feat(clusterlink): update only necessary resources when a link config is changed

apps/emqx_cluster_link/src/emqx_cluster_link_config.erl

@@ -19,6 +19,13 @@
 -define(DEFAULT_ACTOR_TTL, 3_000).
 -endif.
 
+-define(COMMON_FIELDS, [username, password, clientid, server, ssl]).
+%% NOTE: retry_interval, max_inflight may be used for router syncer client as well,
+%% but for now they are not.
+-define(MSG_RES_FIELDS, [resource_opts, pool_size, retry_interval, max_inflight]).
+%% Excludes a special hidden `ps_actor_incarnation` field.
+-define(ACTOR_FIELDS, [topics]).
+
 -export([
     %% General
     update/1,
@@ -196,19 +203,43 @@ update_links(LinksConf) ->
     [update_link(Link) || Link <- LinksConf].
 
 update_link({OldLinkConf, #{enable := true, upstream := Name} = NewLinkConf}) ->
-    _ = ensure_actor_stopped(Name),
-    {ok, _Pid} = emqx_cluster_link_sup:ensure_actor(NewLinkConf),
-    %% TODO: if only msg_fwd resource related config is changed,
-    %% we can skip actor reincarnation/restart.
-    ok = update_msg_fwd_resource(OldLinkConf, NewLinkConf),
-    ok;
+    case what_is_changed(OldLinkConf, NewLinkConf) of
+        both ->
+            _ = ensure_actor_stopped(Name),
+            {ok, _Pid} = emqx_cluster_link_sup:ensure_actor(NewLinkConf),
+            ok = update_msg_fwd_resource(OldLinkConf, NewLinkConf);
+        actor ->
+            _ = ensure_actor_stopped(Name),
+            {ok, _Pid} = emqx_cluster_link_sup:ensure_actor(NewLinkConf),
+            ok;
+        msg_resource ->
+            ok = update_msg_fwd_resource(OldLinkConf, NewLinkConf)
+    end;
 update_link({_OldLinkConf, #{enable := false, upstream := Name} = _NewLinkConf}) ->
     _ = emqx_cluster_link_mqtt:remove_msg_fwd_resource(Name),
     ensure_actor_stopped(Name).
 
-update_msg_fwd_resource(#{pool_size := Old}, #{pool_size := Old} = NewConf) ->
-    {ok, _} = emqx_cluster_link_mqtt:ensure_msg_fwd_resource(NewConf),
-    ok;
+what_is_changed(OldLink, NewLink) ->
+    CommonChanged = are_fields_changed(?COMMON_FIELDS, OldLink, NewLink),
+    ActorChanged = are_fields_changed(?ACTOR_FIELDS, OldLink, NewLink),
+    MsgResChanged = are_fields_changed(?MSG_RES_FIELDS, OldLink, NewLink),
+    AllChanged = ActorChanged andalso MsgResChanged,
+    case CommonChanged orelse AllChanged of
+        true ->
+            both;
+        false ->
+            %% This function is only applicable when it's certain that link conf is changed,
+            %% so if resource fields are the same,
+            %% then some other actor-related fields are definitely changed.
+            case MsgResChanged of
+                true -> msg_resource;
+                false -> actor
+            end
+    end.
+
+are_fields_changed(Fields, OldLink, NewLink) ->
+    maps:with(Fields, OldLink) =/= maps:with(Fields, NewLink).
+
 update_msg_fwd_resource(_, #{upstream := Name} = NewConf) ->
     _ = emqx_cluster_link_mqtt:remove_msg_fwd_resource(Name),
     {ok, _} = emqx_cluster_link_mqtt:ensure_msg_fwd_resource(NewConf),