diff --git a/src/emqx_shared_sub.erl b/src/emqx_shared_sub.erl
index 5c4b46217..65f535059 100644
--- a/src/emqx_shared_sub.erl
+++ b/src/emqx_shared_sub.erl
@@ -75,15 +75,19 @@ mnesia(copy) ->
 start_link() ->
     gen_server:start_link({local, ?SERVER}, ?MODULE, [], []).
 
+-spec(subscribe(emqx_topic:group(), emqx_topic:topic(), pid()) -> ok).
 subscribe(Group, Topic, SubPid) when is_pid(SubPid) ->
     gen_server:call(?SERVER, {subscribe, Group, Topic, SubPid}).
 
+-spec(unsubscribe(emqx_topic:group(), emqx_topic:topic(), pid()) -> ok).
 unsubscribe(Group, Topic, SubPid) when is_pid(SubPid) ->
     gen_server:call(?SERVER, {unsubscribe, Group, Topic, SubPid}).
 
 record(Group, Topic, SubPid) ->
     #emqx_shared_subscription{group = Group, topic = Topic, subpid = SubPid}.
 
+-spec(dispatch(emqx_topic:group(), emqx_topic:topic(), emqx_types:delivery())
+      -> emqx_types:delivery()).
 dispatch(Group, Topic, Delivery) ->
     dispatch(Group, Topic, Delivery, _FailedSubs = []).
 
@@ -173,12 +177,12 @@ get_ack_ref(Msg) ->
 -spec(is_ack_required(emqx_types:message()) -> boolean()).
 is_ack_required(Msg) -> ?no_ack =/= get_ack_ref(Msg).
 
-%% @doc Negative ack dropped message due to message queue being full.
+%% @doc Negative ack dropped message due to inflight window or message queue being full.
 -spec(maybe_nack_dropped(emqx_types:message()) -> ok).
 maybe_nack_dropped(Msg) ->
     case get_ack_ref(Msg) of
         ?no_ack -> ok;
-        {Sender, Ref} -> nack(Sender, Ref, drpped)
+        {Sender, Ref} -> nack(Sender, Ref, dropped)
     end.
 
 %% @doc Negative ack message due to connection down.
@@ -325,9 +329,9 @@ terminate(_Reason, _State) ->
 code_change(_OldVsn, State, _Extra) ->
     {ok, State}.
 
-%%--------------------------------------------------------------------
+%%------------------------------------------------------------------------------
 %% Internal functions
-%%--------------------------------------------------------------------
+%%------------------------------------------------------------------------------
 
 %% keep track of alive remote pids
 maybe_insert_alive_tab(Pid) when ?IS_LOCAL_PID(Pid) -> ok;
diff --git a/src/emqx_topic.erl b/src/emqx_topic.erl
index 2a37a873d..5339a7879 100644
--- a/src/emqx_topic.erl
+++ b/src/emqx_topic.erl
@@ -25,12 +25,13 @@
 -export([systop/1]).
 -export([parse/1, parse/2]).
 
+-type(group() :: binary()).
 -type(topic() :: binary()).
 -type(word() :: '' | '+' | '#' | binary()).
 -type(words() :: list(word())).
 -type(triple() :: {root | binary(), word(), binary()}).
 
--export_type([topic/0, word/0, triple/0]).
+-export_type([group/0, topic/0, word/0, triple/0]).
 
 -define(MAX_TOPIC_LEN, 4096).