gen_server实现

gen_server是erlang otp封装的beheivor模块。

一、gen_server start初始化过程

1、gen_server_start*启动进程，初始化数据，进入loop。gen_server_start*调用gen:start（通过proc_lib:start*创建进程）创建进程。

2、gen执行gen:init_it方法，gen:init_it通过调用gen_server:init_it,gen_server:init_it调用Mod:init实现模块回调，回调成功，执行proc_lib:init_ack，然后进入gen_server:loop等待消息。

3、gen_server:loop收到消息后调用回调模块mod:handle_call mod:handle_cast mod:handle_info处理消息，处理完成后执行gen_server:loop。

4、如果异常，调用gen_server:terminate处理异常，该函数会掉用回调模块mod:terminate执行异常处理，并退出进程exit

二、gen_server:call 

1、gen_server:call通过调用gen:call实现call的。gen:call处理进程信息，然后调用gen:do_call来实现call动作

2、do_call 先调用erlang:monitor监控进程，然后调用erlang:send发送消息，receive reply或者DOWN消息，解除监控erlang:demonitor。如果进程无法监控（crash），监控节点monitor_node(Node,false) ，直接发消息给进程，receive reply或者nodedown，结束节点监控monitor_node(Node,false)。

3、当监控进程时会进行节点连接，所有在发消息时可以认为是已连接状态，设置参数noconnect。

do_call( Process , Label , Request , Timeout ) ->

    %% We trust the arguments to be correct, i.e

    %% Process is either a local or remote pid,

    %% or a {Name, Node} tuple (of atoms) and in this

    %% case this node (node()) _is_ distributed and Node =/= node().

         Node = case Process of

                           { _S , N } when is_atom( N ) ->

                                   N ;

                           _ when is_pid( Process ) ->

                                   node( Process )

                   end ,

         try erlang:monitor(process, Process ) of

                 Mref ->

                         %% If the monitor/2 call failed to set up a connection to a

                         %% remote node, we don't want the '!' operator to attempt

                         %% to set up the connection again. (If the monitor/2 call

                         %% failed due to an expired timeout, '!' too would probably

                         %% have to wait for the timeout to expire.) Therefore,

                         %% use erlang:send/3 with the 'noconnect' option so that it

                         %% will fail immediately if there is no connection to the

                         %% remote node.

                        

                         catch erlang:send(