Value;
lookup(Who, [_|T]) -> lookup(Who, T);
lookup(_, _) -> undefined.

deposit(Who, X, [{Who, Balance}|T]) ->
    [{Who, Balance+X}|T];
deposit(Who, X, [H|T]) ->
    [H|deposit(Who, X, T)];
deposit(Who, X, []) ->
    [{Who, X}].

程序6.1的代码运行于银行总部。而在出纳机（或分行）中执行的是程序6.2，该程序完成与总行服务器的交互。

程序6.2

-module(bank_client).
-export([ask/1, deposit/2, withdraw/2]).

head_office() -> 'bank@super.eua.ericsson.se'.

ask(Who) ->              call_bank({ask, Who}).
deposit(Who, Amount)  -> call_bank({deposit, Who, Amount}).
withdraw(Who, Amount) -> call_bank({withdraw, Who, Amount}).
call_bank(Msg) ->
    Headoffice = head_office(),
    monitor_node(Headoffice, true),
    {bank_server, Headoffice} ! {self(), Msg},
    receive
         {bank_server, Reply} ->
             monitor_node(Headoffice, false),
             Reply;
         {nodedown, Headoffice} ->
             no
    end.

客户端程序定义了三个访问总行服务器的接口函数：

ask(Who)

返回客户Who的余额
deposit(Who, Amount)

给客户Who的帐户里面存入资金数Amount
withdraw(Who, Amount)

尝试从客户Who的帐户里面取出资金数Amount
函数call_bank/1实现了远程过程调用。一旦总行节点停止运作，call_bank/1将会及时发现，并返回no。

总行节点的名称是硬编码在源码中的。在后续章节中我们将展示集中隐藏该信息的手段。