diff --git a/src/couch_db.erl b/src/couch_db.erl
index 5d4619a5..f8c00262 100644
--- a/src/couch_db.erl
+++ b/src/couch_db.erl
@@ -617,12 +617,43 @@ load_validation_funs(#db{main_pid=Pid}=Db) ->
             Doc
     end,
     DDocs = lists:map(OpenDocs, DDocInfos),
-    Funs = lists:flatmap(fun(DDoc) ->
+    Funs0 = lists:flatmap(fun(DDoc) ->
         case couch_doc:get_validate_doc_fun(DDoc) of
             nil -> [];
             Fun -> [Fun]
         end
     end, DDocs),
+    ShardDB = ?l2b(config:get("mem3", "shard_db", "dbs")),
+    Funs = case Db#db.name of
+        ShardDB ->
+            ValidateShardDbUpdate = fun(_ShardDBDoc, nil, _JsonCtx, _SecObj) ->
+                ok;
+            (NewShardMapDoc0, OldShardMapDoc0, _JsonCtx, _SecObj) ->
+                OldShardMapDoc = couch_doc:with_ejson_body(OldShardMapDoc0),
+                NewShardMapDoc = couch_doc:with_ejson_body(NewShardMapDoc0),
+                #doc{body=OldShardMapBody, deleted=OldDeleted} = OldShardMapDoc,
+                #doc{body=NewShardMapBody, deleted=NewDeleted} = NewShardMapDoc,
+                if OldDeleted =:= NewDeleted ->
+                    HashAlgo = couch_util:get_value(
+                        <<"hash_algorithm">>, NewShardMapBody),
+                    OldHashAlgo = couch_util:get_value(
+                        <<"hash_algorithm">>, OldShardMapBody),
+                    if HashAlgo =/= OldHashAlgo ->
+                        throw({
+                            bad_request,
+                            <<"You can't change the hash algorithm.">>
+                        });
+                    true ->
+                        ok
+                    end;
+                true ->
+                    ok
+                end
+            end,
+            [ValidateShardDbUpdate|Funs0];
+        _ ->
+            Funs0
+    end,
     gen_server:cast(Pid, {load_validation_funs, Funs}),
     Funs.
 
diff --git a/src/couch_db_updater.erl b/src/couch_db_updater.erl
index 93ee6e90..236aeeda 100644
--- a/src/couch_db_updater.erl
+++ b/src/couch_db_updater.erl
@@ -625,7 +625,23 @@ refresh_validate_doc_funs(Db0) ->
             Fun -> [Fun]
             end
         end, DesignDocs),
-    Db#db{validate_doc_funs=ProcessDocFuns}.
+
+    DbName = Db0#db.name,
+    couch_log:error("Refreshing validate doc funs", []),
+    ProcessDocFuns1 = case config:get("mem3", "shard_db", "dbs") of
+        DbName ->
+            [fun validate_shard_db_update/4|ProcessDocFuns];
+        _ ->
+            ProcessDocFuns
+    end,
+
+    Db#db{validate_doc_funs=ProcessDocFuns1}.
+
+validate_shard_db_update(Doc, DiskDoc, JsonCtx, SecObj) ->
+    couch_log:error("Doc: ~p", [Doc]),
+    couch_log:error("DiskDoc: ~p", [DiskDoc]),
+    ok.
+
 
 % rev tree functions
 
diff --git a/src/couch_secondary_sup.erl b/src/couch_secondary_sup.erl
index 09e77b7c..e1a9ae77 100644
--- a/src/couch_secondary_sup.erl
+++ b/src/couch_secondary_sup.erl
@@ -26,7 +26,7 @@ init([]) ->
             worker,
             dynamic}
     ],
-    Children = SecondarySupervisors ++ [
+    Children0 = SecondarySupervisors ++ [
         begin
             {ok, {Module, Fun, Args}} = couch_util:parse_term(SpecStr),
 
@@ -39,4 +39,32 @@ init([]) ->
         end
         || {Name, SpecStr}
         <- config:get("daemons"), SpecStr /= ""],
+    Children = [{
+        hash_fun_lru,
+        {ets_lru, start_link, [hash_fun_lru, lru_opts()]},
+        permanent,
+        5000,
+        worker,
+        [ets_lru]
+    }|Children0],
     {ok, {{one_for_one, 50, 3600}, Children}}.
+
+lru_opts() ->
+    case application:get_env(couch, hash_fun_lru_max_objects) of
+        {ok, MxObjs} when is_integer(MxObjs), MxObjs > 0 ->
+            [{max_objects, MxObjs}];
+        _ ->
+            []
+    end ++
+    case application:get_env(couch, hash_fun_lru_max_size) of
+        {ok, MxSize} when is_integer(MxSize), MxSize > 0 ->
+            [{max_size, MxSize}];
+        _ ->
+            []
+    end ++
+    case application:get_env(couch, hash_fun_lru_max_lifetime) of
+        {ok, MxLT} when is_integer(MxLT), MxLT > 0 ->
+            [{max_lifetime, MxLT}];
+        _ ->
+            []
+    end.