Erlang Programming/Using mnesia

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Using Mnesia[edit]

Mnesia is the distributed database written in Erlang, meant to mainly be used by Erlang programs. Simple queries can be written with query list comprehensions (qlc). Realtime queries can decide to not use transactions.

Explain program: art.erl[edit]

The program defines some functions that work similar to SQL. We wish to build a table of artwork named "painting", with fields: index, artist, and title.

Table: painting
 
+---------------------------------------------+ 
| index | artist  | title                     |
+---------------------------------------------+  
| 1     | Dali    | The Ghost of Vermeer      |
| 2     | Dali    | The Persistance of Memory |
| 3     | Vermeer | Girl With Pearl Earring   |
+---------------------------------------------+

art:init(). Starts the database and creates the table: painting.
insert() Puts data into the table
select() Gets data out of the table

Note that the second time we call init() the database is restarted but we get an error when it tries to recreate the table (as it already exists).

Example program: art.erl[edit]

-module(art).
-compile(export_all).
-include_lib("stdlib/include/qlc.hrl"). 
 
-record(painting, {index, artist, title}).
  
init() ->
    mnesia:create_schema([node()]),
    mnesia:start(),
    mnesia:create_table(painting,
        [ {disc_copies, [node()] },
             {attributes,      
                record_info(fields,painting)} ]).
 
insert( Index, Artist, Title) ->
    Fun = fun() ->
         mnesia:write(
         #painting{ index=Index,
                   artist=Artist, 
                        title=Title    } )
               end,
         mnesia:transaction(Fun).
  
select( Index) ->
    Fun = 
        fun() ->
            mnesia:read({painting, Index})
        end,
    {atomic, [Row]}=mnesia:transaction(Fun),
    io:format(" ~p ~p ~n ", [Row#painting.artist, Row#painting.title] ).

select_some( Artist) ->
    Fun = 
        fun() ->
            mnesia:match_object({painting, '_', Artist, '_' } )
        end,
    {atomic, Results} = mnesia:transaction( Fun),
    Results.
 
select_all() -> 
    mnesia:transaction( 
    fun() ->
        qlc:eval( qlc:q(
            [ X || X <- mnesia:table(painting) ] 
        )) 
    end ).
  
select_search( Word ) -> 
    mnesia:transaction( 
    fun() ->
         qlc:eval( qlc:q(
              [ {F0,F1,F2,F3} || 
                   {F0,F1,F2,F3} <- 
                        mnesia:table(painting),
                        (string:str(F2, Word)>0) or  
                        (string:str(F3, Word)>0)
               ] )) 
    end ).

Sample output: art.erl[edit]

%   Sample output:
 
%   6> c(art).
%   {ok,art}
  
%   7> art:init(). 
%   {atomic,ok}
 
%   6> art:insert(1,"Dali","The Ghost of Vermeer").
%   {atomic,ok}
  
%   7> art:select(1).
%   "Dali" "The Ghost of Vermeer"
%   ok
  
%   8> art:insert(2,"Dali","The Persistence of Memory").
%   {atomic,ok}
   
%   9> art:select(2).
%   "Dali" "The Persistence of Memory"
%   ok
   
%   10> art:select(1).
%   "Dali" "The Ghost of Vermeer"
%   ok 
 
%   25> art:insert(3,"Vermeer", "Girl With Pearl Earring").
%   {atomic,ok}
 
%   26> art:select_some("Dali").
%   [{painting,1,"Dali","The Ghost of Vermeer"},
%   {painting,2,"Dali","The Persistence of Memory"}]
 
%   27> art:select_all().
%   {atomic,[{painting,1,"Dali","The Ghost of Vermeer"},
%               {painting,2,"Dali","The Persistence of Memory"},
%               {painting,3,"Vermeer","Girl With Pearl Earring"}]}

%---to run a new session after restarting erlang---
 
%   2> art:init().
%   {aborted,{already_exists,painting}}

%   3> art:select_search("Vermeer").
%   {atomic,[{painting,1,"Dali","The Ghost of Vermeer"},
%         {painting,3,"Vermeer","Girl With Pearl Earring"}]}