mnesia_consensus.erl

-module(mnesia_consensus).

-behaviour(mnesia_access).

%% Mnesia access callbacks.
%% When mnesia:Fun is called inside an activity - it calls a module callback.
%% The callback can forward the call to the next level or do anything.
-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,
     first/3, next/4, prev/4, last/3,
     clear_table/4
       ]).

consensus_transaction(Fun, Args) ->
    start_transaction_context(),
    %% Execute Fun in the mnesia_consensus context
    %% This will collect writes and locks to apply after
    %% transaction
    Res = mnesia:activity(ets, Fun, Args, mnesia_consensus),
    case good_res(Res) of
        true ->
            %% This thing should contain writes and updates. 
            %% May be recovered using ETS or the process dictionary
            TransactionCtx = get_transaction_context()),
            %% Make sure quorum of nodes agrees on the changes

            case consensus_commit(TransactionCtx) of
                ok -> Res;
                %% Retry entire transaction.
                not_ok -> retry_transaction(Fun, Args)
            end;
        false ->
            retry_transaction(Fun, Args)
    end.

%% This can be optimised by aborting the activity on locks. 
%% That will cause activity to re-run and eventually produce
%% a valid transaction context to apply.


%% Example of read function just forwarding the call
read(ActivityId, Opaque, Tab, Key, LockKind) ->
    mnesia:read(ActivityId, Opaque, Tab, Key, LockKind).

%% Example of write function.
%% Do not do an actual write here.
write(ActivityId, Opaque, Tab, Rec, LockKind) ->
    record_consensus_lock(ActivityId, Tab, Rec, LockKind),
    record_consensus_write(ActivityId, Tab, Rec, LockKind).