[erlang-questions] Ridiculuous newbie mnesia question.

Ulf Wiger <>
Mon Oct 25 08:41:47 CEST 2010


Hi Jeffrey,

Looking at the mnesia:info() output, there are a couple of things
to note.

1. 'running db nodes = []' means that while 
   and  might be running at the erlang level, perhaps even  
   with mnesia runnin, they are not connected to 

2. The line '"c:/.../" is NOT used' means that 
    you have not created a persistent schema. Normally, the persistent 
    schema indicates which nodes are part of the mnesia cluster. Any 
    other nodes that want to join should specify -mnesia extra_db_nodes Ns,
    where Ns includes at least some of the nodes listed in the persistent schema.

To create a persistent schema, you run this before starting mnesia:

mnesia:create_schema(Nodes).

...with the nodes in Nodes all up and running. You only need to call it 
from one node; it will create a schema on each node automatically.

BR,
Ulf W

On 25 Oct 2010, at 07:30, Jeffrey Rennie wrote:

> I launched a new erl node with a proper -sname, and I'm still experiencing
> the same problem.  Yes, nodes  and  are up and running.
> 
> ()17> backgammon:init().
> {atomic,ok}
> ()18> backgammon:insert_game(1, "Jeff").
> {aborted,{no_exists,game_record}}
> ()19> backgammon:init().
> {aborted,{already_exists,game_record}}
> ()20> mnesia:info().
> ---> Processes holding locks <--- 
> ---> Processes waiting for locks <--- 
> ---> Participant transactions <--- 
> ---> Coordinator transactions <---
> ---> Uncertain transactions <--- 
> ---> Active tables <--- 
> schema         : with 3        records occupying 656      words of mem
> ===> System info in version "4.4.13", debug level = none <===
> opt_disc. Directory
> "c:/Users/jeff/code/code/scratch/erlang/" is NOT used.
> use fallback at restart = false
> running db nodes   = []
> stopped db nodes   = [] 
> master node tables = []
> remote             = [backgammon_games,game_record]
> ram_copies         = [schema]
> disc_copies        = []
> disc_only_copies   = []
> [] = [backgammon_games,game_record]
> [{,ram_copies}] = [schema]
> 4 transactions committed, 3 aborted, 0 restarted, 0 logged to disc
> 0 held locks, 0 in queue; 0 local transactions, 0 remote
> 0 transactions waits for other nodes: []
> ok
> ()21> mnesia:delete_table(backgammon_games).
> {aborted,{no_exists,backgammon_games}}
> ()22> mnesia:delete_table(game_record).
> {aborted,{no_exists,game_record}}
> ()23> backgammon:init().
> {aborted,{already_exists,game_record}}
> ()24>
> 
> 
>> -----Original Message-----
>> From: Ulf Wiger [mailto:]
>> Sent: Sunday, October 24, 2010 3:02 PM
>> To: Jeffrey Rennie
>> Cc: 
>> Subject: Re: [erlang-questions] Ridiculuous newbie mnesia question.
>> 
>> 
>> Hi Jeffrey,
>> 
>> The problem is that you have specified that the backgammon_games
>> table should have copies on  and , but your
>> Erlang shell has the node name  (i.e. Erlang was
>> started without distribution).
>> 
>> One might argue whether it wouldn't have been better if mnesia had
>> refused to create the table in the first place. As it is now, it
>> believes
>> it exists, because there is an entry in the schema, but it refuses to
>> delete it because it has no active replicas. But since that's not what
>> it
>> complains about, I can understand your confusion. :)
>> 
>> BR,
>> Ulf W
>> 
>> 
>> On 24 Oct 2010, at 23:37, Jeffrey Rennie wrote:
>> 
>>> I seem to be stuck in a state where I can't create a table because it
>>> exists, but I can't delete the table because it doesn't exist!
>>> 
>>> I searched the mnesia documentation, and I couldn't find a definition
>>> of the "Tab" arg passed to many mnesia functions.
>>> 
>>> Here's what I tried:
>>> 
>>> Eshell V5.7.5  (abort with ^G)
>>> 2> c(backgammon).
>>> {ok,backgammon}
>>> 5> c(backgammon).
>>> {ok,backgammon}
>>> 7> mnesia:start().
>>> ok
>>> 8> backgammon:init().
>>> {atomic,ok}
>>> 11> c(backgammon).
>>> {ok,backgammon}
>>> 12> backgammon:dumpfields().
>>> [player_turn,dice,board]
>>> 13> c(backgammon).
>>> {ok,backgammon}
>>> 14> backgammon:init().
>>> {aborted,{already_exists,backgammon_games}}
>>> 15> mnesia:info().
>>> ---> Processes holding locks <---
>>> ---> Processes waiting for locks <---
>>> ---> Participant transactions <---
>>> ---> Coordinator transactions <---
>>> ---> Uncertain transactions <---
>>> ---> Active tables <---
>>> schema         : with 2        records occupying 546      words of
>> mem
>>> ===> System info in version "4.4.13", debug level = none <===
>>> opt_disc. Directory
>>> "c:/Users/jeff/code/code/scratch/erlang/" is NOT
>>> used.
>>> use fallback at restart = false
>>> running db nodes   = []
>>> stopped db nodes   = []
>>> master node tables = []
>>> remote             = [backgammon_games]
>>> ram_copies         = [schema]
>>> disc_copies        = []
>>> disc_only_copies   = []
>>> [] = [backgammon_games]
>>> [{,ram_copies}] = [schema]
>>> 3 transactions committed, 1 aborted, 0 restarted, 0 logged to disc
>>> 0 held locks, 0 in queue; 0 local transactions, 0 remote
>>> 0 transactions waits for other nodes: []
>>> ok
>>> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>>> %% BEGIN CONFUSING BIT
>>> 17> mnesia:delete_table(backgammon_games).
>>> {aborted,{no_exists,backgammon_games}}
>>> 18> backgammon:init().
>>> {aborted,{already_exists,backgammon_games}}
>>> 19>
>>> %% END CONFUSING BIT
>>> %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
>>> 
>>> 
>>> Here's my backgammon.erl:
>>> -module(backgammon).
>>> 
>>> -compile(export_all).
>>> 
>>> -include("backgammon.hrl").
>>> 
>>> create_tables(MnesiaNodes) ->
>>>   mnesia:create_table(backgammon_games,
>>> 			[{attributes, record_info(fields, game_record)},
>>> 			 {ram_copies, MnesiaNodes}]).
>>> 
>>> init() ->
>>>   create_tables([, ]).
>>> 
>>> dumpfields() ->
>>>   record_info(fields, game_state).
>>> 
>>> ________________________________________________________________
>>> erlang-questions (at) erlang.org mailing list.
>>> See http://www.erlang.org/faq.html
>>> To unsubscribe; mailto:
>>> 
>> 
>> Ulf Wiger, CTO, Erlang Solutions, Ltd.
>> http://erlang-solutions.com
>> 
> 
> 
> 

Ulf Wiger, CTO, Erlang Solutions, Ltd.
http://erlang-solutions.com





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