## 10 Appendix C: The Activity Access Call Back Interface

### 10.1 mnesia_access callback behavior

    -module(mnesia_frag).

    %% Callback functions when accessed within an activity
    -export([
             lock/4,
             write/5, delete/5, delete_object/5,
             read/5, match_object/5, all_keys/4,
             select/5,select/6,select_cont/3,
             index_match_object/6, index_read/6,
             foldl/6, foldr/6,
             table_info/4
           ]).

    %% Callback functions which provides transparent
    %% access of fragmented tables from any activity
    %% access context.

    lock(ActivityId, Opaque, {table , Tab}, LockKind) ->
        case frag_names(Tab) of
            [Tab] ->
                mnesia:lock(ActivityId, Opaque, {table, Tab}, LockKind);
            Frags ->
                DeepNs = [mnesia:lock(ActivityId, Opaque, {table, F}, LockKind) ||
                             F <- Frags],
                mnesia_lib:uniq(lists:append(DeepNs))
        end;

    lock(ActivityId, Opaque, LockItem, LockKind) ->
        mnesia:lock(ActivityId, Opaque, LockItem, LockKind).

    write(ActivityId, Opaque, Tab, Rec, LockKind) ->
        Frag = record_to_frag_name(Tab, Rec),
        mnesia:write(ActivityId, Opaque, Frag, Rec, LockKind).

    delete(ActivityId, Opaque, Tab, Key, LockKind) ->
        Frag = key_to_frag_name(Tab, Key),
        mnesia:delete(ActivityId, Opaque, Frag, Key, LockKind).

    delete_object(ActivityId, Opaque, Tab, Rec, LockKind) ->
        Frag = record_to_frag_name(Tab, Rec),
        mnesia:delete_object(ActivityId, Opaque, Frag, Rec, LockKind).

    read(ActivityId, Opaque, Tab, Key, LockKind) ->
        Frag = key_to_frag_name(Tab, Key),
        mnesia:read(ActivityId, Opaque, Frag, Key, LockKind).

    match_object(ActivityId, Opaque, Tab, HeadPat, LockKind) ->
        MatchSpec = [{HeadPat, [], ['$_']}],
        select(ActivityId, Opaque, Tab, MatchSpec, LockKind).

    select(ActivityId, Opaque, Tab, MatchSpec, LockKind) ->
        do_select(ActivityId, Opaque, Tab, MatchSpec, LockKind).

    select(ActivityId, Opaque, Tab, MatchSpec, Limit, LockKind) ->
        init_select(ActivityId, Opaque, Tab, MatchSpec, Limit, LockKind).

    all_keys(ActivityId, Opaque, Tab, LockKind) ->
        Match = [mnesia:all_keys(ActivityId, Opaque, Frag, LockKind)
                 || Frag <- frag_names(Tab)],
        lists:append(Match).

    index_match_object(ActivityId, Opaque, Tab, Pat, Attr, LockKind) ->
        Match =
            [mnesia:index_match_object(ActivityId, Opaque, Frag, Pat, Attr, LockKind)
             || Frag <- frag_names(Tab)],
        lists:append(Match).

    index_read(ActivityId, Opaque, Tab, Key, Attr, LockKind) ->
        Match =
            [mnesia:index_read(ActivityId, Opaque, Frag, Key, Attr, LockKind)
                 || Frag <- frag_names(Tab)],
        lists:append(Match).

    foldl(ActivityId, Opaque, Fun, Acc, Tab, LockKind) ->
        Fun2 = fun(Frag, A) ->
                       mnesia:foldl(ActivityId, Opaque, Fun, A, Frag, LockKind)
               end,
        lists:foldl(Fun2, Acc, frag_names(Tab)).

    foldr(ActivityId, Opaque, Fun, Acc, Tab, LockKind) ->
        Fun2 = fun(Frag, A) ->
                       mnesia:foldr(ActivityId, Opaque, Fun, A, Frag, LockKind)
               end,
        lists:foldr(Fun2, Acc, frag_names(Tab)).

    table_info(ActivityId, Opaque, {Tab, Key}, Item) ->
        Frag = key_to_frag_name(Tab, Key),
        table_info2(ActivityId, Opaque, Tab, Frag, Item);
    table_info(ActivityId, Opaque, Tab, Item) ->
        table_info2(ActivityId, Opaque, Tab, Tab, Item).

    table_info2(ActivityId, Opaque, Tab, Frag, Item) ->
        case Item of
            size ->
                SumFun = fun({_, Size}, Acc) -> Acc + Size end,
                lists:foldl(SumFun, 0, frag_size(ActivityId, Opaque, Tab));
            memory ->
                SumFun = fun({_, Size}, Acc) -> Acc + Size end,
                lists:foldl(SumFun, 0, frag_memory(ActivityId, Opaque, Tab));
            base_table ->
                lookup_prop(Tab, base_table);
            node_pool ->
                lookup_prop(Tab, node_pool);
            n_fragments ->
                FH = lookup_frag_hash(Tab),
                FH#frag_state.n_fragments;
            foreign_key ->
                FH = lookup_frag_hash(Tab),
                FH#frag_state.foreign_key;
            foreigners ->
                lookup_foreigners(Tab);
            n_ram_copies ->
                length(val({Tab, ram_copies}));
            n_disc_copies ->
                length(val({Tab, disc_copies}));
            n_disc_only_copies ->
                length(val({Tab, disc_only_copies}));

            frag_names ->
                frag_names(Tab);
            frag_dist ->
                frag_dist(Tab);
            frag_size ->
                frag_size(ActivityId, Opaque, Tab);
            frag_memory ->
                frag_memory(ActivityId, Opaque, Tab);
            _ ->
                mnesia:table_info(ActivityId, Opaque, Frag, Item)
        end.

mnesia 4.4
Copyright © 1991-2008 Ericsson AB