refactor(broker): simplify external broker behaviour

apps/emqx/src/emqx_broker.erl

@@ -246,7 +246,8 @@ publish(Msg) when is_record(Msg, message) ->
             [];
         Msg1 = #message{} ->
             do_publish(Msg1);
-        Msgs when is_list(Msgs) -> do_publish_many(Msgs)
+        Msgs when is_list(Msgs) ->
+            do_publish_many(Msgs)
     end.
 
 do_publish_many([]) ->
@@ -259,7 +260,7 @@ do_publish(#message{topic = Topic} = Msg) ->
     Routes = aggre(emqx_router:match_routes(Topic)),
     Delivery = delivery(Msg),
     RouteRes = route(Routes, Delivery, PersistRes),
-    ext_route(ext_routes(Topic, Msg), Delivery, RouteRes).
+    do_forward_external(Delivery, RouteRes).
 
 persist_publish(Msg) ->
     case emqx_persistent_message:persist(Msg) of
@@ -344,22 +345,8 @@ aggre([], false, Acc) ->
 aggre([], true, Acc) ->
     lists:usort(Acc).
 
-ext_routes(Topic, Msg) ->
-    case emqx_external_broker:should_route_to_external_dests(Msg) of
-        true -> emqx_external_broker:match_routes(Topic);
-        false -> []
-    end.
-
-ext_route([], _Delivery, RouteRes) ->
-    RouteRes;
-ext_route(ExtRoutes, Delivery, RouteRes) ->
-    lists:foldl(
-        fun(#route{topic = To, dest = ExtDest}, Acc) ->
-            [{ExtDest, To, emqx_external_broker:forward(ExtDest, Delivery)} | Acc]
-        end,
-        RouteRes,
-        ExtRoutes
-    ).
+do_forward_external(Delivery, RouteRes) ->
+    emqx_external_broker:forward(Delivery) ++ RouteRes.
 
 %% @doc Forward message to another node.
 -spec forward(

apps/emqx/src/emqx_external_broker.erl

@@ -16,11 +16,9 @@
 
 -module(emqx_external_broker).
 
--callback forward(dest(), emqx_types:delivery()) ->
+-callback forward(emqx_types:delivery()) ->
     emqx_types:deliver_result().
 
--callback should_route_to_external_dests(emqx_types:message()) -> boolean().
-
 -callback add_route(emqx_types:topic()) -> ok.
 -callback delete_route(emqx_types:topic()) -> ok.
 
@@ -30,23 +28,22 @@
 -callback add_persistent_route(emqx_types:topic(), emqx_persistent_session_ds:id()) -> ok.
 -callback delete_persistent_route(emqx_types:topic(), emqx_persistent_session_ds:id()) -> ok.
 
--callback match_routes(emqx_types:topic()) -> [emqx_types:route()].
-
 -type dest() :: term().
 
 -export([
+    %% Registration
     provider/0,
     register_provider/1,
     unregister_provider/1,
-    forward/2,
-    should_route_to_external_dests/1,
+    %% Forwarding
+    forward/1,
+    %% Routing updates
     add_route/1,
     delete_route/1,
     add_shared_route/2,
     delete_shared_route/2,
     add_persistent_route/2,
-    delete_persistent_route/2,
-    match_routes/1
+    delete_persistent_route/2
 ]).
 
 -export_type([dest/0]).
@@ -111,11 +108,8 @@ provider() ->
 %% Broker API
 %%--------------------------------------------------------------------
 
-forward(ExternalDest, Delivery) ->
-    ?safe_with_provider(?FUNCTION_NAME(ExternalDest, Delivery), {error, unknown_dest}).
-
-should_route_to_external_dests(Message) ->
-    ?safe_with_provider(?FUNCTION_NAME(Message), false).
+forward(Delivery) ->
+    ?safe_with_provider(?FUNCTION_NAME(Delivery), []).
 
 add_route(Topic) ->
     ?safe_with_provider(?FUNCTION_NAME(Topic), ok).
@@ -135,9 +129,6 @@ add_persistent_route(Topic, ID) ->
 delete_persistent_route(Topic, ID) ->
     ?safe_with_provider(?FUNCTION_NAME(Topic, ID), ok).
 
-match_routes(Topic) ->
-    ?safe_with_provider(?FUNCTION_NAME(Topic), ok).
-
 %%--------------------------------------------------------------------
 %% Internal functions
 %%--------------------------------------------------------------------

apps/emqx_cluster_link/src/emqx_cluster_link.erl

@@ -16,9 +16,7 @@
     delete_shared_route/2,
     add_persistent_route/2,
     delete_persistent_route/2,
-    match_routes/1,
-    forward/2,
-    should_route_to_external_dests/1
+    forward/1
 ]).
 
 %% emqx hooks
@@ -73,18 +71,26 @@ add_persistent_route(Topic, ID) ->
 delete_persistent_route(Topic, ID) ->
     maybe_push_route_op(delete, Topic, ?PERSISTENT_ROUTE_ID(Topic, ID), push_persistent_route).
 
-forward(DestCluster, Delivery) ->
-    emqx_cluster_link_mqtt:forward(DestCluster, Delivery).
-
-match_routes(Topic) ->
-    emqx_cluster_link_extrouter:match_routes(Topic).
-
-%% Do not forward any external messages to other links.
-%% Only forward locally originated messages to all the relevant links, i.e. no gossip message forwarding.
-should_route_to_external_dests(#message{extra = #{link_origin := _}}) ->
-    false;
-should_route_to_external_dests(_Msg) ->
-    true.
+forward(#delivery{message = #message{extra = #{link_origin := _}}}) ->
+    %% Do not forward any external messages to other links.
+    %% Only forward locally originated messages to all the relevant links, i.e. no gossip
+    %% message forwarding.
+    [];
+forward(Delivery = #delivery{message = #message{topic = Topic}}) ->
+    Routes = emqx_cluster_link_extrouter:match_routes(Topic),
+    forward(Routes, Delivery).
+
+forward([], _Delivery) ->
+    [];
+forward(Routes, Delivery) ->
+    lists:foldl(
+        fun(#route{topic = To, dest = Cluster}, Acc) ->
+            Result = emqx_cluster_link_mqtt:forward(Cluster, Delivery),
+            [{Cluster, To, Result} | Acc]
+        end,
+        [],
+        Routes
+    ).
 
 %%--------------------------------------------------------------------
 %% EMQX Hooks