diff --git a/apps/emqx_gateway/src/mqttsn/emqx_sn_registry.erl b/apps/emqx_gateway/src/mqttsn/emqx_sn_registry.erl
index 6086ec7d6..448aa8ad5 100644
--- a/apps/emqx_gateway/src/mqttsn/emqx_sn_registry.erl
+++ b/apps/emqx_gateway/src/mqttsn/emqx_sn_registry.erl
@@ -46,6 +46,11 @@
     code_change/3
 ]).
 
+%% Internal exports (RPC)
+-export([
+    do_register/4
+]).
+
 -export([lookup_name/1]).
 
 -define(SN_SHARD, emqx_sn_shard).
@@ -173,33 +178,11 @@ handle_call(
                 TopicId when TopicId >= 16#FFFF ->
                     {reply, {error, too_large}, State};
                 TopicId ->
-                    Fun = fun() ->
-                        mnesia:write(
-                            Tab,
-                            #emqx_sn_registry{
-                                key = {ClientId, next_topic_id},
-                                value = TopicId + 1
-                            },
-                            write
-                        ),
-                        mnesia:write(
-                            Tab,
-                            #emqx_sn_registry{
-                                key = {ClientId, TopicName},
-                                value = TopicId
-                            },
-                            write
-                        ),
-                        mnesia:write(
-                            Tab,
-                            #emqx_sn_registry{
-                                key = {ClientId, TopicId},
-                                value = TopicName
-                            },
-                            write
-                        )
-                    end,
-                    case mria:transaction(?SN_SHARD, Fun) of
+                    case
+                        mria:transaction(?SN_SHARD, fun ?MODULE:do_register/4, [
+                            Tab, ClientId, TopicId, TopicName
+                        ])
+                    of
                         {atomic, ok} ->
                             {reply, TopicId, State};
                         {aborted, Error} ->
@@ -248,6 +231,32 @@ terminate(_Reason, _State) ->
 code_change(_OldVsn, State, _Extra) ->
     {ok, State}.
 
+do_register(Tab, ClientId, TopicId, TopicName) ->
+    mnesia:write(
+        Tab,
+        #emqx_sn_registry{
+            key = {ClientId, next_topic_id},
+            value = TopicId + 1
+        },
+        write
+    ),
+    mnesia:write(
+        Tab,
+        #emqx_sn_registry{
+            key = {ClientId, TopicName},
+            value = TopicId
+        },
+        write
+    ),
+    mnesia:write(
+        Tab,
+        #emqx_sn_registry{
+            key = {ClientId, TopicId},
+            value = TopicName
+        },
+        write
+    ).
+
 %%-----------------------------------------------------------------------------
 
 next_topic_id(Tab, PredefId, ClientId) ->