diff --git a/apps/emqx_management/src/emqx_mgmt_api.erl b/apps/emqx_management/src/emqx_mgmt_api.erl
index e068c5384..d42d2a486 100644
--- a/apps/emqx_management/src/emqx_mgmt_api.erl
+++ b/apps/emqx_management/src/emqx_mgmt_api.erl
@@ -49,17 +49,46 @@ paginate(Tables, Params, RowFun) ->
 
 query_handle(Table) when is_atom(Table) ->
     qlc:q([R|| R <- ets:table(Table)]);
+
+query_handle({Table, Opts}) when is_atom(Table) ->
+    qlc:q([R|| R <- ets:table(Table, Opts)]);
+
 query_handle([Table]) when is_atom(Table) ->
     qlc:q([R|| R <- ets:table(Table)]);
+
+query_handle([{Table, Opts}]) when is_atom(Table) ->
+    qlc:q([R|| R <- ets:table(Table, Opts)]);
+
 query_handle(Tables) ->
-    qlc:append([qlc:q([E || E <- ets:table(T)]) || T <- Tables]).
+    Fold = fun({Table, Opts}, Acc) ->
+                   Handle = qlc:q([R|| R <- ets:table(Table, Opts)]),
+                   [Handle | Acc];
+              (Table, Acc) ->
+                   Handle = qlc:q([R|| R <- ets:table(Table)]),
+                   [Handle | Acc]
+            end,
+    Handles = lists:foldl(Fold, [], Tables),
+    qlc:append(lists:reverse(Handles)).
 
 count(Table) when is_atom(Table) ->
     ets:info(Table, size);
+
+count({Table, _Opts}) when is_atom(Table) ->
+    ets:info(Table, size);
+
 count([Table]) when is_atom(Table) ->
     ets:info(Table, size);
+
+count([{Table, _Opts}]) when is_atom(Table) ->
+    ets:info(Table, size);
+
 count(Tables) ->
-    lists:sum([count(T) || T <- Tables]).
+    Fold = fun({Table, _Opts}, Acc) ->
+                   count(Table) ++ Acc;
+              (Table, Acc) ->
+                   count(Table) ++ Acc
+           end,
+    lists:foldl(Fold, 0, Tables).
 
 count(Table, Nodes) ->
     lists:sum([rpc_call(Node, ets, info, [Table, size], 5000) || Node <- Nodes]).
diff --git a/apps/emqx_plugin_libs/include/emqx_st_statistics.hrl b/apps/emqx_plugin_libs/include/emqx_slow_subs.hrl
similarity index 68%
rename from apps/emqx_plugin_libs/include/emqx_st_statistics.hrl
rename to apps/emqx_plugin_libs/include/emqx_slow_subs.hrl
index 9184c3194..0b5e3a035 100644
--- a/apps/emqx_plugin_libs/include/emqx_st_statistics.hrl
+++ b/apps/emqx_plugin_libs/include/emqx_slow_subs.hrl
@@ -14,12 +14,15 @@
 %% limitations under the License.
 %%--------------------------------------------------------------------
 
--define(LOG_TAB, emqx_st_statistics_log).
--define(TOPK_TAB, emqx_st_statistics_topk).
+-define(TOPK_TAB, emqx_slow_subs_topk).
 
--record(top_k, { rank :: pos_integer()
-               , topic :: emqx_types:topic()
-               , average_count :: number()
-               , average_elapsed :: number()}).
+-define(INDEX(Latency, ClientId), {Latency, ClientId}).
+
+-record(top_k, { index :: index()
+               , type :: emqx_message_latency_stats:latency_type()
+               , last_update_time :: pos_integer()
+               , extra = []
+               }).
 
 -type top_k() :: #top_k{}.
+-type index() :: ?INDEX(non_neg_integer(), emqx_types:clientid()).
diff --git a/apps/emqx_plugin_libs/src/emqx_slow_subs/emqx_slow_subs.erl b/apps/emqx_plugin_libs/src/emqx_slow_subs/emqx_slow_subs.erl
new file mode 100644
index 000000000..a50b5eb0e
--- /dev/null
+++ b/apps/emqx_plugin_libs/src/emqx_slow_subs/emqx_slow_subs.erl
@@ -0,0 +1,319 @@
+%%--------------------------------------------------------------------
+%% Copyright (c) 2020-2021 EMQ Technologies Co., Ltd. All Rights Reserved.
+%%
+%% Licensed under the Apache License, Version 2.0 (the "License");
+%% you may not use this file except in compliance with the License.
+%% You may obtain a copy of the License at
+%%
+%%     http://www.apache.org/licenses/LICENSE-2.0
+%%
+%% Unless required by applicable law or agreed to in writing, software
+%% distributed under the License is distributed on an "AS IS" BASIS,
+%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+%% See the License for the specific language governing permissions and
+%% limitations under the License.
+%%--------------------------------------------------------------------
+
+-module(emqx_slow_subs).
+
+-behaviour(gen_server).
+
+-include_lib("include/emqx.hrl").
+-include_lib("include/logger.hrl").
+-include_lib("emqx_plugin_libs/include/emqx_slow_subs.hrl").
+
+-logger_header("[SLOW Subs]").
+
+-export([ start_link/1, on_stats_update/2, enable/0
+        , disable/0, clear_history/0, init_topk_tab/0
+        ]).
+
+%% gen_server callbacks
+-export([ init/1
+        , handle_call/3
+        , handle_cast/2
+        , handle_info/2
+        , terminate/2
+        , code_change/3
+        ]).
+
+-compile(nowarn_unused_type).
+
+-type state() :: #{ config := proplist:proplist()
+                  , enable := boolean()
+                  , last_tick_at := pos_integer()
+                  }.
+
+-type log() :: #{ rank := pos_integer()
+                , clientid := emqx_types:clientid()
+                , latency := non_neg_integer()
+                , type := emqx_message_latency_stats:latency_type()
+                }.
+
+-type window_log() :: #{ last_tick_at := pos_integer()
+                       , logs := [log()]
+                       }.
+
+-type message() :: #message{}.
+
+-import(proplists, [get_value/2]).
+
+-type stats_update_args() :: #{ clientid := emqx_types:clientid()
+                              , latency := non_neg_integer()
+                              , type := emqx_message_latency_stats:latency_type()
+                              , last_insert_value := non_neg_integer()
+                              , update_time := timer:time()
+                              }.
+
+-type stats_update_env() :: #{max_size := pos_integer()}.
+
+-ifdef(TEST).
+-define(EXPIRE_CHECK_INTERVAL, timer:seconds(1)).
+-else.
+-define(EXPIRE_CHECK_INTERVAL, timer:seconds(10)).
+-endif.
+
+-define(NOW, erlang:system_time(millisecond)).
+-define(NOTICE_TOPIC_NAME, "slow_subs").
+-define(DEF_CALL_TIMEOUT, timer:seconds(10)).
+
+%% erlang term order
+%% number < atom < reference < fun < port < pid < tuple < list < bit string
+
+%% ets ordered_set is ascending by term order
+
+%%--------------------------------------------------------------------
+%%--------------------------------------------------------------------
+%% APIs
+%%--------------------------------------------------------------------
+%% @doc Start the st_statistics
+-spec(start_link(Env :: list()) -> emqx_types:startlink_ret()).
+start_link(Env) ->
+    gen_server:start_link({local, ?MODULE}, ?MODULE, [Env], []).
+
+%% XXX NOTE:pay attention to the performance here
+-spec on_stats_update(stats_update_args(), stats_update_env()) -> true.
+on_stats_update(#{clientid := ClientId,
+                  latency := Latency,
+                  type := Type,
+                  last_insert_value := LIV,
+                  update_time := Ts},
+                #{max_size := MaxSize}) ->
+
+    LastIndex = ?INDEX(LIV, ClientId),
+    Index = ?INDEX(Latency, ClientId),
+
+    %% check whether the client is in the table
+    case ets:lookup(?TOPK_TAB, LastIndex) of
+        [#top_k{index = Index}] ->
+            %% if last value == the new value, return
+            true;
+        [_] ->
+            %% if Latency > minimum value, we should update it
+            %% if Latency < minimum value, maybe it can replace the minimum value
+            %% so alwyas update at here
+            %% do we need check if Latency == minimum ???
+            ets:insert(?TOPK_TAB,
+                       #top_k{index = Index, type = Type, last_update_time = Ts}),
+            ets:delete(?TOPK_TAB, LastIndex);
+        [] ->
+            %% try to insert
+            try_insert_to_topk(MaxSize, Index, Latency, Type, Ts)
+    end.
+
+clear_history() ->
+    gen_server:call(?MODULE, ?FUNCTION_NAME, ?DEF_CALL_TIMEOUT).
+
+enable() ->
+    gen_server:call(?MODULE, {enable, true}, ?DEF_CALL_TIMEOUT).
+
+disable() ->
+    gen_server:call(?MODULE, {enable, false}, ?DEF_CALL_TIMEOUT).
+
+init_topk_tab() ->
+    case ets:whereis(?TOPK_TAB) of
+        undefined ->
+            ?TOPK_TAB = ets:new(?TOPK_TAB,
+                                [ ordered_set, public, named_table
+                                , {keypos, #top_k.index}, {write_concurrency, true}
+                                , {read_concurrency, true}
+                                ]);
+        _ ->
+            ?TOPK_TAB
+    end.
+
+%%--------------------------------------------------------------------
+%% gen_server callbacks
+%%--------------------------------------------------------------------
+
+init([Conf]) ->
+    notice_tick(Conf),
+    expire_tick(Conf),
+    MaxSize = get_value(top_k_num, Conf),
+    load(MaxSize),
+    {ok, #{config => Conf,
+           last_tick_at => ?NOW,
+           enable => true}}.
+
+handle_call({enable, Enable}, _From,
+            #{config := Cfg, enable := IsEnable} = State) ->
+    State2 = case Enable of
+                 IsEnable ->
+                     State;
+                 true ->
+                     MaxSize = get_value(max_topk_num, Cfg),
+                     load(MaxSize),
+                     State#{enable := true};
+                 _ ->
+                     unload(),
+                     State#{enable := false}
+             end,
+    {reply, ok, State2};
+
+handle_call(clear_history, _, State) ->
+    ets:delete_all_objects(?TOPK_TAB),
+    {reply, ok, State};
+
+handle_call(Req, _From, State) ->
+    ?LOG(error, "Unexpected call: ~p", [Req]),
+    {reply, ignored, State}.
+
+handle_cast(Msg, State) ->
+    ?LOG(error, "Unexpected cast: ~p", [Msg]),
+    {noreply, State}.
+
+handle_info(expire_tick, #{config := Cfg} = State) ->
+    expire_tick(Cfg),
+    Logs = ets:tab2list(?TOPK_TAB),
+    do_clear(Cfg, Logs),
+    {noreply, State};
+
+handle_info(notice_tick, #{config := Cfg} = State) ->
+    notice_tick(Cfg),
+    Logs = ets:tab2list(?TOPK_TAB),
+    do_notification(Logs, State),
+    {noreply, State#{last_tick_at := ?NOW}};
+
+handle_info(Info, State) ->
+    ?LOG(error, "Unexpected info: ~p", [Info]),
+    {noreply, State}.
+
+terminate(_Reason, _) ->
+    unload(),
+    ok.
+
+code_change(_OldVsn, State, _Extra) ->
+    {ok, State}.
+
+%%--------------------------------------------------------------------
+%% Internal functions
+%%--------------------------------------------------------------------
+expire_tick(_) ->
+    erlang:send_after(?EXPIRE_CHECK_INTERVAL, self(), ?FUNCTION_NAME).
+
+notice_tick(Cfg) ->
+    case get_value(notice_interval, Cfg) of
+        0 -> ok;
+        Interval ->
+            erlang:send_after(Interval, self(), ?FUNCTION_NAME),
+            ok
+    end.
+
+-spec do_notification(list(), state()) -> ok.
+do_notification([], _) ->
+    ok;
+
+do_notification(Logs, #{last_tick_at := LastTickTime, config := Cfg}) ->
+    start_publish(Logs, LastTickTime, Cfg),
+    ok.
+
+start_publish(Logs, TickTime, Cfg) ->
+    emqx_pool:async_submit({fun do_publish/4, [Logs, erlang:length(Logs), TickTime, Cfg]}).
+
+do_publish([], _, _, _) ->
+    ok;
+
+do_publish(Logs, Rank, TickTime, Cfg) ->
+    BatchSize = get_value(notice_batch_size, Cfg),
+    do_publish(Logs, BatchSize, Rank, TickTime, Cfg, []).
+
+do_publish([Log | T], Size, Rank, TickTime, Cfg, Cache) when Size > 0 ->
+    Cache2 = [convert_to_notice(Rank, Log) | Cache],
+    do_publish(T, Size - 1, Rank - 1, TickTime, Cfg, Cache2);
+
+do_publish(Logs, Size, Rank, TickTime, Cfg, Cache) when Size =:= 0 ->
+    publish(TickTime, Cfg, Cache),
+    do_publish(Logs, Rank, TickTime, Cfg);
+
+do_publish([], _, _Rank, TickTime, Cfg, Cache) ->
+    publish(TickTime, Cfg, Cache),
+    ok.
+
+convert_to_notice(Rank, #top_k{index = ?INDEX(Latency, ClientId),
+                               type = Type,
+                               last_update_time = Ts}) ->
+    #{rank => Rank,
+      clientid => ClientId,
+      latency => Latency,
+      type => Type,
+      timestamp => Ts}.
+
+publish(TickTime, Cfg, Notices) ->
+    WindowLog = #{last_tick_at => TickTime,
+                  logs => lists:reverse(Notices)},
+    Payload = emqx_json:encode(WindowLog),
+    Msg = #message{ id = emqx_guid:gen()
+                  , qos = get_value(notice_qos, Cfg)
+                  , from = ?MODULE
+                  , topic = emqx_topic:systop(?NOTICE_TOPIC_NAME)
+                  , payload = Payload
+                  , timestamp = ?NOW
+                  },
+    _ = emqx_broker:safe_publish(Msg),
+    ok.
+
+load(MaxSize) ->
+    _ = emqx:hook('message.slow_subs_stats',
+                  fun ?MODULE:on_stats_update/2,
+                  [#{max_size => MaxSize}]),
+    ok.
+
+unload() ->
+    emqx:unhook('message.slow_subs_stats', fun ?MODULE:on_stats_update/2).
+
+do_clear(Cfg, Logs) ->
+    Now = ?NOW,
+    Interval = get_value(expire_interval, Cfg),
+    Each = fun(#top_k{index = Index, last_update_time = Ts}) ->
+                   case Now - Ts >= Interval of
+                       true ->
+                           ets:delete(?TOPK_TAB, Index);
+                       _ ->
+                           true
+               end
+           end,
+    lists:foreach(Each, Logs).
+
+try_insert_to_topk(MaxSize, Index, Latency, Type, Ts) ->
+    case ets:info(?TOPK_TAB, size) of
+        Size when Size < MaxSize ->
+            %% if the size is under limit, insert it directly
+            ets:insert(?TOPK_TAB,
+                       #top_k{index = Index, type = Type, last_update_time = Ts});
+        _Size ->
+            %% find the minimum value
+            ?INDEX(Min, _) = First =
+                case ets:first(?TOPK_TAB) of
+                    ?INDEX(_, _) = I ->  I;
+                    _ -> ?INDEX(Latency - 1, <<>>)
+                end,
+
+            case Latency =< Min of
+                true -> true;
+                _ ->
+                    ets:insert(?TOPK_TAB,
+                               #top_k{index = Index, type = Type, last_update_time = Ts}),
+
+                    ets:delete(?TOPK_TAB, First)
+            end
+    end.
diff --git a/apps/emqx_plugin_libs/src/emqx_slow_subs/emqx_slow_subs_api.erl b/apps/emqx_plugin_libs/src/emqx_slow_subs/emqx_slow_subs_api.erl
new file mode 100644
index 000000000..a7c9fc050
--- /dev/null
+++ b/apps/emqx_plugin_libs/src/emqx_slow_subs/emqx_slow_subs_api.erl
@@ -0,0 +1,57 @@
+%%--------------------------------------------------------------------
+%% Copyright (c) 2021 EMQ Technologies Co., Ltd. All Rights Reserved.
+%%
+%% Licensed under the Apache License, Version 2.0 (the "License");
+%% you may not use this file except in compliance with the License.
+%% You may obtain a copy of the License at
+%%
+%%     http://www.apache.org/licenses/LICENSE-2.0
+%%
+%% Unless required by applicable law or agreed to in writing, software
+%% distributed under the License is distributed on an "AS IS" BASIS,
+%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+%% See the License for the specific language governing permissions and
+%% limitations under the License.
+%%--------------------------------------------------------------------
+
+-module(emqx_slow_subs_api).
+
+-rest_api(#{name   => clear_history,
+            method => 'DELETE',
+            path   => "/slow_subscriptions",
+            func   => clear_history,
+            descr  => "Clear current data and re count slow topic"}).
+
+-rest_api(#{name   => get_history,
+            method => 'GET',
+            path   => "/slow_subscriptions",
+            func   => get_history,
+            descr  => "Get slow topics statistics record data"}).
+
+-export([ clear_history/2
+        , get_history/2
+        ]).
+
+-include("include/emqx_slow_subs.hrl").
+
+-import(minirest, [return/1]).
+
+%%--------------------------------------------------------------------
+%% HTTP API
+%%--------------------------------------------------------------------
+
+clear_history(_Bindings, _Params) ->
+    ok = emqx_slow_subs:clear_history(),
+    return(ok).
+
+get_history(_Bindings, Params) ->
+    RowFun = fun(#top_k{index = ?INDEX(Latency, ClientId),
+                        type = Type,
+                        last_update_time = Ts}) ->
+                 [{clientid, ClientId},
+                  {latency, Latency},
+                  {type, Type},
+                  {last_update_time, Ts}]
+             end,
+    Return = emqx_mgmt_api:paginate({?TOPK_TAB, [{traverse, last_prev}]}, Params, RowFun),
+    return({ok, Return}).
diff --git a/apps/emqx_plugin_libs/src/emqx_st_statistics/emqx_st_statistics.erl b/apps/emqx_plugin_libs/src/emqx_st_statistics/emqx_st_statistics.erl
deleted file mode 100644
index 668cb3926..000000000
--- a/apps/emqx_plugin_libs/src/emqx_st_statistics/emqx_st_statistics.erl
+++ /dev/null
@@ -1,379 +0,0 @@
-%%--------------------------------------------------------------------
-%% Copyright (c) 2020-2021 EMQ Technologies Co., Ltd. All Rights Reserved.
-%%
-%% Licensed under the Apache License, Version 2.0 (the "License");
-%% you may not use this file except in compliance with the License.
-%% You may obtain a copy of the License at
-%%
-%%     http://www.apache.org/licenses/LICENSE-2.0
-%%
-%% Unless required by applicable law or agreed to in writing, software
-%% distributed under the License is distributed on an "AS IS" BASIS,
-%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-%% See the License for the specific language governing permissions and
-%% limitations under the License.
-%%--------------------------------------------------------------------
-
--module(emqx_st_statistics).
-
--behaviour(gen_server).
-
--include_lib("include/emqx.hrl").
--include_lib("include/logger.hrl").
--include_lib("emqx_plugin_libs/include/emqx_st_statistics.hrl").
-
--logger_header("[SLOW TOPICS]").
-
--export([ start_link/1, on_publish_done/3, enable/0
-        , disable/0, clear_history/0
-        ]).
-
-%% gen_server callbacks
--export([ init/1
-        , handle_call/3
-        , handle_cast/2
-        , handle_info/2
-        , terminate/2
-        , code_change/3
-        ]).
-
--compile(nowarn_unused_type).
-
--type state() :: #{ config := proplist:proplist()
-                  , period := pos_integer()
-                  , last_tick_at := pos_integer()
-                  , counter := counters:counters_ref()
-                  , enable := boolean()
-                  }.
-
--type log() :: #{ topic := emqx_types:topic()
-                , count := pos_integer()
-                , average := float()
-                }.
-
--type window_log() :: #{ last_tick_at := pos_integer()
-                       , logs := [log()]
-                       }.
-
--record(slow_log, { topic :: emqx_types:topic()
-                  , count :: integer()     %% 0 will be used in initial value
-                  , elapsed :: integer()
-                  }).
-
--type message() :: #message{}.
-
--import(proplists, [get_value/2]).
-
--define(NOW, erlang:system_time(millisecond)).
--define(QUOTA_IDX, 1).
-
--type slow_log() :: #slow_log{}.
--type top_k_map() :: #{emqx_types:topic() => top_k()}.
-
--type publish_done_env() :: #{ ignore_before_create := boolean()
-                             , threshold := pos_integer()
-                             , counter := counters:counters_ref()
-                             }.
-
--type publish_done_args() :: #{session_rebirth_time => pos_integer()}.
-
--ifdef(TEST).
--define(TOPK_ACCESS, public).
--else.
--define(TOPK_ACCESS, protected).
--endif.
-
-%% erlang term order
-%% number < atom < reference < fun < port < pid < tuple < list < bit string
-
-%% ets ordered_set is ascending by term order
-
-%%--------------------------------------------------------------------
-%%--------------------------------------------------------------------
-%% APIs
-%%--------------------------------------------------------------------
-%% @doc Start the st_statistics
--spec(start_link(Env :: list()) -> emqx_types:startlink_ret()).
-start_link(Env) ->
-    gen_server:start_link({local, ?MODULE}, ?MODULE, [Env], []).
-
--spec on_publish_done(message(), publish_done_args(), publish_done_env()) -> ok.
-on_publish_done(#message{timestamp = Timestamp},
-                #{session_rebirth_time := Created},
-                #{ignore_before_create := IgnoreBeforeCreate})
-  when IgnoreBeforeCreate, Timestamp < Created ->
-    ok;
-
-on_publish_done(#message{timestamp = Timestamp} = Msg,
-                _,
-                #{threshold := Threshold, counter := Counter}) ->
-    case ?NOW - Timestamp of
-        Elapsed when Elapsed > Threshold ->
-            case get_log_quota(Counter) of
-                true ->
-                    update_log(Msg, Elapsed);
-                _ ->
-                    ok
-            end;
-        _ ->
-            ok
-    end.
-
-clear_history() ->
-    gen_server:call(?MODULE, ?FUNCTION_NAME).
-
-enable() ->
-    gen_server:call(?MODULE, {enable, true}).
-
-disable() ->
-    gen_server:call(?MODULE, {enable, false}).
-
-%%--------------------------------------------------------------------
-%% gen_server callbacks
-%%--------------------------------------------------------------------
-
-init([Env]) ->
-    erlang:process_flag(trap_exit, true),
-    init_log_tab(Env),
-    init_topk_tab(Env),
-    notification_tick(Env),
-    Counter = counters:new(1, [write_concurrency]),
-    set_log_quota(Env, Counter),
-    Threshold = get_value(threshold_time, Env),
-    IgnoreBeforeCreate = get_value(ignore_before_create, Env),
-    load(IgnoreBeforeCreate, Threshold, Counter),
-    {ok, #{config => Env,
-           period => 1,
-           last_tick_at => ?NOW,
-           counter => Counter,
-           enable => true}}.
-
-handle_call({enable, Enable}, _From,
-            #{config := Cfg, counter := Counter, enable := IsEnable} = State) ->
-    State2 = case Enable of
-                 IsEnable ->
-                     State;
-                 true ->
-                     Threshold = get_value(threshold_time, Cfg),
-                     IgnoreBeforeCreate = get_value(ignore_before_create, Cfg),
-                     load(IgnoreBeforeCreate, Threshold, Counter),
-                     State#{enable := true};
-                 _ ->
-                     unload(),
-                     State#{enable := false}
-             end,
-    {reply, ok, State2};
-
-handle_call(clear_history, _, State) ->
-    ets:delete_all_objects(?TOPK_TAB),
-    {reply, ok, State};
-
-handle_call(Req, _From, State) ->
-    ?LOG(error, "Unexpected call: ~p", [Req]),
-    {reply, ignored, State}.
-
-handle_cast(Msg, State) ->
-    ?LOG(error, "Unexpected cast: ~p", [Msg]),
-    {noreply, State}.
-
-handle_info(notification_tick, #{config := Cfg, period := Period} = State) ->
-    notification_tick(Cfg),
-    do_notification(State),
-    {noreply, State#{last_tick_at := ?NOW,
-                     period := Period + 1}};
-
-handle_info(Info, State) ->
-    ?LOG(error, "Unexpected info: ~p", [Info]),
-    {noreply, State}.
-
-terminate(_Reason, _) ->
-    unload(),
-    ok.
-
-code_change(_OldVsn, State, _Extra) ->
-    {ok, State}.
-
-%%--------------------------------------------------------------------
-%% Internal functions
-%%--------------------------------------------------------------------
-notification_tick(Env) ->
-    TimeWindow = get_value(time_window, Env),
-    erlang:send_after(TimeWindow, self(), ?FUNCTION_NAME).
-
-init_log_tab(_) ->
-    ?LOG_TAB = ets:new(?LOG_TAB, [ set, public, named_table
-                                 , {keypos, #slow_log.topic}, {write_concurrency, true}
-                                 , {read_concurrency, true}
-                                 ]).
-
-init_topk_tab(_) ->
-    ?TOPK_TAB = ets:new(?TOPK_TAB, [ set, ?TOPK_ACCESS, named_table
-                                   , {keypos, #top_k.rank}, {write_concurrency, false}
-                                   , {read_concurrency, true}
-                                   ]).
-
--spec get_log_quota(counters:counters_ref()) -> boolean().
-get_log_quota(Counter) ->
-    case counters:get(Counter, ?QUOTA_IDX) of
-        Quota when Quota > 0 ->
-            counters:sub(Counter, ?QUOTA_IDX, 1),
-            true;
-        _ ->
-            false
-    end.
-
--spec set_log_quota(proplists:proplist(), counters:counters_ref()) -> ok.
-set_log_quota(Cfg, Counter) ->
-    MaxLogNum = get_value(max_log_num, Cfg),
-    counters:put(Counter, ?QUOTA_IDX, MaxLogNum).
-
--spec update_log(message(), pos_integer()) -> ok.
-update_log(#message{topic = Topic}, Elapsed) ->
-    _ = ets:update_counter(?LOG_TAB,
-                           Topic,
-                           [{#slow_log.count, 1}, {#slow_log.elapsed, Elapsed}],
-                           #slow_log{topic = Topic,
-                                     count = 0,
-                                     elapsed = 0}),
-    ok.
-
--spec do_notification(state()) -> true.
-do_notification(#{last_tick_at := TickTime,
-                  config := Cfg,
-                  period := Period,
-                  counter := Counter}) ->
-    Logs = ets:tab2list(?LOG_TAB),
-    ets:delete_all_objects(?LOG_TAB),
-    start_publish(Logs, TickTime, Cfg),
-    set_log_quota(Cfg, Counter),
-    MaxRecord = get_value(top_k_num, Cfg),
-    update_topk(Logs, MaxRecord, Period).
-
--spec update_topk(list(slow_log()), pos_integer(), pos_integer()) -> true.
-update_topk(Logs, MaxRecord, Period) ->
-    TopkMap = get_topk_map(Period),
-    TopkMap2 = update_topk_map(Logs, Period, TopkMap),
-    SortFun = fun(A, B) ->
-                      A#top_k.average_count > B#top_k.average_count
-              end,
-    TopkL = lists:sort(SortFun, maps:values(TopkMap2)),
-    TopkL2 = lists:sublist(TopkL, 1, MaxRecord),
-    update_topk_tab(TopkL2).
-
--spec update_topk_map(list(slow_log()), pos_integer(), top_k_map()) -> top_k_map().
-update_topk_map([#slow_log{topic = Topic,
-                           count = LogTimes,
-                           elapsed = LogElapsed} | T], Period, TopkMap) ->
-    case maps:get(Topic, TopkMap, undefined) of
-        undefined ->
-            Record = #top_k{rank = 1,
-                            topic = Topic,
-                            average_count = LogTimes,
-                            average_elapsed = LogElapsed},
-            TopkMap2 = TopkMap#{Topic => Record},
-            update_topk_map(T, Period, TopkMap2);
-        #top_k{average_count = AvgCount,
-               average_elapsed = AvgElapsed} = Record ->
-            NewPeriod = Period + 1,
-            %% (a + b) / c = a / c + b / c
-            %% average_count(elapsed) dived NewPeriod in function get_topk_maps
-            AvgCount2 = AvgCount + LogTimes / NewPeriod,
-            AvgElapsed2 = AvgElapsed + LogElapsed / NewPeriod,
-            Record2 = Record#top_k{average_count = AvgCount2,
-                                   average_elapsed = AvgElapsed2},
-            update_topk_map(T, Period, TopkMap#{Topic := Record2})
-    end;
-
-update_topk_map([], _, TopkMap) ->
-    TopkMap.
-
--spec update_topk_tab(list(top_k())) -> true.
-update_topk_tab(Records) ->
-    Zip = fun(Rank, Item) -> Item#top_k{rank = Rank} end,
-    Len = erlang:length(Records),
-    RankedTopics = lists:zipwith(Zip, lists:seq(1, Len), Records),
-    ets:insert(?TOPK_TAB, RankedTopics).
-
-start_publish(Logs, TickTime, Cfg) ->
-    emqx_pool:async_submit({fun do_publish/3, [Logs, TickTime, Cfg]}).
-
-do_publish([], _, _) ->
-    ok;
-
-do_publish(Logs, TickTime, Cfg) ->
-    BatchSize = get_value(notice_batch_size, Cfg),
-    do_publish(Logs, BatchSize, TickTime, Cfg, []).
-
-do_publish([Log | T], Size, TickTime, Cfg, Cache) when Size > 0 ->
-    Cache2 = [convert_to_notice(Log) | Cache],
-    do_publish(T, Size - 1, TickTime, Cfg, Cache2);
-
-do_publish(Logs, Size, TickTime, Cfg, Cache) when Size =:= 0 ->
-    publish(TickTime, Cfg, Cache),
-    do_publish(Logs, TickTime, Cfg);
-
-do_publish([], _, TickTime, Cfg, Cache) ->
-    publish(TickTime, Cfg, Cache),
-    ok.
-
-convert_to_notice(#slow_log{topic = Topic,
-                            count = Count,
-                            elapsed = Elapsed}) ->
-    #{topic => Topic,
-      count => Count,
-      average => Elapsed / Count}.
-
-publish(TickTime, Cfg, Notices) ->
-    WindowLog = #{last_tick_at => TickTime,
-                  logs => Notices},
-    Payload = emqx_json:encode(WindowLog),
-    _ = emqx:publish(#message{ id = emqx_guid:gen()
-                             , qos = get_value(notice_qos, Cfg)
-                             , from = ?MODULE
-                             , topic = get_topic(Cfg)
-                             , payload = Payload
-                             , timestamp = ?NOW
-                             }),
-    ok.
-
-load(IgnoreBeforeCreate, Threshold, Counter) ->
-    _ = emqx:hook('message.publish_done',
-                  fun ?MODULE:on_publish_done/3,
-                  [#{ignore_before_create => IgnoreBeforeCreate,
-                     threshold => Threshold,
-                     counter => Counter}
-                  ]),
-    ok.
-
-unload() ->
-    emqx:unhook('message.publish_done', fun ?MODULE:on_publish_done/3).
-
--spec get_topic(proplists:proplist()) -> binary().
-get_topic(Cfg) ->
-    case get_value(notice_topic, Cfg) of
-        Topic when is_binary(Topic) ->
-            Topic;
-        Topic ->
-            erlang:list_to_binary(Topic)
-    end.
-
--spec get_topk_map(pos_integer()) -> top_k_map().
-get_topk_map(Period) ->
-    Size = ets:info(?TOPK_TAB, size),
-    get_topk_map(1, Size, Period, #{}).
-
--spec get_topk_map(pos_integer(),
-                   non_neg_integer(), pos_integer(), top_k_map()) -> top_k_map().
-get_topk_map(Index, Size, _, TopkMap) when Index > Size ->
-    TopkMap;
-get_topk_map(Index, Size, Period, TopkMap) ->
-    [#top_k{topic = Topic,
-            average_count = AvgCount,
-            average_elapsed = AvgElapsed} = R] = ets:lookup(?TOPK_TAB, Index),
-    NewPeriod = Period + 1,
-    TotalTimes = AvgCount * Period,
-    AvgCount2 = TotalTimes / NewPeriod,
-    AvgElapsed2 = TotalTimes * AvgElapsed / NewPeriod,
-    TopkMap2 = TopkMap#{Topic => R#top_k{average_count = AvgCount2,
-                                         average_elapsed = AvgElapsed2}},
-    get_topk_map(Index + 1, Size, Period, TopkMap2).
diff --git a/apps/emqx_plugin_libs/src/emqx_st_statistics/emqx_st_statistics_api.erl b/apps/emqx_plugin_libs/src/emqx_st_statistics/emqx_st_statistics_api.erl
deleted file mode 100644
index dc74dcf36..000000000
--- a/apps/emqx_plugin_libs/src/emqx_st_statistics/emqx_st_statistics_api.erl
+++ /dev/null
@@ -1,83 +0,0 @@
-%%--------------------------------------------------------------------
-%% Copyright (c) 2021 EMQ Technologies Co., Ltd. All Rights Reserved.
-%%
-%% Licensed under the Apache License, Version 2.0 (the "License");
-%% you may not use this file except in compliance with the License.
-%% You may obtain a copy of the License at
-%%
-%%     http://www.apache.org/licenses/LICENSE-2.0
-%%
-%% Unless required by applicable law or agreed to in writing, software
-%% distributed under the License is distributed on an "AS IS" BASIS,
-%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-%% See the License for the specific language governing permissions and
-%% limitations under the License.
-%%--------------------------------------------------------------------
-
--module(emqx_st_statistics_api).
-
--rest_api(#{name   => clear_history,
-            method => 'DELETE',
-            path   => "/slow_topic",
-            func   => clear_history,
-            descr  => "Clear current data and re count slow topic"}).
-
--rest_api(#{name   => get_history,
-            method => 'GET',
-            path   => "/slow_topic",
-            func   => get_history,
-            descr  => "Get slow topics statistics record data"}).
-
--export([ clear_history/2
-        , get_history/2
-        ]).
-
--include("include/emqx_st_statistics.hrl").
-
--import(minirest, [return/1]).
-
-%%--------------------------------------------------------------------
-%% HTTP API
-%%--------------------------------------------------------------------
-
-clear_history(_Bindings, _Params) ->
-    ok = emqx_st_statistics:clear_history(),
-    return(ok).
-
-get_history(_Bindings, Params) ->
-    PageT = proplists:get_value(<<"_page">>, Params),
-    LimitT = proplists:get_value(<<"_limit">>, Params),
-    Page = erlang:binary_to_integer(PageT),
-    Limit = erlang:binary_to_integer(LimitT),
-    Start = (Page - 1) * Limit + 1,
-    Size = ets:info(?TOPK_TAB, size),
-    End = Start + Limit - 1,
-    {HasNext, Count, Infos} = get_history(Start, End, Size),
-    return({ok, #{meta => #{page => Page,
-                            limit => Limit,
-                            hasnext => HasNext,
-                            count => Count},
-                  data => Infos}}).
-
-
-get_history(Start, _End, Size) when Start > Size ->
-    {false, 0, []};
-
-get_history(Start, End, Size) when End > Size ->
-    get_history(Start, Size, Size);
-
-get_history(Start, End, Size) ->
-    Fold = fun(Rank, Acc) ->
-                   [#top_k{topic = Topic
-                          , average_count = Count
-                          , average_elapsed = Elapsed}] = ets:lookup(?TOPK_TAB, Rank),
-
-                   Info = [ {rank, Rank}
-                          , {topic, Topic}
-                          , {count, Count}
-                          , {elapsed, Elapsed}],
-
-                   [Info | Acc]
-           end,
-    Infos = lists:foldl(Fold, [], lists:seq(Start, End)),
-    {End < Size, End - Start + 1, Infos}.
diff --git a/etc/emqx.conf b/etc/emqx.conf
index 78d383930..de5c062a4 100644
--- a/etc/emqx.conf
+++ b/etc/emqx.conf
@@ -2217,47 +2217,34 @@ module.presence.qos = 1
 ## module.rewrite.sub.rule.1 = y/+/z/# ^y/(.+)/z/(.+)$ y/z/$2
 
 ##--------------------------------------------------------------------
-## Slow Topic Statistics Module
+## Slow Subscribers Statistics Module
 
-## Threshold time of slow topics statistics
-##
-## Default: 10 seconds
-#module.st_statistics.threshold_time = 10S
-
-## ignore the messages that before than session created
-##
-## Default: true
-#module.st_statistics.ignore_before_create = true
-
-## Time window of slow topics statistics
+## the expire time of the record which in topk
 ##
 ## Value: 5 minutes
-#module.st_statistics.time_window = 5M
+#module.slow_subs.expire_interval = 5m
 
-## Maximum of slow topics log, log will clear when enter new time window
+## maximum number of Top-K record
 ##
-## Value: 500
-#module.st_statistics.max_log_num = 500
+## Value: 10
+#module.slow_subs.top_k_num = 10
 
-## Top-K record for slow topics, update from logs
+## enable notification
+## publish topk list to $SYS/brokers/${node}/slow_subs per notice_interval
+## publish is disabled if set to 0s.
 ##
-## Value: 500
-#module.st_statistics.top_k_num = 500
-
-## Topic of notification
-##
-## Defaut: $slow_topics
-#module.st_statistics.notice_topic = $slow_topics
+## Defaut: 0s
+#module.slow_subs.notice_interval = 0s
 
 ## QoS of notification message in notice topic
 ##
 ## Defaut: 0
-#module.st_statistics.notice_qos = 0
+#module.slow_subs.notice_qos = 0
 
 ## Maximum information number in one notification
 ##
-## Default: 500
-#module.st_statistics.notice_batch_size = 500
+## Default: 100
+#module.slow_subs.notice_batch_size = 100
 
 ## CONFIG_SECTION_END=modules ==================================================
 
diff --git a/lib-ce/emqx_modules/src/emqx_mod_st_statistics.erl b/lib-ce/emqx_modules/src/emqx_mod_slow_subs.erl
similarity index 90%
rename from lib-ce/emqx_modules/src/emqx_mod_st_statistics.erl
rename to lib-ce/emqx_modules/src/emqx_mod_slow_subs.erl
index d6796122c..b9117fe8b 100644
--- a/lib-ce/emqx_modules/src/emqx_mod_st_statistics.erl
+++ b/lib-ce/emqx_modules/src/emqx_mod_slow_subs.erl
@@ -14,14 +14,14 @@
 %% limitations under the License.
 %%--------------------------------------------------------------------
 
--module(emqx_mod_st_statistics).
+-module(emqx_mod_slow_subs).
 
 -behaviour(emqx_gen_mod).
 
 -include_lib("include/emqx.hrl").
 -include_lib("include/logger.hrl").
 
--logger_header("[SLOW TOPICS]").
+-logger_header("[SLOW Subs]").
 
 %% emqx_gen_mod callbacks
 -export([ load/1
@@ -29,7 +29,7 @@
         , description/0
         ]).
 
--define(LIB, emqx_st_statistics).
+-define(LIB, emqx_slow_subs).
 
 %%--------------------------------------------------------------------
 %% Load/Unload
@@ -46,4 +46,4 @@ unload(_Env) ->
     ok.
 
 description() ->
-    "EMQ X Slow Topic Statistics Module".
+    "EMQ X Slow Subscribers Statistics Module".
diff --git a/lib-ce/emqx_modules/src/emqx_mod_sup.erl b/lib-ce/emqx_modules/src/emqx_mod_sup.erl
index 109564f65..4c8f54679 100644
--- a/lib-ce/emqx_modules/src/emqx_mod_sup.erl
+++ b/lib-ce/emqx_modules/src/emqx_mod_sup.erl
@@ -69,6 +69,7 @@ stop_child(ChildId) ->
 
 init([]) ->
     ok = emqx_tables:new(emqx_modules, [set, public, {write_concurrency, true}]),
+    emqx_slow_subs:init_topk_tab(),
     {ok, {{one_for_one, 10, 100}, []}}.
 
 %%--------------------------------------------------------------------
diff --git a/lib-ce/emqx_modules/test/emqx_st_statistics_SUITE.erl b/lib-ce/emqx_modules/test/emqx_slow_subs_SUITE.erl
similarity index 53%
rename from lib-ce/emqx_modules/test/emqx_st_statistics_SUITE.erl
rename to lib-ce/emqx_modules/test/emqx_slow_subs_SUITE.erl
index 8df07f8b8..46f144a57 100644
--- a/lib-ce/emqx_modules/test/emqx_st_statistics_SUITE.erl
+++ b/lib-ce/emqx_modules/test/emqx_slow_subs_SUITE.erl
@@ -14,18 +14,18 @@
 %% limitations under the License.
 %%--------------------------------------------------------------------
 
--module(emqx_st_statistics_SUITE).
+-module(emqx_slow_subs_SUITE).
 
 -compile(export_all).
 -compile(nowarn_export_all).
 
 -include_lib("eunit/include/eunit.hrl").
+-include("include/emqx_mqtt.hrl").
 -include_lib("include/emqx.hrl").
 
 %-define(LOGT(Format, Args), ct:pal(Format, Args)).
 
--define(LOG_TAB, emqx_st_statistics_log).
--define(TOPK_TAB, emqx_st_statistics_topk).
+-define(TOPK_TAB, emqx_slow_subs_topk).
 -define(NOW, erlang:system_time(millisecond)).
 
 all() -> emqx_ct:all(?MODULE).
@@ -39,11 +39,11 @@ end_per_suite(Config) ->
     Config.
 
 init_per_testcase(_, Config) ->
-    emqx_mod_st_statistics:load(base_conf()),
+    emqx_mod_slow_subs:load(base_conf()),
     Config.
 
 end_per_testcase(_, _) ->
-    emqx_mod_st_statistics:unload(undefined),
+    emqx_mod_slow_subs:unload([]),
     ok.
 
 %%--------------------------------------------------------------------
@@ -51,62 +51,74 @@ end_per_testcase(_, _) ->
 %%--------------------------------------------------------------------
 t_log_and_pub(_) ->
     %% Sub topic first
-    SubBase = "/test",
-    emqx:subscribe("$slow_topics"),
-    Clients = start_client(SubBase),
+    Subs = [{<<"/test1/+">>, ?QOS_1}, {<<"/test2/+">>, ?QOS_2}],
+    Clients = start_client(Subs),
+    emqx:subscribe("$SYS/brokers/+/slow_subs"),
     timer:sleep(1000),
     Now = ?NOW,
     %% publish
-    ?assert(ets:info(?LOG_TAB, size) =:= 0),
+
     lists:foreach(fun(I) ->
-                          Topic = list_to_binary(io_lib:format("~s~p", [SubBase, I])),
-                          Msg = emqx_message:make(Topic, <<"Hello">>),
-                          emqx:publish(Msg#message{timestamp = Now - 1000})
+                      Topic = list_to_binary(io_lib:format("/test1/~p", [I])),
+                      Msg = emqx_message:make(undefined, ?QOS_1, Topic, <<"Hello">>),
+                      emqx:publish(Msg#message{timestamp = Now - 500})
                   end,
                   lists:seq(1, 10)),
 
-    timer:sleep(2400),
+    lists:foreach(fun(I) ->
+                      Topic = list_to_binary(io_lib:format("/test2/~p", [I])),
+                      Msg = emqx_message:make(undefined, ?QOS_2, Topic, <<"Hello">>),
+                      emqx:publish(Msg#message{timestamp = Now - 500})
+                  end,
+                  lists:seq(1, 10)),
 
-    ?assert(ets:info(?LOG_TAB, size) =:= 0),
-    ?assert(ets:info(?TOPK_TAB, size) =:= 3),
-    try_receive(3),
-    try_receive(2),
+    timer:sleep(1000),
+    Size = ets:info(?TOPK_TAB, size),
+    %% some time record maybe delete due to it expired
+    ?assert(Size =< 6 andalso Size >= 4),
+
+    timer:sleep(1500),
+    Recs = try_receive([]),
+    RecSum = lists:sum(Recs),
+    ?assert(RecSum >= 5),
+    ?assert(lists:all(fun(E) -> E =< 3 end, Recs)),
+
+    timer:sleep(2000),
+    ?assert(ets:info(?TOPK_TAB, size) =:= 0),
     [Client ! stop || Client <- Clients],
     ok.
-
 base_conf() ->
-    [{top_k_num, 3},
-     {threshold_time, 10},
-     {notice_qos, 0},
-     {notice_batch_size, 3},
-     {notice_topic,"$slow_topics"},
-     {time_window, 2000},
-     {max_log_num, 5}].
+    [ {top_k_num, 5}
+    , {expire_interval, timer:seconds(3)}
+    , {notice_interval, 1500}
+    , {notice_qos, 0}
+    , {notice_batch_size, 3}
+    ].
 
-start_client(Base) ->
-    [spawn(fun() ->
-                   Topic = list_to_binary(io_lib:format("~s~p", [Base, I])),
-                   client(Topic)
-           end)
-     || I <- lists:seq(1, 10)].
+start_client(Subs) ->
+    [spawn(fun() -> client(I, Subs) end) || I <- lists:seq(1, 10)].
 
-client(Topic) ->
+client(I, Subs) ->
     {ok, C} = emqtt:start_link([{host,      "localhost"},
-                                {clientid,  Topic},
+                                {clientid,  io_lib:format("slow_subs_~p", [I])},
                                 {username,  <<"plain">>},
                                 {password,  <<"plain">>}]),
     {ok, _} = emqtt:connect(C),
-    {ok, _, _} = emqtt:subscribe(C, Topic),
+
+    Len = erlang:length(Subs),
+    Sub = lists:nth(I rem Len + 1, Subs),
+    _ = emqtt:subscribe(C, Sub),
+
     receive
         stop ->
             ok
     end.
 
-try_receive(L) ->
+try_receive(Acc) ->
     receive
         {deliver, _, #message{payload = Payload}} ->
             #{<<"logs">> := Logs} =  emqx_json:decode(Payload, [return_maps]),
-            ?assertEqual(length(Logs), L)
+            try_receive([length(Logs) | Acc])
     after 500 ->
-            ?assert(false)
+            Acc
     end.
diff --git a/lib-ce/emqx_modules/test/emqx_st_statistics_api_SUITE.erl b/lib-ce/emqx_modules/test/emqx_slow_subs_api_SUITE.erl
similarity index 59%
rename from lib-ce/emqx_modules/test/emqx_st_statistics_api_SUITE.erl
rename to lib-ce/emqx_modules/test/emqx_slow_subs_api_SUITE.erl
index 0fb9e904b..2efc7230a 100644
--- a/lib-ce/emqx_modules/test/emqx_st_statistics_api_SUITE.erl
+++ b/lib-ce/emqx_modules/test/emqx_slow_subs_api_SUITE.erl
@@ -12,9 +12,9 @@
 %% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 %% See the License for the specific language governing permissions and
 %% limitations under the License.
-%%--------------------------------------------------------------------
+%%--------------------------------------------------------------------n
 
--module(emqx_st_statistics_api_SUITE).
+-module(emqx_slow_subs_api_SUITE).
 
 -compile(export_all).
 -compile(nowarn_export_all).
@@ -24,7 +24,7 @@
 -include_lib("emqx/include/emqx.hrl").
 -include_lib("emqx/include/emqx_mqtt.hrl").
 -include_lib("emqx_management/include/emqx_mgmt.hrl").
--include_lib("emqx_plugin_libs/include/emqx_st_statistics.hrl").
+-include_lib("emqx_plugin_libs/include/emqx_slow_subs.hrl").
 
 -define(CONTENT_TYPE, "application/x-www-form-urlencoded").
 
@@ -33,6 +33,7 @@
 -define(API_VERSION, "v4").
 
 -define(BASE_PATH, "api").
+-define(NOW, erlang:system_time(millisecond)).
 
 all() ->
     emqx_ct:all(?MODULE).
@@ -48,78 +49,52 @@ end_per_suite(Config) ->
     Config.
 
 init_per_testcase(_, Config) ->
-    emqx_mod_st_statistics:load(emqx_st_statistics_SUITE:base_conf()),
+    emqx_mod_slow_subs:load(base_conf()),
     Config.
 
 end_per_testcase(_, Config) ->
-    emqx_mod_st_statistics:unload(undefined),
+    emqx_mod_slow_subs:unload([]),
     Config.
 
+base_conf() ->
+    [ {top_k_num, 5}
+    , {expire_interval, timer:seconds(60)}
+    , {notice_interval, 0}
+    , {notice_qos, 0}
+    , {notice_batch_size, 3}
+    ].
+
 t_get_history(_) ->
-    ets:insert(?TOPK_TAB, #top_k{rank = 1,
-                                 topic = <<"test">>,
-                                 average_count = 12,
-                                 average_elapsed = 1500}),
+    Now = ?NOW,
+    Each = fun(I) ->
+               ClientId = erlang:list_to_binary(io_lib:format("test_~p", [I])),
+               ets:insert(?TOPK_TAB, #top_k{index = ?INDEX(I, ClientId),
+                                            type = average,
+                                            last_update_time = Now})
+           end,
 
-    {ok, Data} = request_api(get, api_path(["slow_topic"]), "_page=1&_limit=10",
+    lists:foreach(Each, lists:seq(1, 5)),
+
+    {ok, Data} = request_api(get, api_path(["slow_subscriptions"]), "_page=1&_limit=10",
                              auth_header_()),
+    #{meta := Meta, data := [First | _]} = decode(Data),
 
-    ShouldRet = #{meta => #{page => 1,
-                            limit => 10,
-                            hasnext => false,
-                            count => 1},
-                   data => [#{topic => <<"test">>,
-                              rank => 1,
-                              elapsed => 1500,
-                              count => 12}],
-                   code => 0},
+    RMeta = #{page => 1, limit => 10, count => 5},
+    ?assertEqual(RMeta, Meta),
 
-    Ret = decode(Data),
+    RFirst = #{clientid => <<"test_5">>,
+               latency => 5,
+               type => <<"average">>,
+               last_update_time => Now},
 
-    ?assertEqual(ShouldRet, Ret).
-
-t_rank_range(_) ->
-    Insert = fun(Rank) ->
-                     ets:insert(?TOPK_TAB,
-                                #top_k{rank = Rank,
-                                       topic = <<"test">>,
-                                       average_count = 12,
-                                       average_elapsed = 1500})
-             end,
-    lists:foreach(Insert, lists:seq(1, 15)),
-
-    timer:sleep(100),
-
-    {ok, Data} = request_api(get, api_path(["slow_topic"]), "_page=1&_limit=10",
-                             auth_header_()),
-
-    Meta1 = #{page => 1, limit => 10, hasnext => true, count => 10},
-    Ret1 = decode(Data),
-    ?assertEqual(Meta1, maps:get(meta, Ret1)),
-
-    %% End > Size
-    {ok, Data2} = request_api(get, api_path(["slow_topic"]), "_page=2&_limit=10",
-                              auth_header_()),
-
-    Meta2 = #{page => 2, limit => 10, hasnext => false, count => 5},
-    Ret2 = decode(Data2),
-    ?assertEqual(Meta2, maps:get(meta, Ret2)),
-
-    %% Start > Size
-    {ok, Data3} = request_api(get, api_path(["slow_topic"]), "_page=3&_limit=10",
-                              auth_header_()),
-
-    Meta3 = #{page => 3, limit => 10, hasnext => false, count => 0},
-    Ret3 = decode(Data3),
-    ?assertEqual(Meta3, maps:get(meta, Ret3)).
+    ?assertEqual(RFirst, First).
 
 t_clear(_) ->
-    ets:insert(?TOPK_TAB, #top_k{rank = 1,
-                                 topic = <<"test">>,
-                                 average_count = 12,
-                                 average_elapsed = 1500}),
+    ets:insert(?TOPK_TAB, #top_k{index = ?INDEX(1, <<"test">>),
+                                 type = average,
+                                 last_update_time = ?NOW}),
 
-    {ok, _} = request_api(delete, api_path(["slow_topic"]), [],
+    {ok, _} = request_api(delete, api_path(["slow_subscriptions"]), [],
                           auth_header_()),
 
     ?assertEqual(0, ets:info(?TOPK_TAB, size)).
diff --git a/priv/emqx.schema b/priv/emqx.schema
index 15f0324cd..61a98f824 100644
--- a/priv/emqx.schema
+++ b/priv/emqx.schema
@@ -1007,6 +1007,18 @@ end}.
   {datatype, {duration, s}}
 ]}.
 
+%% @doc the number of smaples for calculate the average latency of delivery
+{mapping, "zone.$name.latency_samples", "emqx.zones", [
+  {default, 10},
+  {datatype, integer}
+]}.
+
+%% @doc Threshold for slow subscription statistics
+{mapping, "zone.$name.latency_stats_threshold", "emqx.zones", [
+  {default, "100ms"},
+  {datatype, {duration, ms}}
+]}.
+
 %% @doc Max Packets that Awaiting PUBREL, 0 means no limit
 {mapping, "zone.$name.max_awaiting_rel", "emqx.zones", [
   {default, 0},
@@ -2218,43 +2230,28 @@ end}.
   {datatype, string}
 ]}.
 
-{mapping, "module.st_statistics.threshold_time", "emqx.modules", [
-  {default, "10s"},
+{mapping, "module.slow_subs.expire_interval", "emqx.modules", [
+  {default, "5m"},
   {datatype, {duration, ms}}
 ]}.
 
-{mapping, "module.st_statistics.ignore_before_create", "emqx.modules", [
-  {default, true},
-  {datatype, {enum, [true, false]}}
-]}.
-
-{mapping, "module.st_statistics.time_window", "emqx.modules", [
-  {default, "5M"},
-  {datatype, {duration, ms}}
-]}.
-
-{mapping, "module.st_statistics.max_log_num", "emqx.modules", [
+{mapping, "module.slow_subs.top_k_num", "emqx.modules", [
   {default, 500},
   {datatype, integer}
 ]}.
 
-{mapping, "module.st_statistics.top_k_num", "emqx.modules", [
-  {default, 500},
-  {datatype, integer}
+{mapping, "module.slow_subs.notice_interval", "emqx.modules", [
+  {default, "0s"},
+  {datatype, {duration, ms}}
 ]}.
 
-{mapping, "module.st_statistics.notice_topic", "emqx.modules", [
-  {default, "$slow_topics"},
-  {datatype, string}
-]}.
-
-{mapping, "module.st_statistics.notice_qos", "emqx.modules", [
+{mapping, "module.slow_subs.notice_qos", "emqx.modules", [
   {default, 0},
   {datatype, integer},
   {validators, ["range:0-1"]}
 ]}.
 
-{mapping, "module.st_statistics.notice_batch_size", "emqx.modules", [
+{mapping, "module.slow_subs.notice_batch_size", "emqx.modules", [
   {default, 500},
   {datatype, integer}
 ]}.
@@ -2283,8 +2280,8 @@ end}.
                 end, TotalRules)
   end,
 
-  SlowTopic = fun() ->
-                   List = cuttlefish_variable:filter_by_prefix("module.st_statistics", Conf),
+  SlowSubs = fun() ->
+                   List = cuttlefish_variable:filter_by_prefix("module.slow_subs", Conf),
                    [{erlang:list_to_atom(Key), Value} || {[_, _, Key], Value} <- List]
               end,
 
@@ -2295,7 +2292,7 @@ end}.
     [{emqx_mod_topic_metrics, []}],
     [{emqx_mod_delayed, []}],
     [{emqx_mod_trace, []}],
-    [{emqx_mod_st_statistics, SlowTopic()}],
+    [{emqx_mod_slow_subs, SlowSubs()}],
     [{emqx_mod_acl_internal, [{acl_file, cuttlefish:conf_get("acl_file", Conf1)}]}]
   ])
 end}.
diff --git a/rebar.config.erl b/rebar.config.erl
index 1000a2c92..6c14b6ae8 100644
--- a/rebar.config.erl
+++ b/rebar.config.erl
@@ -106,7 +106,7 @@ test_plugins() ->
 
 test_deps() ->
     [ {bbmustache, "1.10.0"}
-    , {emqx_ct_helpers, {git, "https://github.com/emqx/emqx-ct-helpers", {tag, "1.3.9"}}}
+    , {emqx_ct_helpers, {git, "https://github.com/emqx/emqx-ct-helpers", {tag, "1.3.11"}}}
     , meck
     ].
 
diff --git a/src/emqx_session.erl b/src/emqx_session.erl
index 6122982ae..3e1e48a13 100644
--- a/src/emqx_session.erl
+++ b/src/emqx_session.erl
@@ -95,6 +95,8 @@
 -import(emqx_zone, [get_env/3]).
 
 -record(session, {
+          %% Client's id
+          clientid :: emqx_types:clientid(),
           %% Client’s Subscriptions.
           subscriptions :: map(),
           %% Max subscriptions allowed
@@ -121,8 +123,15 @@
           %% Awaiting PUBREL Timeout (Unit: millsecond)
           await_rel_timeout :: timeout(),
           %% Created at
-          created_at :: pos_integer()
-         }).
+          created_at :: pos_integer(),
+          %% Message deliver latency stats
+          latency_stats :: emqx_message_latency_stats:stats()
+          }).
+
+%% in the previous code, we will replace the message record with the pubrel atom
+%% in the pubrec function, this will lose the creation time of the message,
+%% but now we need this time to calculate latency, so now pubrel atom is changed to this record
+-record(pubrel_await, {timestamp :: non_neg_integer()}).
 
 -type(session() :: #session{}).
 
@@ -148,19 +157,26 @@
                      mqueue_dropped,
                      next_pkt_id,
                      awaiting_rel_cnt,
-                     awaiting_rel_max
+                     awaiting_rel_max,
+                     latency_stats
                     ]).
 
 -define(DEFAULT_BATCH_N, 1000).
 
+-ifdef(TEST).
+-define(GET_CLIENT_ID(C), maps:get(clientid, C, <<>>)).
+-else.
+-define(GET_CLIENT_ID(C), maps:get(clientid, C)).
+-endif.
 
 %%--------------------------------------------------------------------
 %% Init a Session
 %%--------------------------------------------------------------------
 
 -spec(init(emqx_types:clientinfo(), emqx_types:conninfo()) -> session()).
-init(#{zone := Zone}, #{receive_maximum := MaxInflight}) ->
-    #session{max_subscriptions = get_env(Zone, max_subscriptions, 0),
+init(#{zone := Zone} = CInfo, #{receive_maximum := MaxInflight}) ->
+    #session{clientid          = ?GET_CLIENT_ID(CInfo),
+             max_subscriptions = get_env(Zone, max_subscriptions, 0),
              subscriptions     = #{},
              upgrade_qos       = get_env(Zone, upgrade_qos, false),
              inflight          = emqx_inflight:new(MaxInflight),
@@ -170,7 +186,8 @@ init(#{zone := Zone}, #{receive_maximum := MaxInflight}) ->
              awaiting_rel      = #{},
              max_awaiting_rel  = get_env(Zone, max_awaiting_rel, 100),
              await_rel_timeout = timer:seconds(get_env(Zone, await_rel_timeout, 300)),
-             created_at        = erlang:system_time(millisecond)
+             created_at        = erlang:system_time(millisecond),
+             latency_stats     = emqx_message_latency_stats:new(Zone)
             }.
 
 %% @private init mq
@@ -227,7 +244,9 @@ info(awaiting_rel_max, #session{max_awaiting_rel = Max}) ->
 info(await_rel_timeout, #session{await_rel_timeout = Timeout}) ->
     Timeout div 1000;
 info(created_at, #session{created_at = CreatedAt}) ->
-    CreatedAt.
+    CreatedAt;
+info(latency_stats, #session{latency_stats = Stats}) ->
+    emqx_message_latency_stats:latency(Stats).
 
 %% @doc Get stats of the session.
 -spec(stats(session()) -> emqx_types:stats()).
@@ -317,13 +336,12 @@ is_awaiting_full(#session{awaiting_rel = AwaitingRel,
       -> {ok, emqx_types:message(), session()}
        | {ok, emqx_types:message(), replies(), session()}
        | {error, emqx_types:reason_code()}).
-puback(PacketId, Session = #session{inflight = Inflight, created_at = CreatedAt}) ->
+puback(PacketId, Session = #session{inflight = Inflight}) ->
     case emqx_inflight:lookup(PacketId, Inflight) of
         {value, {Msg, _Ts}} when is_record(Msg, message) ->
-            emqx:run_hook('message.publish_done',
-                          [Msg, #{session_rebirth_time => CreatedAt}]),
             Inflight1 = emqx_inflight:delete(PacketId, Inflight),
-            return_with(Msg, dequeue(Session#session{inflight = Inflight1}));
+            Session2 = update_latency(Msg, Session),
+            return_with(Msg, dequeue(Session2#session{inflight = Inflight1}));
         {value, {_Pubrel, _Ts}} ->
             {error, ?RC_PACKET_IDENTIFIER_IN_USE};
         none ->
@@ -343,15 +361,13 @@ return_with(Msg, {ok, Publishes, Session}) ->
 -spec(pubrec(emqx_types:packet_id(), session())
       -> {ok, emqx_types:message(), session()}
        | {error, emqx_types:reason_code()}).
-pubrec(PacketId, Session = #session{inflight = Inflight, created_at = CreatedAt}) ->
+pubrec(PacketId, Session = #session{inflight = Inflight}) ->
     case emqx_inflight:lookup(PacketId, Inflight) of
         {value, {Msg, _Ts}} when is_record(Msg, message) ->
-            %% execute hook here, because message record will be replaced by pubrel
-            emqx:run_hook('message.publish_done',
-                          [Msg, #{session_rebirth_time => CreatedAt}]),
-            Inflight1 = emqx_inflight:update(PacketId, with_ts(pubrel), Inflight),
+            Update = with_ts(#pubrel_await{timestamp = Msg#message.timestamp}),
+            Inflight1 = emqx_inflight:update(PacketId, Update, Inflight),
             {ok, Msg, Session#session{inflight = Inflight1}};
-        {value, {pubrel, _Ts}} ->
+        {value, {_PUBREL, _Ts}} ->
             {error, ?RC_PACKET_IDENTIFIER_IN_USE};
         none ->
             {error, ?RC_PACKET_IDENTIFIER_NOT_FOUND}
@@ -380,9 +396,10 @@ pubrel(PacketId, Session = #session{awaiting_rel = AwaitingRel}) ->
        | {error, emqx_types:reason_code()}).
 pubcomp(PacketId, Session = #session{inflight = Inflight}) ->
     case emqx_inflight:lookup(PacketId, Inflight) of
-        {value, {pubrel, _Ts}} ->
+        {value, {Pubrel, _Ts}} when is_record(Pubrel, pubrel_await) ->
+            Session2 = update_latency(Pubrel, Session),
             Inflight1 = emqx_inflight:delete(PacketId, Inflight),
-            dequeue(Session#session{inflight = Inflight1});
+            dequeue(Session2#session{inflight = Inflight1});
         {value, _Other} ->
             {error, ?RC_PACKET_IDENTIFIER_IN_USE};
         none ->
@@ -550,11 +567,16 @@ await(PacketId, Msg, Session = #session{inflight = Inflight}) ->
 %%--------------------------------------------------------------------
 
 -spec(retry(session()) -> {ok, session()} | {ok, replies(), timeout(), session()}).
-retry(Session = #session{inflight = Inflight}) ->
+retry(Session = #session{inflight = Inflight, retry_interval = RetryInterval}) ->
     case emqx_inflight:is_empty(Inflight) of
         true  -> {ok, Session};
-        false -> retry_delivery(emqx_inflight:to_list(sort_fun(), Inflight),
-                                [], erlang:system_time(millisecond), Session)
+        false ->
+            Now = erlang:system_time(millisecond),
+            Session2 = check_expire_latency(Now, RetryInterval, Session),
+            retry_delivery(emqx_inflight:to_list(sort_fun(), Inflight),
+                           [],
+                           Now,
+                           Session2)
     end.
 
 retry_delivery([], Acc, _Now, Session = #session{retry_interval = Interval}) ->
@@ -581,8 +603,8 @@ retry_delivery(PacketId, Msg, Now, Acc, Inflight) when is_record(Msg, message) -
             {[{PacketId, Msg1} | Acc], Inflight1}
     end;
 
-retry_delivery(PacketId, pubrel, Now, Acc, Inflight) ->
-    Inflight1 = emqx_inflight:update(PacketId, {pubrel, Now}, Inflight),
+retry_delivery(PacketId, Pubrel, Now, Acc, Inflight) ->
+    Inflight1 = emqx_inflight:update(PacketId, {Pubrel, Now}, Inflight),
     {[{pubrel, PacketId} | Acc], Inflight1}.
 
 %%--------------------------------------------------------------------
@@ -626,10 +648,10 @@ resume(ClientInfo = #{clientid := ClientId}, Session = #session{subscriptions =
 
 -spec(replay(session()) -> {ok, replies(), session()}).
 replay(Session = #session{inflight = Inflight}) ->
-    Pubs = lists:map(fun({PacketId, {pubrel, _Ts}}) ->
-                             {pubrel, PacketId};
+    Pubs = lists:map(fun({PacketId, {Pubrel,  _Ts}}) when is_record(Pubrel, pubrel_await) ->
+                         {pubrel, PacketId};
                         ({PacketId, {Msg, _Ts}}) ->
-                             {PacketId, emqx_message:set_flag(dup, true, Msg)}
+                         {PacketId, emqx_message:set_flag(dup, true, Msg)}
                      end, emqx_inflight:to_list(Inflight)),
     case dequeue(Session) of
         {ok, NSession} -> {ok, Pubs, NSession};
@@ -677,6 +699,35 @@ next_pkt_id(Session = #session{next_pkt_id = ?MAX_PACKET_ID}) ->
 next_pkt_id(Session = #session{next_pkt_id = Id}) ->
     Session#session{next_pkt_id = Id + 1}.
 
+%%--------------------------------------------------------------------
+%% Message Latency Stats
+%%--------------------------------------------------------------------
+update_latency(Msg,
+               #session{clientid = ClientId,
+                        latency_stats = Stats,
+                        created_at = CreateAt} = S) ->
+    case get_birth_timestamp(Msg, CreateAt) of
+        0 -> S;
+        Ts ->
+            Latency = erlang:system_time(millisecond) - Ts,
+            Stats2 = emqx_message_latency_stats:update(ClientId, Latency, Stats),
+            S#session{latency_stats = Stats2}
+    end.
+
+check_expire_latency(Now, Interval,
+                     #session{clientid = ClientId, latency_stats = Stats} = S) ->
+    Stats2 = emqx_message_latency_stats:check_expire(ClientId, Now, Interval, Stats),
+    S#session{latency_stats = Stats2}.
+
+get_birth_timestamp(#message{timestamp = Ts}, CreateAt) when CreateAt =< Ts ->
+    Ts;
+
+get_birth_timestamp(#pubrel_await{timestamp = Ts}, CreateAt) when CreateAt =< Ts ->
+    Ts;
+
+get_birth_timestamp(_, _) ->
+    0.
+
 %%--------------------------------------------------------------------
 %% Helper functions
 %%--------------------------------------------------------------------
diff --git a/src/emqx_slow_subs/emqx_message_latency_stats.erl b/src/emqx_slow_subs/emqx_message_latency_stats.erl
new file mode 100644
index 000000000..f9661ab91
--- /dev/null
+++ b/src/emqx_slow_subs/emqx_message_latency_stats.erl
@@ -0,0 +1,105 @@
+%%--------------------------------------------------------------------
+%% Copyright (c) 2021 EMQ Technologies Co., Ltd. All Rights Reserved.
+%%
+%% Licensed under the Apache License, Version 2.0 (the "License");
+%% you may not use this file except in compliance with the License.
+%% You may obtain a copy of the License at
+%%
+%%     http://www.apache.org/licenses/LICENSE-2.0
+%%
+%% Unless required by applicable law or agreed to in writing, software
+%% distributed under the License is distributed on an "AS IS" BASIS,
+%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+%% See the License for the specific language governing permissions and
+%% limitations under the License.
+%%--------------------------------------------------------------------
+
+-module(emqx_message_latency_stats).
+
+%% API
+-export([ new/1, new/2, update/3
+        , check_expire/4, latency/1]).
+
+-define(NOW, erlang:system_time(millisecond)).
+-define(MINIMUM_INSERT_INTERVAL, 1000).
+-define(MINIMUM_THRESHOLD, 100).
+
+-opaque stats() :: #{ threshold := number()
+                    , ema := emqx_moving_average:ema()
+                    , last_update_time := timestamp()
+                    , last_access_time := timestamp()  %% timestamp of last access top-k
+                    , last_insert_value := non_neg_integer()
+                    }.
+
+-type timestamp() :: non_neg_integer().
+-type timespan() :: number().
+
+-type latency_type() :: average
+                      | expire.
+
+-import(emqx_zone, [get_env/3]).
+
+-export_type([stats/0, latency_type/0]).
+
+%%--------------------------------------------------------------------
+%% API
+%%--------------------------------------------------------------------
+-spec new(emqx_types:zone()) -> stats().
+new(Zone) ->
+    Samples = get_env(Zone, latency_samples, 1),
+    Threshold = get_env(Zone, latency_stats_threshold, ?MINIMUM_THRESHOLD),
+    new(Samples, Threshold).
+
+-spec new(non_neg_integer(), number()) -> stats().
+new(SamplesT, ThresholdT) ->
+    Samples = erlang:max(1, SamplesT),
+    Threshold = erlang:max(?MINIMUM_THRESHOLD, ThresholdT),
+    #{ ema => emqx_moving_average:new(exponential, #{period => Samples})
+     , threshold => Threshold
+     , last_update_time => 0
+     , last_access_time => 0
+     , last_insert_value => 0
+     }.
+
+-spec update(emqx_types:clientid(), number(), stats()) -> stats().
+update(ClientId, Val, #{ema := EMA} = Stats) ->
+    Now = ?NOW,
+    #{average := Latency} = EMA2 = emqx_moving_average:update(Val, EMA),
+    Stats2 = call_hook(ClientId, Now, average, Latency, Stats),
+    Stats2#{ ema := EMA2
+           , last_update_time := ?NOW}.
+
+-spec check_expire(emqx_types:clientid(), timestamp(), timespan(), stats()) -> stats().
+check_expire(_, Now, Interval, #{last_update_time := LUT} = S)
+  when LUT >= Now - Interval ->
+    S;
+
+check_expire(ClientId, Now, _Interval, #{last_update_time := LUT} = S) ->
+    Latency = Now - LUT,
+    call_hook(ClientId, Now, expire, Latency, S).
+
+-spec latency(stats()) -> number().
+latency(#{ema := #{average := Average}}) ->
+    Average.
+
+%%--------------------------------------------------------------------
+%%  Internal functions
+%%--------------------------------------------------------------------
+-spec call_hook(emqx_types:clientid(), timestamp(), latency_type(), timespan(), stats()) -> stats().
+call_hook(_, Now, _, _, #{last_access_time := LIT} = S)
+  when LIT >= Now - ?MINIMUM_INSERT_INTERVAL ->
+    S;
+
+call_hook(_, _, _, Latency, #{threshold := Threshold} = S)
+  when Latency =< Threshold ->
+    S;
+
+call_hook(ClientId, Now, Type, Latency, #{last_insert_value := LIV} = Stats) ->
+    Arg = #{clientid => ClientId,
+            latency => erlang:floor(Latency),
+            type => Type,
+            last_insert_value => LIV,
+            update_time => Now},
+    emqx:run_hook('message.slow_subs_stats', [Arg]),
+    Stats#{last_insert_value := Latency,
+           last_access_time := Now}.
diff --git a/src/emqx_slow_subs/emqx_moving_average.erl b/src/emqx_slow_subs/emqx_moving_average.erl
new file mode 100644
index 000000000..64c73f987
--- /dev/null
+++ b/src/emqx_slow_subs/emqx_moving_average.erl
@@ -0,0 +1,90 @@
+%%--------------------------------------------------------------------
+%% Copyright (c) 2021 EMQ Technologies Co., Ltd. All Rights Reserved.
+%%
+%% Licensed under the Apache License, Version 2.0 (the "License");
+%% you may not use this file except in compliance with the License.
+%% You may obtain a copy of the License at
+%%
+%%     http://www.apache.org/licenses/LICENSE-2.0
+%%
+%% Unless required by applicable law or agreed to in writing, software
+%% distributed under the License is distributed on an "AS IS" BASIS,
+%% WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+%% See the License for the specific language governing permissions and
+%% limitations under the License.
+%%--------------------------------------------------------------------
+
+%% @see https://en.wikipedia.org/wiki/Moving_average
+
+-module(emqx_moving_average).
+
+%% API
+-export([new/0, new/1, new/2, update/2]).
+
+-type type() :: cumulative
+              | exponential.
+
+-type ema() :: #{ type := exponential
+                , average := 0 | float()
+                , coefficient := float()
+                }.
+
+-type cma() :: #{ type := cumulative
+                , average := 0 | float()
+                , count := non_neg_integer()
+                }.
+
+-type moving_average() :: ema()
+                        | cma().
+
+-define(DEF_EMA_ARG, #{period => 10}).
+-define(DEF_AVG_TYPE, exponential).
+
+-export_type([type/0, moving_average/0, ema/0, cma/0]).
+
+%%--------------------------------------------------------------------
+%% API
+%%--------------------------------------------------------------------
+-spec new() -> moving_average().
+new() ->
+    new(?DEF_AVG_TYPE, #{}).
+
+-spec new(type()) -> moving_average().
+new(Type) ->
+    new(Type, #{}).
+
+-spec new(type(), Args :: map()) -> moving_average().
+new(cumulative, _) ->
+    #{ type => cumulative
+     , average => 0
+     , count => 0
+     };
+
+new(exponential, Arg) ->
+    #{period := Period} = maps:merge(?DEF_EMA_ARG, Arg),
+    #{ type => exponential
+     , average => 0
+       %% coefficient = 2/(N+1) is a common convention, see the wiki link for details
+     , coefficient => 2 / (Period + 1)
+     }.
+
+-spec update(number(), moving_average()) -> moving_average().
+
+update(Val, #{average := 0} = Avg) ->
+    Avg#{average := Val};
+
+update(Val, #{ type := cumulative
+             , average := Average
+             , count := Count} = CMA) ->
+    NewCount = Count + 1,
+    CMA#{average := (Count * Average + Val) / NewCount,
+         count := NewCount};
+
+update(Val, #{ type := exponential
+             , average := Average
+             , coefficient := Coefficient} = EMA) ->
+    EMA#{average := Coefficient * Val + (1 - Coefficient) * Average}.
+
+%%--------------------------------------------------------------------
+%%  Internal functions
+%%--------------------------------------------------------------------
diff --git a/test/emqx_session_SUITE.erl b/test/emqx_session_SUITE.erl
index cb7c10cae..f838d052a 100644
--- a/test/emqx_session_SUITE.erl
+++ b/test/emqx_session_SUITE.erl
@@ -180,7 +180,8 @@ t_puback_with_dequeue(_) ->
     ?assertEqual(<<"t2">>, emqx_message:topic(Msg3)).
 
 t_puback_error_packet_id_in_use(_) ->
-    Inflight = emqx_inflight:insert(1, {pubrel, ts(millisecond)}, emqx_inflight:new()),
+    Now = ts(millisecond),
+    Inflight = emqx_inflight:insert(1, {{pubrel_await, Now}, Now}, emqx_inflight:new()),
     {error, ?RC_PACKET_IDENTIFIER_IN_USE} =
         emqx_session:puback(1, session(#{inflight => Inflight})).
 
@@ -192,10 +193,11 @@ t_pubrec(_) ->
     Inflight = emqx_inflight:insert(2, {Msg, ts(millisecond)}, emqx_inflight:new()),
     Session = session(#{inflight => Inflight}),
     {ok, Msg, Session1} = emqx_session:pubrec(2, Session),
-    ?assertMatch([{pubrel, _}], emqx_inflight:values(emqx_session:info(inflight, Session1))).
+    ?assertMatch([{{pubrel_await, _}, _}], emqx_inflight:values(emqx_session:info(inflight, Session1))).
 
 t_pubrec_packet_id_in_use_error(_) ->
-    Inflight = emqx_inflight:insert(1, {pubrel, ts(millisecond)}, emqx_inflight:new()),
+    Now = ts(millisecond),
+    Inflight = emqx_inflight:insert(1, {{pubrel_await, Now}, Now}, emqx_inflight:new()),
     {error, ?RC_PACKET_IDENTIFIER_IN_USE} =
         emqx_session:pubrec(1, session(#{inflight => Inflight})).
 
@@ -211,7 +213,8 @@ t_pubrel_error_packetid_not_found(_) ->
     {error, ?RC_PACKET_IDENTIFIER_NOT_FOUND} = emqx_session:pubrel(1, session()).
 
 t_pubcomp(_) ->
-    Inflight = emqx_inflight:insert(1, {pubrel, ts(millisecond)}, emqx_inflight:new()),
+    Now = ts(millisecond),
+    Inflight = emqx_inflight:insert(1, {{pubrel_await, Now}, Now}, emqx_inflight:new()),
     Session = session(#{inflight => Inflight}),
     {ok, Session1} = emqx_session:pubcomp(1, Session),
     ?assertEqual(0, emqx_session:info(inflight_cnt, Session1)).
@@ -260,7 +263,7 @@ t_deliver_qos0(_) ->
 t_deliver_qos1(_) ->
     ok = meck:expect(emqx_broker, subscribe, fun(_, _, _) -> ok end),
     {ok, Session} = emqx_session:subscribe(
-                       clientinfo(), <<"t1">>, subopts(#{qos => ?QOS_1}), session()),
+                      clientinfo(), <<"t1">>, subopts(#{qos => ?QOS_1}), session()),
     Delivers = [delivery(?QOS_1, T) || T <- [<<"t1">>, <<"t2">>]],
     {ok, [{1, Msg1}, {2, Msg2}], Session1} = emqx_session:deliver(Delivers, Session),
     ?assertEqual(2, emqx_session:info(inflight_cnt, Session1)),
@@ -373,7 +376,7 @@ mqueue(Opts) ->
 session() -> session(#{}).
 session(InitFields) when is_map(InitFields) ->
     maps:fold(fun(Field, Value, Session) ->
-                      emqx_session:set_field(Field, Value, Session)
+                  emqx_session:set_field(Field, Value, Session)
               end,
               emqx_session:init(#{zone => channel}, #{receive_maximum => 0}),
               InitFields).
@@ -396,4 +399,3 @@ ts(second) ->
     erlang:system_time(second);
 ts(millisecond) ->
     erlang:system_time(millisecond).
-