Add {majority, boolean()} per-table option.

With {majority, true} set for a table, write transactions will
abort if they cannot commit to a majority of the nodes that
have a copy of the table. Currently, the implementation hooks
into the prepare_commit, and forces an asymmetric transaction
if the commit set affects any table with the majority flag set.
In the commit itself, the transaction will abort if it cannot
satisfy the majority requirement for all tables involved in the
thransaction.

A future optimization might be to abort already when a write
lock is attempted on such a table (/-object) and the lock cannot
be set on enough nodes.

This functionality introduces the possibility to automatically
"fence off" a table in the presence of failures.

This is a first implementation. Only basic tests have been
performed.

1 files changed, 14 insertions, 1 deletions

diff --git a/lib/mnesia/src/mnesia_schema.erl b/lib/mnesia/src/mnesia_schema.erl
index d1d892a387..a6c8ffec01 100644
--- a/lib/mnesia/src/mnesia_schema.erl
+++ b/lib/mnesia/src/mnesia_schema.erl
@@ -178,6 +178,7 @@ do_set_schema(Tab, Cs) ->
     set({Tab, disc_only_copies}, Cs#cstruct.disc_only_copies),
     set({Tab, load_order}, Cs#cstruct.load_order),
     set({Tab, access_mode}, Cs#cstruct.access_mode),
+    set({Tab, majority}, Cs#cstruct.majority),
     set({Tab, snmp}, Cs#cstruct.snmp),
     set({Tab, user_properties}, Cs#cstruct.user_properties),
     [set({Tab, user_property, element(1, P)}, P) || P <- Cs#cstruct.user_properties],
@@ -651,6 +652,7 @@ list2cs(List) when is_list(List) ->
     Snmp = pick(Name, snmp, List, []),
     LoadOrder = pick(Name, load_order, List, 0),
     AccessMode = pick(Name, access_mode, List, read_write),
+    Majority = pick(Name, majority, List, false),
     UserProps = pick(Name, user_properties, List, []),
     verify({alt, [nil, list]}, mnesia_lib:etype(UserProps),
 	   {bad_type, Name, {user_properties, UserProps}}),
@@ -676,6 +678,7 @@ list2cs(List) when is_list(List) ->
              snmp = Snmp,
              load_order = LoadOrder,
              access_mode = AccessMode,
+	     majority = Majority,
              local_content = LC,
 	     record_name = RecName,
              attributes = Attrs,
@@ -809,7 +812,16 @@ verify_cstruct(Cs) when is_record(Cs, cstruct) ->
     Access = Cs#cstruct.access_mode,
     verify({alt, [read_write, read_only]}, Access,
 	   {bad_type, Tab, {access_mode, Access}}),
-
+    Majority = Cs#cstruct.majority,
+    verify({alt, [true, false]}, Majority,
+	   {bad_type, Tab, {majority, Majority}}),
+    case Majority of
+	true ->
+	    verify(false, LC,
+		   {combine_error, Tab, [{local_content,true},{majority,true}]});
+	false ->
+	    ok
+    end,
     Snmp = Cs#cstruct.snmp,
     verify(true, mnesia_snmp_hook:check_ustruct(Snmp),
 	   {badarg, Tab, {snmp, Snmp}}),
@@ -2971,6 +2983,7 @@ merge_versions(AnythingNew, Cs, RemoteCs, Force) ->
 	Cs#cstruct.index == RemoteCs#cstruct.index,
 	Cs#cstruct.snmp == RemoteCs#cstruct.snmp,
 	Cs#cstruct.access_mode == RemoteCs#cstruct.access_mode,
+	Cs#cstruct.majority == RemoteCs#cstruct.majority,
 	Cs#cstruct.load_order == RemoteCs#cstruct.load_order,
 	Cs#cstruct.user_properties == RemoteCs#cstruct.user_properties ->
 	    do_merge_versions(AnythingNew, Cs, RemoteCs);