diff --git a/apps/emqx/src/emqx_broker.erl b/apps/emqx/src/emqx_broker.erl
index 103e358f6..856a3f7ae 100644
--- a/apps/emqx/src/emqx_broker.erl
+++ b/apps/emqx/src/emqx_broker.erl
@@ -159,6 +159,14 @@ do_subscribe(Topic, SubPid, SubOpts) when is_binary(Topic) ->
     %% https://emqx.atlassian.net/browse/EMQX-10214
     I = emqx_broker_helper:get_sub_shard(SubPid, Topic),
     true = ets:insert(?SUBOPTION, {{Topic, SubPid}, with_shard_idx(I, SubOpts)}),
+    %% NOTE
+    %% We are relying on the local state to minimize global routing state changes,
+    %% thus it's important that some operations on ETS tables on the same topic
+    %% should not be interleaved: `ets:member/2` + `ets:insert/2` that are part of
+    %% broker's `subscribe` codepath, and `ets:delete_object/2` that's part of
+    %% `unsubscribe` codepath. So we have to pick a worker according to the topic,
+    %% but not shard. If there are topics with high number of shards, then the
+    %% load across the pool will be unbalanced.
     call(pick(Topic), {subscribe, Topic, SubPid, I});
 do_subscribe(Topic = #share{group = Group, topic = RealTopic}, SubPid, SubOpts) when
     is_binary(RealTopic)
@@ -204,6 +212,14 @@ do_unsubscribe2(Topic, SubPid, SubOpts) when
         0 -> emqx_exclusive_subscription:unsubscribe(Topic, SubOpts);
         _ -> ok
     end,
+    %% NOTE
+    %% We are relying on the local state to minimize global routing state changes,
+    %% thus it's important that some operations on ETS tables on the same topic
+    %% should not be interleaved: `ets:member/2` + `ets:insert/2` that are part of
+    %% broker's `subscribe` codepath, and `ets:delete_object/2` that's part of
+    %% `unsubscribe` codepath. So we have to pick a worker according to the topic,
+    %% but not shard. If there are topics with high number of shards, then the
+    %% load across the pool will be unbalanced.
     cast(pick(Topic), {unsubscribed, Topic, SubPid, I});
 do_unsubscribe2(#share{group = Group, topic = Topic}, SubPid, _SubOpts) when
     is_binary(Group), is_binary(Topic), is_pid(SubPid)