Mnesia ramblings

README.md

http://www.tamewildsystems.com/2009/11/consistent-hashing-for-mnesia-fragments.html http://www.tamewildsystems.com/2009/11/consistent-hashing-for-mnesia-fragments_19.html

http://www.tamewildsystems.com/2010/05/mnesia-one-year-later.html http://www.tamewildsystems.com/2010/05/mnesia-one-year-later-part-2.html http://www.tamewildsystems.com/2010/05/mnesia-one-year-later-part-3.html

master.erl

-module(master).

-include("record.hrl").

-export([start/0, read/1, write/2, frag_dist/0]).

% mnesia:table_info(book_info, frag_properties).

start() ->
    mnesia:start(),
    net_adm:ping('lr2@Luis-Rascao-MPT'),
    {atomic, ok} = mnesia:create_table(book_info,
                                       [{ram_copies, ['lr1@Luis-Rascao-MPT']},
                                        {index, [name]},
                                        {frag_properties, [{node_pool, [node()]},
                                                           {n_fragments, 4},
                                                           {hash_module, mnesia_frag_hash2}]},
                                        {attributes, record_info(fields, book_info)}]).

read(Id) ->
    F = fun(I) ->
           mnesia:read(book_info, I)
        end,
    mnesia:activity(transaction, F, [Id], mnesia_frag).

write(Id, Name) ->
    F = fun(I, N) ->
           mnesia:write(book_info, #book_info{id = I, name = N}, write)
        end,
    mnesia:activity(transaction, F, [Id, Name], mnesia_frag).

frag_dist() ->
    FragProperties = mnesia:table_info(book_info, frag_properties),
    NFragments = proplists:get_value(n_fragments, FragProperties),
    L = lists:map(fun(Fragment) ->
                    FragId = list_to_atom(atom_to_list(book_info_frag) ++ integer_to_list(Fragment)),
                    NodeFrag = mnesia:table_info(FragId, where_to_read),
                    {NodeFrag, FragId}
                  end, lists:seq(2, NFragments)),
    lists:map(fun(Node) ->
                Frags = proplists:get_all_values(Node, L),
                {Node, Frags}
              end, proplists:get_keys(L)).

mnesia_frag_hash2.erl

%%
%% %CopyrightBegin%
%% 
%% Copyright Ericsson AB 2002-2011. All Rights Reserved.
%% 
%% The contents of this file are subject to the Erlang Public License,
%% Version 1.1, (the "License"); you may not use this file except in
%% compliance with the License. You should have received a copy of the
%% Erlang Public License along with this software. If not, it can be
%% retrieved online at http://www.erlang.org/.
%% 
%% Software distributed under the License is distributed on an "AS IS"
%% basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
%% the License for the specific language governing rights and limitations
%% under the License.
%% 
%% %CopyrightEnd%
%%

%%
%%%----------------------------------------------------------------------
%%% Purpose : Implements hashing functionality for fragmented tables
%%%----------------------------------------------------------------------

%header_doc_include
-module(mnesia_frag_hash2).

%% Fragmented Table Hashing callback functions
-export([
	 init_state/2,
	 add_frag/1,
	 del_frag/1,
	 key_to_frag_number/2,
	 match_spec_to_frag_numbers/2
	]).

%header_doc_include
%%-behaviour(mnesia_frag_hash).

%impl_doc_include
-record(hash_state,
	{n_fragments,
	 next_n_to_split,
	 n_doubles,
	 function}).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

init_state(_Tab, State) when State == undefined ->
    #hash_state{n_fragments     = 1,
		next_n_to_split = 1,
		n_doubles       = 0,
		function        = phash2}.

convert_old_state({hash_state, N, P, L}) ->
    #hash_state{n_fragments     = N,
		next_n_to_split = P,
		n_doubles       = L,
		function        = phash}.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

add_frag(#hash_state{next_n_to_split = SplitN, n_doubles = L, n_fragments = N} = State) ->
    P = SplitN + 1,
    NewN = N + 1,
    State2 = case power2(L) + 1 of
		 P2 when P2 == P ->
		     State#hash_state{n_fragments      = NewN,
				      n_doubles        = L + 1,
				      next_n_to_split = 1};
		 _ ->
		     State#hash_state{n_fragments     = NewN,
				      next_n_to_split = P}
	     end,
    {State2, [SplitN], [NewN]};
add_frag(OldState) ->
    State = convert_old_state(OldState),
    add_frag(State).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

del_frag(#hash_state{next_n_to_split = SplitN, n_doubles = L, n_fragments = N} = State) ->
    P = SplitN - 1,
    if
	P < 1 ->
	    L2 = L - 1,
	    MergeN = power2(L2),
	    State2 = State#hash_state{n_fragments     = N - 1,
				      next_n_to_split = MergeN,
				      n_doubles       = L2},
	    {State2, [N], [MergeN]};
	true ->
	    MergeN = P,
	    State2 = State#hash_state{n_fragments     = N - 1,
				      next_n_to_split = MergeN},
	    {State2, [N], [MergeN]}
	end;
del_frag(OldState) ->
    State = convert_old_state(OldState),
    del_frag(State).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
key_to_frag_number(State, Key) ->
    FragNumber = key_to_frag_number0(State, Key),
    io:format("frag #: ~p~n", [FragNumber]),
    FragNumber.

key_to_frag_number0(#hash_state{function = phash, n_fragments = N, n_doubles = L}, Key) ->
    A = erlang:phash(Key, power2(L + 1)),
    if
	A > N ->
	    A - power2(L);
	true ->
	    A
    end;
key_to_frag_number0(#hash_state{function = phash2, n_fragments = N, n_doubles = L}, Key) ->
    A = erlang:phash2(Key, power2(L + 1)) + 1,
    if
	A > N ->
	    A - power2(L);
	true ->
	    A
    end;
key_to_frag_number0(OldState, Key) ->
    State = convert_old_state(OldState),
    key_to_frag_number(State, Key).

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

match_spec_to_frag_numbers(#hash_state{n_fragments = N} = State, MatchSpec) ->
    case MatchSpec of
	[{HeadPat, _, _}] when is_tuple(HeadPat), tuple_size(HeadPat) > 2 ->
	    KeyPat = element(2, HeadPat),
	    case has_var(KeyPat) of
		false ->
		    [key_to_frag_number(State, KeyPat)];
		true ->
		    lists:seq(1, N)
	    end;
	_ -> 
	    lists:seq(1, N)
    end;
match_spec_to_frag_numbers(OldState, MatchSpec) ->
    State = convert_old_state(OldState),
    match_spec_to_frag_numbers(State, MatchSpec).

power2(Y) ->
    1 bsl Y. % trunc(math:pow(2, Y)).

%impl_doc_include

has_var(Pat) ->
    mnesia:has_var(Pat).

record.hrl

-record(book_info, {id, name}).

slave.erl

-module(slave).

-include("record.hrl").

-export([start/0, read/1, write/2, frag_dist/0]).

% mnesia:table_info(book_info, where_to_read).
% mnesia:table_info(book_info, frag_dist).

start() ->
    mnesia:start(),
    mnesia:change_config(extra_db_nodes, ['lr1@Luis-Rascao-MPT']),
    {atomic, ok} = mnesia:change_table_frag(book_info, {add_node, node()}),
    {atomic, ok} = mnesia:change_table_frag(book_info, {add_frag, [node()]}),
    {atomic, ok} = mnesia:change_table_frag(book_info, {add_frag, [node()]}),
    {atomic, ok} = mnesia:change_table_frag(book_info, {add_frag, [node()]}),
    {atomic, ok} = mnesia:change_table_frag(book_info, {add_frag, [node()]}),
    % mnesia:add_table_copy(book_info, node(), ram_copies),
    ok.

read(Id) ->
    F = fun(I) ->
           mnesia:read(book_info, I)
        end,
    mnesia:activity(transaction, F, [Id], mnesia_frag).

write(Id, Name) ->
    F = fun(I, N) ->
           mnesia:write(book_info, #book_info{id = I, name = N}, write)
        end,
    mnesia:activity(transaction, F, [Id, Name], mnesia_frag).

frag_dist() ->
    FragProperties = mnesia:table_info(book_info, frag_properties),
    NFragments = proplists:get_value(n_fragments, FragProperties),
    L = lists:map(fun(Fragment) ->
                    FragId = list_to_atom(atom_to_list(book_info_frag) ++ integer_to_list(Fragment)),
                    NodeFrag = mnesia:table_info(FragId, where_to_read),
                    {NodeFrag, FragId}
                  end, lists:seq(2, NFragments)),
    lists:map(fun(Node) ->
                Frags = proplists:get_all_values(Node, L),
                {Node, Frags}
              end, proplists:get_keys(L)).