gen_server在erlang otp程式設計中的地位是無可撼動的，幾乎都是gen_server或者gen_fsm的模型。那麼程式執行起來的時候 我們如何檢視gen_server的內部狀態呢。有2種方法：
1. 自己寫個類似於info這樣的函式，來獲取狀態。
2. 利用系統現有的架構。sasl應用帶了一個si的東西 全名是status inspector, 這個東西就是設計來幫使用者解決這個問題的。

實驗開始:

[email protected]:~# cat abc.erl

-module(abc).
-behaviour(gen_server).
-export([start_link/0]).
-export([init/1, handle_call/3, handle_cast/2, handle_info/2,
         terminate/2, code_change/3]).

-export([format_status/2]).
-export([test/0]).

-record(state, {a, b}).

-define(SERVER, ?MODULE).

start_link() ->
    gen_server:start_link({local, ?SERVER}, ?MODULE, [], []).

test()->
    gen_server:call(?SERVER, {test, "param1"}).

init([]) ->
    {ok, #state{a=hello, b=world}}.

handle_call({test, _} = Request, _From, State) ->
    io:format("got msg ~p~n", [Request]),
    {reply, ok, State};

handle_call(_Request, _From, State) ->
    Reply = ok,
    {reply, Reply, State}.

handle_cast(_Msg, State) ->
    {noreply, State}.

handle_info(_Info, State) ->
    {noreply, State}.

terminate(_Reason, _State) ->
    ok.

code_change(_OldVsn, State, _Extra) ->
    {ok, State}.

format_status(_Opt, [_PDict, #state{a=A,
                                    b = B
                                    }]) ->
   
    [{data, [{"a===", A},
             {"b===", B}]}].

[email protected]:~# erl -boot start_sasl
Erlang R13B03 (erts-5.7.4) [/source] [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]


=PROGRESS REPORT==== 29-Oct-2009::16:07:24 ===
          supervisor: {local,sasl_safe_sup}
             started: [{pid,<0.35.0>},
                       {name,alarm_handler},
                       {mfa,{alarm_handler,start_link,[]}},
                       {restart_type,permanent},
                       {shutdown,2000},
                       {child_type,worker}]

=PROGRESS REPORT==== 29-Oct-2009::16:07:24 ===
          supervisor: {local,sasl_safe_sup}
             started: [{pid,<0.36.0>},
                       {name,overload},
                       {mfa,{overload,start_link,[]}},
                       {restart_type,permanent},
                       {shutdown,2000},
                       {child_type,worker}]

=PROGRESS REPORT==== 29-Oct-2009::16:07:24 ===
          supervisor: {local,sasl_sup}
             started: [{pid,<0.34.0>},
                       {name,sasl_safe_sup},
                       {mfa,
                           {supervisor,start_link,
                               [{local,sasl_safe_sup},sasl,safe]}},
                       {restart_type,permanent},
                       {shutdown,infinity},
                       {child_type,supervisor}]

=PROGRESS REPORT==== 29-Oct-2009::16:07:24 ===
          supervisor: {local,sasl_sup}
             started: [{pid,<0.37.0>},
                       {name,release_handler},
                       {mfa,{release_handler,start_link,[]}},
                       {restart_type,permanent},
                       {shutdown,2000},
                       {child_type,worker}]

=PROGRESS REPORT==== 29-Oct-2009::16:07:24 ===
         application: sasl
          started_at: 
 
[email protected]
Eshell V5.7.4  (abort with ^G)
1> si:start(). %必須手動啟動

=PROGRESS REPORT==== 29-Oct-2009::16:07:31 ===
          supervisor: {local,sasl_sup}
             started: [{pid,<0.43.0>},
                       {name,si_server},
                       {mfa,{si_sasl_supp,start_link,[[]]}},
                       {restart_type,temporary},
                       {shutdown,brutal_kill},
                       {child_type,worker}]
{ok,<0.43.0>}
2> si:help().

Status Inspection tool - usage
==============================
    For all these functions, Opt is an optional argument
    which can be 'normal' or 'all'; default is 'normal'.
    If 'all', all information will be printed.
    A Pid can be: "<A.B.C>", {A, B, C}, B, a registered_name or an abbrev.
ANY PROCESS
si:pi([Opt,] Pid)   - Formatted information about any process that
                      SI recognises.
si:pi([Opt,] A,B,C) - Same as si:pi({A, B, C}).
si:ppi(Pid)         - Pretty formating of process_info.
                      Works for any process.
MISC
si:abbrevs()        - Lists valid abbreviations.
si:start_log(Filename) - Logging to file.
si:stop_log()
si:start()          - Starts Status Inspection (the si_server).
si:start([{start_log, FileName}])
si:stop()           - Shut down SI.
ok
3> abc:start_link().
{ok,<0.46.0>}
4> abc:test().
got msg {test,"param1"}
ok
5> sys:log(abc,true). %開啟gen_server的訊息log功能
ok
6> abc:test().  %這個請求訊息被記錄
got msg {test,"param1"}
ok
7> si:pi(abc). %好戲開始

Status for generic server abc                                                 
===============================================================================
Pid                                                                    <0.46.0>
Status                                                                  running
Parent                                                                 <0.41.0>
Logged events  %這個是log到的訊息
         {10,
         [{{out,ok,<0.41.0>,{state,hello,world}},
           abc,
           {gen_server,print_event}},
          {{in,{'$gen_call',{<0.41.0>,#Ref<0.0.0.85>},{test,"param1"}}},
           abc,
           {gen_server,print_event}}]}

%這個是format_status的結果  如果沒有format_status那麼匯出是 {a=hello, b=world}

a===                                                                      hello
b===                                                                      world

ok
8> si:ppi(abc).

Pretty Process Info
-------------------
[{registered_name,abc},
{current_function,{gen_server,loop,6}},
{initial_call,{proc_lib,init_p,5}},
{status,waiting},
{message_queue_len,0},
{messages,[]},
{links,[<0.41.0>]},
{dictionary,[{'$ancestors',[<0.41.0>]},{'$initial_call',{abc,init,1}}]},
{trap_exit,false},
{error_handler,error_handler},
{priority,normal},
{group_leader,<0.25.0>},
{total_heap_size,233},
{heap_size,233},
{stack_size,9},
{reductions,117},
{garbage_collection,[{fullsweep_after,65535},{minor_gcs,0}]},
{suspending,[]}]
ok

9> sys:get_status(abc).
{status,<0.46.0>,
        {module,gen_server},
        [[{'$ancestors',[<0.41.0>]},{'$initial_call',{abc,init,1}}],
         running,<0.41.0>,
         [{log,{10,
                [{{out,ok,<0.41.0>,{state,hello,world}},
                  abc,
                  {gen_server,print_event}},
                 {{in,{'$gen_call',{<0.41.0>,#Ref<0.0.0.85>},
                                   {test,"param1"}}},
                  abc,
                  {gen_server,print_event}}]}}],
         [abc,{state,hello,world},abc,infinity]]}

結論：
這個是文件未公開的功能。上面演示瞭如何sys開啟log, 如何察看gen_server的狀態

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