first commit

README

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+unsplit - Resolve conflicts in Mnesia tables after network split
+
+Author: Ulf Wiger, Erlang Solutions Ltd
+
+
+NOTE: This application is currently a fairly crude prototype,
+      and is not intended for mission-critical tasks (yet).
+
+Documentation will follow, as soon as the approach has been
+verified to work. So far, no Mnesia patches are used, but 
+for it to work really well, some added functionality in
+Mnesia may well be needed.

commands.txt

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+erl -boot start_sasl -kernel dist_auto_connect once -sname n1 -pa ~/src/unsplit-0.1/ebin -mnesia debug trace
+erl -boot start_sasl -kernel dist_auto_connect once -sname n2 -pa ~/src/unsplit-0.1/ebin -mnesia debug trace
+
+mnesia:start().
+application:start(unsplit).
+rd(test,{key,modified=erlang:now(),value}).
+
+On n2@debian:
+
+mnesia:create_schema([n1@debian,n2@debian]).
+
+mnesia:delete_table(test).
+mnesia:create_table(test,[{ram_copies,[n1@debian,n2@debian]},{attributes,[key,modified,value]},{user_properties,[{unsplit_method,{unsplit_lib,last_modified,[]}}]}]).
+
+mnesia:transaction(fun() -> mnesia:write(#test{key=1,value=a}) end).
+mnesia:transaction(fun() -> mnesia:write(#test{key=2,value=a}) end).
+
+ets:tab2list(test).
+
+
+On n1@debian:
+
+disconnect_node(n2@debian).
+mnesia:transaction(fun() -> mnesia:write(#test{key=2,value=b}) end).
+timer:sleep(3000).
+net_kernel:connect_node(n2@debian).

ebin/unsplit.app

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+{application, unsplit,
+ [{vsn, "0.1"},
+  {description, "Merges mnesia tables after net split"},
+  {applications, [mnesia]},
+  {mod, {unsplit, []}},
+  {env, []}
+ ]}.

src/unsplit.erl

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+-module(unsplit).
+
+-behaviour(application).
+-behaviour(supervisor).
+
+-export([start/2, stop/1]).
+
+
+-export([init/1]).
+
+start(_, _) ->
+    supervisor:start_link({local,?MODULE}, ?MODULE, []).
+
+stop(_) ->
+    ok.
+
+%% Supervisor callback:    
+
+init([]) ->
+    Children = [{unsplit_server, {unsplit_server, start_link, []},
+                 permanent, 3000, worker, [unsplit_server]}],
+    {ok, {{one_for_one, 3, 10}, Children}}.

src/unsplit_lib.erl

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+-module(unsplit_lib).
+-export([no_action/2,
+         last_modified/2,
+         last_version/2]).
+
+
+
+no_action(start, [Tab|_]) ->
+    error_logger:format("Will not merge table ~p~n", [Tab]),
+    stop.
+
+
+last_modified(init, S0) ->
+    last_version(init, S0 ++ [modified]);
+last_modified(Other, S) ->
+    last_version(Other, S).
+
+last_version(init, [Tab, Attrs, Attr]) ->
+    case lists:member(Attr, Attrs) of
+        false ->
+            error_logger:format("Cannot merge table ~p."
+                                "Missing ~p attribute~n", [Tab, Attr]),
+            stop;
+        true ->
+            io:fwrite("Starting merge of ~p (~p)~n", [Tab, Attrs]),
+            {ok, {Tab, pos(Attr, Tab, Attrs)}}
+    end;
+last_version(done, _) ->
+    ok;
+last_version(Objs, {T, P} = S) when is_list(Objs) ->
+    Actions = lists:map(fun(Obj) ->
+                                last_version_entry(Obj, T, P)
+                        end, Objs),
+    {ok, Actions, same, S}.
+
+
+last_version_entry(Obj, T, P) ->
+    io:fwrite("last_version_entry(~p)~n", [Obj]),
+    case Obj of
+        {A, []} -> {write, A};
+        {[], B} -> {write, B};
+        {[A], [B]} ->
+            ModA = element(P, A),
+            ModB = element(P, B),
+            io:fwrite("ModA = ~p, ModB = ~p~n", [ModA, ModB]),
+            if ModA < ModB ->
+                    {write, B};
+               ModA > ModB ->
+                    {write, A};
+               ModA == ModB ->
+                    if A =/= B ->
+                            mnesia:abort({undecided,T,A,B});
+                       true ->
+                            {write, A}
+                    end
+            end
+    end.
+
+
+
+pos(A, T, L) ->
+    pos(A, T, L, 2).  % record tag is the 1st element in the tuple
+
+pos(H, _, [H|_], P) ->
+    P;
+pos(H, Tab, [_|T], P) ->
+    pos(H, Tab, T, P+1);
+pos(A, Tab, [], _) ->
+    mnesia:abort({missing_attribute, Tab, A}).