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 typehandle_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),1end 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