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[Erlang 0051] Using ETS in Erlang Shell

2017-12-03 1276
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简介：
在Erlang Shell中调试的时候经常会遇到的一个问题就是在Shell中遇到异常会导致ETS表丢失,需要反复去创建ETS表,调试比较麻烦.这是由于Erlang Shell在遇到异常之后会重建,ETS表依赖于创建它的进程,如果创建它的进程崩溃了ETS表也就销毁了(不是绝对的,后面可以看到);看下官方文档的描述:
  Note that there is no automatic garbage collection for tables. Even if there are no references to a table from any process,it will not automatically be destroyed unless the owner process terminates.It can be destroyed explicitly by using delete/1.The default owner is the process that created the table. 
 
  我们先来解决这个Erlang Shell调试的问题,首先一个很简单的方法就是解除对Shell进程的依赖,我们在别的进程里面创建ETS表.
复制代码 
Eshell V5.9  (abort with ^G)

1> ets:new(test,[named_table]). %%% 首先我们模拟一下出错的情景

test

2> ets:i().   %%% 查看一下ets表 的信息

id              name              type  size   mem      owner

----------------------------------------------------------------------------

13              code              set   258    9657     code_server

4110            code_names        set   55     6983     code_server

8207            shell_records     ordered_set 0      106      <0.24.0>

ac_tab          ac_tab            set   6      852      application_controller

file_io_servers file_io_servers   set   0      316      file_server_2

global_locks    global_locks      set   0      316      global_name_server

global_names    global_names      set   0      316      global_name_server

global_names_ext global_names_ext  set   0      316      global_name_server

global_pid_ids  global_pid_ids    bag   0      316      global_name_server

global_pid_names global_pid_names  bag   0      316      global_name_server

inet_cache      inet_cache        bag   0      316      inet_db

inet_db         inet_db           set   23     503      inet_db

inet_hosts_byaddr inet_hosts_byaddr bag   0      316      inet_db

inet_hosts_byname inet_hosts_byname bag   0      316      inet_db

inet_hosts_file_byaddr inet_hosts_file_byaddr bag   0      316      inet_db

inet_hosts_file_byname inet_hosts_file_byname bag   0      316      inet_db

test            test              set   0      316      <0.30.0>

ok

3> 1/0.    %%% 异常一下 让Erlang Shell重建

** exception error: bad argument in an arithmetic expression

in operator  '/'/2

called as 1 / 0

4> ets:i(). %%% 再一次查看ETS表,可以看到test表已经不存在了

id              name              type  size   mem      owner

----------------------------------------------------------------------------

13              code              set   259    9760     code_server

4110            code_names        set   55     6983     code_server

8207            shell_records     ordered_set 0      106      <0.24.0>

ac_tab          ac_tab            set   6      852      application_controller

file_io_servers file_io_servers   set   0      316      file_server_2

global_locks    global_locks      set   0      316      global_name_server

global_names    global_names      set   0      316      global_name_server

global_names_ext global_names_ext  set   0      316      global_name_server

global_pid_ids  global_pid_ids    bag   0      316      global_name_server

global_pid_names global_pid_names  bag   0      316      global_name_server

inet_cache      inet_cache        bag   0      316      inet_db

inet_db         inet_db           set   23     503      inet_db

inet_hosts_byaddr inet_hosts_byaddr bag   0      316      inet_db

inet_hosts_byname inet_hosts_byname bag   0      316      inet_db

inet_hosts_file_byaddr inet_hosts_file_byaddr bag   0      316      inet_db

inet_hosts_file_byname inet_hosts_file_byname bag   0      316      inet_db

ok



5> spawn(fun() -> ets:new(test,[named_table]), receive after infinity->ok end end). %%% 在别的进程创建ETS

<0.39.0>

6> ets:i().  %%% 查看ETS

id              name              type  size   mem      owner

----------------------------------------------------------------------------

13              code              set   259    9760     code_server

4110            code_names        set   55     6983     code_server

8207            shell_records     ordered_set 0      106      <0.24.0>

ac_tab          ac_tab            set   6      852      application_controller

file_io_servers file_io_servers   set   0      316      file_server_2

global_locks    global_locks      set   0      316      global_name_server

global_names    global_names      set   0      316      global_name_server

global_names_ext global_names_ext  set   0      316      global_name_server

global_pid_ids  global_pid_ids    bag   0      316      global_name_server

global_pid_names global_pid_names  bag   0      316      global_name_server

inet_cache      inet_cache        bag   0      316      inet_db

inet_db         inet_db           set   23     503      inet_db

inet_hosts_byaddr inet_hosts_byaddr bag   0      316      inet_db

inet_hosts_byname inet_hosts_byname bag   0      316      inet_db

inet_hosts_file_byaddr inet_hosts_file_byaddr bag   0      316      inet_db

inet_hosts_file_byname inet_hosts_file_byname bag   0      316      inet_db

test            test              set   0      316      <0.39.0>

ok

7> 1/0.   %%% 搞崩Erlang Shell 

** exception error: bad argument in an arithmetic expression

in operator  '/'/2

called as 1 / 0

8> ets:i().  %%% 查看ETS表信息 test表还在

id              name              type  size   mem      owner

----------------------------------------------------------------------------

13              code              set   259    9760     code_server

4110            code_names        set   55     6983     code_server

8207            shell_records     ordered_set 0      106      <0.24.0>

ac_tab          ac_tab            set   6      852      application_controller

file_io_servers file_io_servers   set   0      316      file_server_2

global_locks    global_locks      set   0      316      global_name_server

global_names    global_names      set   0      316      global_name_server

global_names_ext global_names_ext  set   0      316      global_name_server

global_pid_ids  global_pid_ids    bag   0      316      global_name_server

global_pid_names global_pid_names  bag   0      316      global_name_server

inet_cache      inet_cache        bag   0      316      inet_db

inet_db         inet_db           set   23     503      inet_db

inet_hosts_byaddr inet_hosts_byaddr bag   0      316      inet_db

inet_hosts_byname inet_hosts_byname bag   0      316      inet_db

inet_hosts_file_byaddr inet_hosts_file_byaddr bag   0      316      inet_db

inet_hosts_file_byname inet_hosts_file_byname bag   0      316      inet_db

test            test              set   0      316      <0.39.0>

ok

9>
复制代码
   第二种方法创建的ets依赖于进程<0.39.0>解除了对Erlang Shell进程的依赖,Erlang Shell重建之后,ets表依然存在;我们可以把这个方法封装到user_default.erl中方便调试使用.可以参考: [Erlang 0027] Using Record in Erlang Shell 
 
   可以注意到文档里面还提到了一种方法来指定ETS表的所有权:
   Table ownership can be transferred at process termination by using the heir option or explicitly by calling give_away/3. 
   文档描述中可以把ETS的拥有权移交出去,所以我们试一下give_away的方式:
复制代码 
Eshell V5.9  (abort with ^G)

1> ets:new(test,[named_table]).

test

2> ets:i().

id              name              type  size   mem      owner

----------------------------------------------------------------------------

13              code              set   259    10181    code_server

4110            code_names        set   54     7151     code_server

8207            shell_records     ordered_set 0      97       <0.26.0>

ac_tab          ac_tab            set   6      843      application_controller

file_io_servers file_io_servers   set   0      307      file_server_2

global_locks    global_locks      set   0      307      global_name_server

global_names    global_names      set   0      307      global_name_server

global_names_ext global_names_ext  set   0      307      global_name_server

global_pid_ids  global_pid_ids    bag   0      307      global_name_server

global_pid_names global_pid_names  bag   0      307      global_name_server

inet_cache      inet_cache        bag   0      307      inet_db

inet_db         inet_db           set   29     630      inet_db

inet_hosts_byaddr inet_hosts_byaddr bag   0      307      inet_db

inet_hosts_byname inet_hosts_byname bag   0      307      inet_db

inet_hosts_file_byaddr inet_hosts_file_byaddr bag   0      307      inet_db

inet_hosts_file_byname inet_hosts_file_byname bag   0      307      inet_db

test            test              set   0      307      <0.32.0>

ok

3> 1/0.

** exception error: bad argument in an arithmetic expression

in operator  '/'/2

called as 1 / 0

4> ets:i().

id              name              type  size   mem      owner

----------------------------------------------------------------------------

13              code              set   260    10290    code_server

4110            code_names        set   54     7151     code_server

8207            shell_records     ordered_set 0      97       <0.26.0>

ac_tab          ac_tab            set   6      843      application_controller

file_io_servers file_io_servers   set   0      307      file_server_2

global_locks    global_locks      set   0      307      global_name_server

global_names    global_names      set   0      307      global_name_server

global_names_ext global_names_ext  set   0      307      global_name_server

global_pid_ids  global_pid_ids    bag   0      307      global_name_server

global_pid_names global_pid_names  bag   0      307      global_name_server

inet_cache      inet_cache        bag   0      307      inet_db

inet_db         inet_db           set   29     630      inet_db

inet_hosts_byaddr inet_hosts_byaddr bag   0      307      inet_db

inet_hosts_byname inet_hosts_byname bag   0      307      inet_db

inet_hosts_file_byaddr inet_hosts_file_byaddr bag   0      307      inet_db

inet_hosts_file_byname inet_hosts_file_byname bag   0      307      inet_db

ok

5> P=spwan(fun-> receive infinity -> ok end end ).

* 1: syntax error before: '->'

5> P=spwan(fun-> receive  after infinity -> ok end end ).

* 1: syntax error before: '->'

5> P=spwan(fun()-> receive  after infinity -> ok end end ).

** exception error: undefined shell command spwan/1

6> P=spawn(fun()-> receive  after infinity -> ok end end ).

<0.43.0>

7> ets:new(test,[named_table]).                            

test

8> ets:i().                                                

id              name              type  size   mem      owner

----------------------------------------------------------------------------

13              code              set   261    10419    code_server

4110            code_names        set   54     7151     code_server

8207            shell_records     ordered_set 0      97       <0.26.0>

ac_tab          ac_tab            set   6      843      application_controller

file_io_servers file_io_servers   set   0      307      file_server_2

global_locks    global_locks      set   0      307      global_name_server

global_names    global_names      set   0      307      global_name_server

global_names_ext global_names_ext  set   0      307      global_name_server

global_pid_ids  global_pid_ids    bag   0      307      global_name_server

global_pid_names global_pid_names  bag   0      307      global_name_server

inet_cache      inet_cache        bag   0      307      inet_db

inet_db         inet_db           set   29     630      inet_db

inet_hosts_byaddr inet_hosts_byaddr bag   0      307      inet_db

inet_hosts_byname inet_hosts_byname bag   0      307      inet_db

inet_hosts_file_byaddr inet_hosts_file_byaddr bag   0      307      inet_db

inet_hosts_file_byname inet_hosts_file_byname bag   0      307      inet_db

test            test              set   0      307      <0.41.0>

ok

9> self().

<0.41.0>

10> ets:give_away(test,P,[]).

true

11> 1/0.

** exception error: bad argument in an arithmetic expression

in operator  '/'/2

called as 1 / 0

12> self().

<0.49.0>

13> ets:i().                 

id              name              type  size   mem      owner

----------------------------------------------------------------------------

13              code              set   261    10419    code_server

4110            code_names        set   54     7151     code_server

8207            shell_records     ordered_set 0      97       <0.26.0>

ac_tab          ac_tab            set   6      843      application_controller

file_io_servers file_io_servers   set   0      307      file_server_2

global_locks    global_locks      set   0      307      global_name_server

global_names    global_names      set   0      307      global_name_server

global_names_ext global_names_ext  set   0      307      global_name_server

global_pid_ids  global_pid_ids    bag   0      307      global_name_server

global_pid_names global_pid_names  bag   0      307      global_name_server

inet_cache      inet_cache        bag   0      307      inet_db

inet_db         inet_db           set   29     630      inet_db

inet_hosts_byaddr inet_hosts_byaddr bag   0      307      inet_db

inet_hosts_byname inet_hosts_byname bag   0      307      inet_db

inet_hosts_file_byaddr inet_hosts_file_byaddr bag   0      307      inet_db

inet_hosts_file_byname inet_hosts_file_byname bag   0      307      inet_db

test            test              set   0      307      <0.43.0>

ok

14>
复制代码


  效果和我们预期的一样,这个问题可以告一段落;既然ETS表会对创建的进程存在生命周期的依赖,那么还有什么方法可以控制ETS的生命周期呢?
 
ETS Owner
 
  另外可以在创建ETS表的时候指定一个继任者,当创建的进程死掉之后,ETS的所有权转移到继任者:
 
{heir,Pid,HeirData} | {heir,none}
Set  a  process  as  heir.  The  heir  will  inherit  the  table  if  the  owner  terminates.  The  message  'ETS-TRANSFER',tid(),FromPid,HeirData} will be sent to the heir when that happens. The heir must be a local process. Default heir is none, which will destroy the table when the owner terminates.
复制代码 
19> HP=spawn(fun()-> receive  after infinity -> ok end end ).                                                   

<0.60.0>

20> ets:new(demo_ets,[named_table,{heir,HP,[]}]).            

demo_ets

21> ets:i().

id              name              type  size   mem      owner

----------------------------------------------------------------------------

13              code              set   262    10534    code_server

4110            code_names        set   54     7151     code_server

8207            shell_records     ordered_set 0      97       <0.26.0>

ac_tab          ac_tab            set   6      843      application_controller

demo_ets        demo_ets          set   0      307      <0.56.0>

file_io_servers file_io_servers   set   0      307      file_server_2

global_locks    global_locks      set   0      307      global_name_server

global_names    global_names      set   0      307      global_name_server

global_names_ext global_names_ext  set   0      307      global_name_server

global_pid_ids  global_pid_ids    bag   0      307      global_name_server

global_pid_names global_pid_names  bag   0      307      global_name_server

inet_cache      inet_cache        bag   0      307      inet_db

inet_db         inet_db           set   29     630      inet_db

inet_hosts_byaddr inet_hosts_byaddr bag   0      307      inet_db

inet_hosts_byname inet_hosts_byname bag   0      307      inet_db

inet_hosts_file_byaddr inet_hosts_file_byaddr bag   0      307      inet_db

inet_hosts_file_byname inet_hosts_file_byname bag   0      307      inet_db

test            test              set   0      307      <0.43.0>

ok

22> self().                                      

<0.56.0>

23> 1/0.   

** exception error: bad argument in an arithmetic expression

in operator  '/'/2

called as 1 / 0

24> ets:i().

id              name              type  size   mem      owner

----------------------------------------------------------------------------

13              code              set   262    10534    code_server

4110            code_names        set   54     7151     code_server

8207            shell_records     ordered_set 0      97       <0.26.0>

ac_tab          ac_tab            set   6      843      application_controller

demo_ets        demo_ets          set   0      307      <0.60.0>

file_io_servers file_io_servers   set   0      307      file_server_2

global_locks    global_locks      set   0      307      global_name_server

global_names    global_names      set   0      307      global_name_server

global_names_ext global_names_ext  set   0      307      global_name_server

global_pid_ids  global_pid_ids    bag   0      307      global_name_server

global_pid_names global_pid_names  bag   0      307      global_name_server

inet_cache      inet_cache        bag   0      307      inet_db

inet_db         inet_db           set   29     630      inet_db

inet_hosts_byaddr inet_hosts_byaddr bag   0      307      inet_db

inet_hosts_byname inet_hosts_byname bag   0      307      inet_db

inet_hosts_file_byaddr inet_hosts_file_byaddr bag   0      307      inet_db

inet_hosts_file_byname inet_hosts_file_byname bag   0      307      inet_db

ok

25>
复制代码
  
   下面这篇文章,很好的总结了ETS表几种管理方式 Don’t Lose Your ets Tables 我们上面基本上都涉及到了,作者提到的第三种方法也很容易想到,专门有一个进程,这个进程的职责就是"own the table".

Don’t Lose Your ets Tables 


Name an Heir

When you create an ets table you can also name a process to inherit the table should the creating process die:

TableId = ets:new(my_table, [{heir, SomeOtherProcess, HeirData}]),

If the creating process dies, the process SomeOtherProcess will receive a message of the form

{'ETS-TRANSFER', TableId, OldOwner, HeirData}

where TableId is the table identifier returned from ets:new, OldOwner is the pid of the process that owned the table, and HeirData is the data provided with the heir option passed to ets:new. Once it receives this message, SomeOtherProcess owns the table.

Give It Away

Alternatively, you can create an ets table and then give it to some other process to keep it:

TableId = ets:new(my_table, []),ets:give_away(TableId, SomeOtherProcess, GiftData),

 

If the creating process dies, the process SomeOtherProcess will receive a message of the form

{'ETS-TRANSFER', TableId, OldOwner, GiftData}

where TableId is the table identifier returned from ets:new, OldOwner is the pid of the process that owned the table, and GiftData is the data provided in the ets:give_away call. Once it receives this message, SomeOtherProcess owns the table.

Table Manager

Instead of naming an heir or giving a table away, you can just have your Erlang supervisorprocess create a child process whose sole task is to own the table. This process creates the table as a named public table, thus allowing other processes to know its name and read/write it directly, with ets built-in concurrency protection dealing with any concurrency issues. Since the owner process does nothing more than create the table and then wait to be told to shut down, the likelihood of it crashing and taking the table with it is practically nil. The drawback here, though, is that the process actually using the table may have to coordinate with the owner process to ensure the table is available, and worse, it ends up using what is essentially a global variable — the table name — which can make code harder to read and maintain.

A Combination Approach

A nice way of managing ets tables, though, is to use a combination of the three previous techniques:

  1. The Erlang supervisor creates a table manager process. Since all this process does is manage the table, the likelihood of it crashing is very low.
  2. The table manager links itself to the table user process and traps exits, allowing it to receive an EXIT message if the table user process dies unexpectedly.
  3. The table manager creates a table, names itself (self()) as the heir, and then gives it away to the table user process.
  4. If the table user process dies, the table manager is informed of the process death and also inherits the table back.

Once it inherits the table, the table manager can then for example wait until the supervisorrecreates the table user process, and then repeat the steps above to give the table to the new table user process. Other variations on this approach, like maybe a small pool of child process clones that cooperate to transfer the table between them in case of error, are of course also possible. Even though there are still process coordination issues here (but nothing difficult), I like this approach because it avoids global named tables and takes advantage of Erlang's supervision hierarchy.

The title of my QCon talk was "Let It Crash...Except When You Shouldn't." This scenario is an example of "when you shouldn't" — losing ets data due to a process crash is easily avoided.

 

2012-12-20 17:04:48 补充 如果仅仅是解决Erlang Shell异常死掉导致ETS崩溃的问题,还有一个最简单的方法:不让Shell死掉就行了  多谢@[京]汪(84871172) 的提醒

复制代码 
Eshell V5.9  (abort with ^G)
1> catch_exception(true).
false
2> self().
<0.30.0>
3> 1/0.
* exception error: bad argument in an arithmetic expression
    in operator  '/'/2
       called as 1 / 0
4> self().
复制代码
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