[Ericsson AB]

11 Appendix D: The Fragmented Table Hashing Call Back Interface

11.1 mnesia_frag_hash callback behavior

-module(mnesia_frag_hash).

%% 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
        ]).


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

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

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

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

add_frag(State) when record(State, hash_state) ->
    SplitN = State#hash_state.next_n_to_split,
    P = SplitN + 1,
    L = State#hash_state.n_doubles,
    NewN = State#hash_state.n_fragments + 1,
    State2 = case trunc(math:pow(2, 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]}.

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

del_frag(State) when record(State, hash_state) ->
    P = State#hash_state.next_n_to_split - 1,
    L = State#hash_state.n_doubles,
    N = State#hash_state.n_fragments,
    if
        P < 1 ->
            L2 = L - 1,
            MergeN = trunc(math:pow(2, 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.

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

key_to_frag_number(State, Key) when record(State, hash_state) ->
    L = State#hash_state.n_doubles,
    A = erlang:phash(Key, trunc(math:pow(2, L))),
    P = State#hash_state.next_n_to_split,
    if
        A < P ->
            erlang:phash(Key, trunc(math:pow(2, L + 1)));
        true ->
            A
    end.

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

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



mnesia 4.3.5
Copyright © 1991-2007 Ericsson AB