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。这里我用的是自己机器上的两个不同的节点，客户端为：client@wae01020199woobest.com 服务端为server@wae01020199woobest.com。两个节点分别处理客户端与服务端的逻辑。
客户端：主要封装了三个接口，ask（查询某人的存储信息），desposit（存储某人的信息，{name，money}），withdraw（取钱操作，{name，money}）。这里用了monitor_node（node，true）来检查网络的连接，只要未连接上，被连接进程将向该进程发送{nodedown， nodeid}，只有才第二个参数为true的时候才会进行监听，如果连接不存在，且monitor_node/2被调用，系统将尝试建立连接；若连接建立失败则投递一个nodedown消息。若针对同一节点连续两次调用monitor_node/2则在节点失败时将投递两条nodedown消息。
-module(bank_client). -export([ask/1, deposit/2, withdraw/2]). head_office() -> 'server@wae01020199.woobest.com'. ask(Who) -> call_back({ask, Who}). deposit(Who, Amount) -> call_back({deposit, Who, Amount}). withdraw(Who, Amount) -> call_back({withdraw, Who, Amount}). call_back(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.服务端：首先start（）方法注册一个bank_serser进程，执行server（[]），此时参数为[]。在server中接受消息，ask，deposit，withdraw，例如接受ask（Who）后，返回client（From） {bank_server，lookup（Who，Data）}，即为在Data中查找Who的数值。然后在client中接受后关闭监听进程，并将数据输出到控制台。在接受deposit，withdraw时处理方式类似。
-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}]. 这里说明下自己创建两个节点的过程。client：在bank_client所在目录下以管理员权限运行cmd，然后输入erl -name client。。。以client命名方式打开erlang shell。同样在bank_server中，打开cmd输入erl -name server。。。打开erlang shell。这里说一下，启动可以用 erl -name server ，也可以用erl -sname server，区别在于 The node name will be Name@Host, where Host is the fully qualified host name of the current host. For short names, use flag -sname instead.
