gen_server在erlang otp编程中的地位是无可撼动的，几乎都是gen_server或者gen_fsm的模型。那么程序运行起来的时候 我们如何查看gen_server的内部状态呢。有2种方法：
1. 自己写个类似于info这样的函数，来获取状态。
2. 利用系统现有的架构。sasl应用带了一个si的东西 全名是status inspector, 这个东西就是设计来帮用户解决这个问题的。
实验开始:
root@nd-desktop:~# 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}]}].
root@nd-desktop:~# erl -boot start_sasl
Erlang R13B03 (erts-5.7.4) [source][/source][/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: nonode@nohost
Eshell V5.7.4 (abort with ^G)
1> si:start(). %必须手动启动
=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). %打开gen_server的消息log功能
ok
6> abc:test(). %这个请求消息被记录
got msg {test,"param1"}
ok
7> si:pi(abc). %好戏开始
Status for generic server abc
===============================================================================
Pid <0.46.0>
Status running
Parent <0.41.0>
Logged events %这个是log到的消息
{10,
[{{out,ok,<0.41.0>,{state,hello,world}},
abc,
{gen_server,print_event}},
{{in,{'$gen_call',{<0.41.0>,#Ref<0.0.0.85>},{test,"param1"}}},
abc,
{gen_server,print_event}}]}
%这个是format_status的结果 如果没有format_status那么导出是 {a=hello, b=world}
a=== hello
b=== world
ok
8> si:ppi(abc).
Pretty Process Info
-------------------
[{registered_name,abc},
{current_function,{gen_server,loop,6}},
{initial_call,{proc_lib,init_p,5}},
{status,waiting},
{message_queue_len,0},
{messages,[]},
{links,[<0.41.0>]},
{dictionary,[{'$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]]}
结论：
这个是文档未公开的功能。上面演示了如何sys打开log, 如何察看gen_server的状态
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