Erlang mnesia(一) 生成sequence 查错过程

整了段代码，想用mnesia数据库来实现类似于Oracle中sequence 的功能。

原代码如下：

%% Author: Administrator
%% Created: 2012-2-16
%% Description: TODO: Add description to id_generator
-module(id_generator).
-behavior(gen_server).
%%
%% Include files
%%
%%
%% Exported Functions
%%
-export([start_link/0,getnewid/1]).
-export([init/1,handle_call/3,handle_cast/2,handle_info/2,terminate/2,code_change/3]).
-record(ids,{idtype,ids}).
-record(state,{}).
%%
%% API Functions
%%
start_link()->
	gen_server:start_link({local,?MODULE}, ?MODULE, [],[])
.
init([])->
	mnesia:start(),
	io:format("Started"),
	mnesia:create_schema([node()]),
	case mnesia:create_table(ids,[{type,ordered_set},
									  {attributes,record_info(fields,ids)},
									  {disc_copies,[]}
									 ]) of
		{atomic,ok}->
			{atomic,ok};
		{error,Reason}->
			io:format("create table error")
	end,
	{ok,#state{}}
.
getnewid(IdType)->
	io:format("waiting ~n"),
	%case mnesia:wait_for_tables([tbl_clientid], 5000) of
	%	ok->
	%		gen_server:call(?MODULE, {getid,IdType});
	%	{timeout,_BadList}->
	%		{timeout,_BadList};
	%	{error,Reason}->
	%		{error,Reason}
	%end
    mnesia:force_load_table(ids),
	gen_server:call(?MODULE, {getid,IdType})
.
%%generate new Id with given type
handle_call({getid,IdType},From,State)->
	F=fun()->
		Result=mnesia:read(ids,IdType,write),
		case Result of
			[S]->
				io:format("have ~n"),
				Id=S#ids.ids,
				NewClumn=S#ids{ids=Id+1},
				mnesia:write(ids,NewClumn,write),
				Id;
			[]->
				io:format("none ~n"),
				NewClumn=#ids{idtype=IdType,ids=2},
				mnesia:write(ids,NewClumn,write),
				1
		end
	 end,
	 case mnesia:transaction(F)of
	  {atomic,Id}->
		  {atomic,Id};
	  {aborted,Reason}->
		  io:format("run transaction error ~1000.p ~n",[Reason]),
		  Id=0;
	   _Els->
          Id=1000
	 end,
	 {reply,Id,State}
.

handle_cast(_From,State)->
	ok.
handle_info(_From,State)->
	ok.
terminate(_From,State)->
	ok.
code_change(_OldVer,State,Ext)->
	{ok,State}.
%%
%% Local Functions
%%

使用方法：

id_generator:start_link().

id_generator:getnewid(aaa).

 

总结：

1.要注意将mnsia创建表时的参数写对

2.直接将表的列定义为一个record ，并且第二个字段与表同名

3.表未创建成功，或者有问题时mnesia:wait_for_tables 会超时，最好不要用mnesia:wait_for_tables([mytable],infinity)，一旦出错快速结束很重要

4.针对可能异常的地方多判断，做好异常处理