diff --git a/src/emqttd.erl b/src/emqttd.erl
index f012fdc23..65739952f 100644
--- a/src/emqttd.erl
+++ b/src/emqttd.erl
@@ -136,17 +136,17 @@ subscribed(Topic, Subscriber) ->
 %%--------------------------------------------------------------------
 
 -spec(hook(atom(), function() | {emqttd_hooks:hooktag(), function()}, list(any()))
-      -> ok | {error, any()}).
+      -> ok | {error, term()}).
 hook(Hook, TagFunction, InitArgs) ->
     emqttd_hooks:add(Hook, TagFunction, InitArgs).
 
 -spec(hook(atom(), function() | {emqttd_hooks:hooktag(), function()}, list(any()), integer())
-      -> ok | {error, any()}).
+      -> ok | {error, term()}).
 hook(Hook, TagFunction, InitArgs, Priority) ->
     emqttd_hooks:add(Hook, TagFunction, InitArgs, Priority).
 
 -spec(unhook(atom(), function() | {emqttd_hooks:hooktag(), function()})
-      -> ok | {error, any()}).
+      -> ok | {error, term()}).
 unhook(Hook, TagFunction) ->
     emqttd_hooks:delete(Hook, TagFunction).
 
diff --git a/src/emqttd_access_control.erl b/src/emqttd_access_control.erl
index 6cfcc03cf..601fd263f 100644
--- a/src/emqttd_access_control.erl
+++ b/src/emqttd_access_control.erl
@@ -43,12 +43,12 @@
 %%--------------------------------------------------------------------
 
 %% @doc Start access control server.
--spec(start_link() -> {ok, pid()} | ignore | {error, any()}).
+-spec(start_link() -> {ok, pid()} | ignore | {error, term()}).
 start_link() ->
     gen_server:start_link({local, ?SERVER}, ?MODULE, [], []).
 
 %% @doc Authenticate MQTT Client.
--spec(auth(Client :: mqtt_client(), Password :: password()) -> ok | {ok, boolean()} | {error, any()}).
+-spec(auth(Client :: mqtt_client(), Password :: password()) -> ok | {ok, boolean()} | {error, term()}).
 auth(Client, Password) when is_record(Client, mqtt_client) ->
     auth(Client, Password, lookup_mods(auth)).
 auth(_Client, _Password, []) ->
@@ -88,16 +88,16 @@ reload_acl() ->
     [Mod:reload_acl(State) || {Mod, State, _Seq} <- lookup_mods(acl)].
 
 %% @doc Register Authentication or ACL module.
--spec(register_mod(auth | acl, atom(), list()) -> ok | {error, any()}).
+-spec(register_mod(auth | acl, atom(), list()) -> ok | {error, term()}).
 register_mod(Type, Mod, Opts) when Type =:= auth; Type =:= acl->
     register_mod(Type, Mod, Opts, 0).
 
--spec(register_mod(auth | acl, atom(), list(), non_neg_integer()) -> ok | {error, any()}).
+-spec(register_mod(auth | acl, atom(), list(), non_neg_integer()) -> ok | {error, term()}).
 register_mod(Type, Mod, Opts, Seq) when Type =:= auth; Type =:= acl->
     gen_server:call(?SERVER, {register_mod, Type, Mod, Opts, Seq}).
 
 %% @doc Unregister authentication or ACL module
--spec(unregister_mod(Type :: auth | acl, Mod :: atom()) -> ok | {error, any()}).
+-spec(unregister_mod(Type :: auth | acl, Mod :: atom()) -> ok | {error, not_found | term()}).
 unregister_mod(Type, Mod) when Type =:= auth; Type =:= acl ->
     gen_server:call(?SERVER, {unregister_mod, Type, Mod}).
 
diff --git a/src/emqttd_acl_mod.erl b/src/emqttd_acl_mod.erl
index 12e949afe..4ed07b369 100644
--- a/src/emqttd_acl_mod.erl
+++ b/src/emqttd_acl_mod.erl
@@ -32,7 +32,7 @@
                      PubSub :: pubsub(),
                      Topic  :: binary()}, State :: any()) -> allow | deny | ignore).
 
--callback(reload_acl(State :: any()) -> ok | {error, any()}).
+-callback(reload_acl(State :: any()) -> ok | {error, term()}).
 
 -callback(description() -> string()).
 
diff --git a/src/emqttd_bridge_sup.erl b/src/emqttd_bridge_sup.erl
index d28b2274c..75138332f 100644
--- a/src/emqttd_bridge_sup.erl
+++ b/src/emqttd_bridge_sup.erl
@@ -23,7 +23,7 @@
 %%--------------------------------------------------------------------
 
 %% @doc Start bridge pool supervisor
--spec(start_link(atom(), binary(), [emqttd_bridge:option()]) -> {ok, pid()} | {error, any()}).
+-spec(start_link(atom(), binary(), [emqttd_bridge:option()]) -> {ok, pid()} | {error, term()}).
 start_link(Node, Topic, Options) ->
     MFA = {emqttd_bridge, start_link, [Node, Topic, Options]},
     emqttd_pool_sup:start_link({bridge, Node, Topic}, random, MFA).
diff --git a/src/emqttd_bridge_sup_sup.erl b/src/emqttd_bridge_sup_sup.erl
index 1d47bd003..11679aba8 100644
--- a/src/emqttd_bridge_sup_sup.erl
+++ b/src/emqttd_bridge_sup_sup.erl
@@ -40,11 +40,11 @@ bridges() ->
                              <- supervisor:which_children(?MODULE)].
 
 %% @doc Start a bridge
--spec(start_bridge(atom(), binary()) -> {ok, pid()} | {error, any()}).
+-spec(start_bridge(atom(), binary()) -> {ok, pid()} | {error, term()}).
 start_bridge(Node, Topic) when is_atom(Node) andalso is_binary(Topic) ->
     start_bridge(Node, Topic, []).
 
--spec(start_bridge(atom(), binary(), [emqttd_bridge:option()]) -> {ok, pid()} | {error, any()}).
+-spec(start_bridge(atom(), binary(), [emqttd_bridge:option()]) -> {ok, pid()} | {error, term()}).
 start_bridge(Node, _Topic, _Options) when Node =:= node() ->
     {error, bridge_to_self};
 start_bridge(Node, Topic, Options) when is_atom(Node) andalso is_binary(Topic) ->
diff --git a/src/emqttd_broker.erl b/src/emqttd_broker.erl
index 9f939a45e..0161720f2 100644
--- a/src/emqttd_broker.erl
+++ b/src/emqttd_broker.erl
@@ -61,7 +61,7 @@
 %%--------------------------------------------------------------------
 
 %% @doc Start emqttd broker
--spec(start_link() -> {ok, pid()} | ignore | {error, any()}).
+-spec(start_link() -> {ok, pid()} | ignore | {error, term()}).
 start_link() ->
     gen_server:start_link({local, ?SERVER}, ?MODULE, [], []).
 
diff --git a/src/emqttd_cli.erl b/src/emqttd_cli.erl
index a4029d46f..c8836230f 100644
--- a/src/emqttd_cli.erl
+++ b/src/emqttd_cli.erl
@@ -248,7 +248,6 @@ subscriptions(["show", ClientId]) ->
         Records -> [print(subscription, Subscription) || Subscription <- Records]
     end;
 
-
 subscriptions(["add", ClientId, Topic, QoS]) ->
    Add = fun(IntQos) ->
            case emqttd:subscribe(bin(Topic), bin(ClientId), [{qos, IntQos}]) of
@@ -260,22 +259,14 @@ subscriptions(["add", ClientId, Topic, QoS]) ->
          end,
    if_valid_qos(QoS, Add);
 
-
-
-subscriptions(["del", ClientId]) ->
-   Ok = emqttd:subscriber_down(bin(ClientId)),
-   ?PRINT("~p~n", [Ok]);
-
 subscriptions(["del", ClientId, Topic]) ->
    Ok = emqttd:unsubscribe(bin(Topic), bin(ClientId)),
    ?PRINT("~p~n", [Ok]);
 
-
 subscriptions(_) ->
     ?USAGE([{"subscriptions list",                         "List all subscriptions"},
             {"subscriptions show <ClientId>",              "Show subscriptions of a client"},
             {"subscriptions add <ClientId> <Topic> <QoS>", "Add a static subscription manually"},
-            {"subscriptions del <ClientId>",               "Delete static subscriptions manually"},
             {"subscriptions del <ClientId> <Topic>",       "Delete a static subscription manually"}]).
 
 % if_could_print(Tab, Fun) ->
diff --git a/src/emqttd_cm.erl b/src/emqttd_cm.erl
index 2e57ebe5b..c5f0ff145 100644
--- a/src/emqttd_cm.erl
+++ b/src/emqttd_cm.erl
@@ -47,7 +47,7 @@
 %%--------------------------------------------------------------------
 
 %% @doc Start Client Manager
--spec(start_link(atom(), pos_integer(), fun()) -> {ok, pid()} | ignore | {error, any()}).
+-spec(start_link(atom(), pos_integer(), fun()) -> {ok, pid()} | ignore | {error, term()}).
 start_link(Pool, Id, StatsFun) ->
     gen_server2:start_link(?MODULE, [Pool, Id, StatsFun], []).
 
diff --git a/src/emqttd_gen_mod.erl b/src/emqttd_gen_mod.erl
index 824b65f4c..f8d690024 100644
--- a/src/emqttd_gen_mod.erl
+++ b/src/emqttd_gen_mod.erl
@@ -24,9 +24,9 @@
 
 -ifdef(use_specs).
 
--callback(load(Opts :: any()) -> ok | {error, any()}).
+-callback(load(Opts :: any()) -> ok | {error, term()}).
 
--callback(unload(State :: any()) -> any()).
+-callback(unload(State :: term()) -> term()).
 
 -else.
 
diff --git a/src/emqttd_keepalive.erl b/src/emqttd_keepalive.erl
index 5d26ea8a6..a0458038a 100644
--- a/src/emqttd_keepalive.erl
+++ b/src/emqttd_keepalive.erl
@@ -29,7 +29,7 @@
 -export_type([keepalive/0]).
 
 %% @doc Start a keepalive
--spec(start(fun(), integer(), any()) -> {ok, keepalive()} | {error, any()}).
+-spec(start(fun(), integer(), any()) -> {ok, keepalive()} | {error, term()}).
 start(_, 0, _) ->
     {ok, #keepalive{}};
 start(StatFun, TimeoutSec, TimeoutMsg) ->
@@ -43,7 +43,7 @@ start(StatFun, TimeoutSec, TimeoutMsg) ->
     end.
 
 %% @doc Check keepalive, called when timeout.
--spec(check(keepalive()) -> {ok, keepalive()} | {error, any()}).
+-spec(check(keepalive()) -> {ok, keepalive()} | {error, term()}).
 check(KeepAlive = #keepalive{statfun = StatFun, statval = LastVal, repeat = Repeat}) ->
     case StatFun() of
         {ok, NewVal} ->
diff --git a/src/emqttd_metrics.erl b/src/emqttd_metrics.erl
index c21d274a0..17e6e96d4 100644
--- a/src/emqttd_metrics.erl
+++ b/src/emqttd_metrics.erl
@@ -96,8 +96,8 @@
 %% API
 %%--------------------------------------------------------------------
 
-%% @doc Start metrics server
--spec(start_link() -> {ok, pid()} | ignore | {error, any()}).
+%% @doc Start the metrics server
+-spec(start_link() -> {ok, pid()} | ignore | {error, term()}).
 start_link() ->
     gen_server:start_link({local, ?SERVER}, ?MODULE, [], []).
 
diff --git a/src/emqttd_mod_sup.erl b/src/emqttd_mod_sup.erl
index 586d5af1a..749b84a42 100644
--- a/src/emqttd_mod_sup.erl
+++ b/src/emqttd_mod_sup.erl
@@ -42,7 +42,7 @@ start_child(ChildSpec) when is_tuple(ChildSpec) ->
 start_child(Mod, Type) when is_atom(Mod) andalso is_atom(Type) ->
     supervisor:start_child(?MODULE, ?CHILD(Mod, Type)).
 
--spec(stop_child(any()) -> ok | {error, any()}).
+-spec(stop_child(any()) -> ok | {error, term()}).
 stop_child(ChildId) ->
     case supervisor:terminate_child(?MODULE, ChildId) of
         ok    -> supervisor:delete_child(?MODULE, ChildId);
diff --git a/src/emqttd_parser.erl b/src/emqttd_parser.erl
index 27be18a25..4699b3f77 100644
--- a/src/emqttd_parser.erl
+++ b/src/emqttd_parser.erl
@@ -39,7 +39,7 @@ initial_state(MaxSize) ->
 
 %% @doc Parse MQTT Packet
 -spec(parse(binary(), {none, pos_integer()} | fun())
-            -> {ok, mqtt_packet()} | {error, any()} | {more, fun()}).
+            -> {ok, mqtt_packet()} | {error, term()} | {more, fun()}).
 parse(<<>>, {none, MaxLen}) ->
     {more, fun(Bin) -> parse(Bin, {none, MaxLen}) end};
 parse(<<Type:4, Dup:1, QoS:2, Retain:1, Rest/binary>>, {none, Limit}) ->
diff --git a/src/emqttd_plugins.erl b/src/emqttd_plugins.erl
index 73f184a3a..81ff61a4d 100644
--- a/src/emqttd_plugins.erl
+++ b/src/emqttd_plugins.erl
@@ -47,7 +47,7 @@ init_config(CfgFile) ->
                   end, AppsEnv).
 
 %% @doc Load all plugins when the broker started.
--spec(load() -> list() | {error, any()}).
+-spec(load() -> list() | {error, term()}).
 load() ->
     case emqttd:env(plugins_loaded_file) of
         {ok, File} ->
@@ -80,7 +80,7 @@ load_plugins(Names, Persistent) ->
     [load_plugin(find_plugin(Name, Plugins), Persistent) || Name <- NeedToLoad].
 
 %% @doc Unload all plugins before broker stopped.
--spec(unload() -> list() | {error, any()}).
+-spec(unload() -> list() | {error, term()}).
 unload() ->
     case emqttd:env(plugins_loaded_file) of
         {ok, File} ->
@@ -119,7 +119,7 @@ plugin(CfgFile) ->
     #mqtt_plugin{name = AppName, version = Ver, descr = Descr}.
 
 %% @doc Load a Plugin
--spec(load(atom()) -> ok | {error, any()}).
+-spec(load(atom()) -> ok | {error, term()}).
 load(PluginName) when is_atom(PluginName) ->
     case lists:member(PluginName, names(started_app)) of
         true ->
@@ -172,7 +172,7 @@ find_plugin(Name, Plugins) ->
     lists:keyfind(Name, 2, Plugins). 
 
 %% @doc UnLoad a Plugin
--spec(unload(atom()) -> ok | {error, any()}).
+-spec(unload(atom()) -> ok | {error, term()}).
 unload(PluginName) when is_atom(PluginName) ->
     case {lists:member(PluginName, names(started_app)), lists:member(PluginName, names(plugin))} of
         {true, true} ->
diff --git a/src/emqttd_pool_sup.erl b/src/emqttd_pool_sup.erl
index fa1d6aace..87654bcff 100644
--- a/src/emqttd_pool_sup.erl
+++ b/src/emqttd_pool_sup.erl
@@ -36,12 +36,12 @@ spec(ChildId, Args) ->
     {ChildId, {?MODULE, start_link, Args},
         transient, infinity, supervisor, [?MODULE]}.
 
--spec(start_link(atom() | tuple(), atom(), mfa()) -> {ok, pid()} | {error, any()}).
+-spec(start_link(atom() | tuple(), atom(), mfa()) -> {ok, pid()} | {error, term()}).
 start_link(Pool, Type, MFA) ->
     Schedulers = erlang:system_info(schedulers),
     start_link(Pool, Type, Schedulers, MFA).
 
--spec(start_link(atom(), atom(), pos_integer(), mfa()) -> {ok, pid()} | {error, any()}).
+-spec(start_link(atom(), atom(), pos_integer(), mfa()) -> {ok, pid()} | {error, term()}).
 start_link(Pool, Type, Size, MFA) ->
     supervisor:start_link(?MODULE, [Pool, Type, Size, MFA]).
 
diff --git a/src/emqttd_pooler.erl b/src/emqttd_pooler.erl
index 1b73d138c..a74e01fec 100644
--- a/src/emqttd_pooler.erl
+++ b/src/emqttd_pooler.erl
@@ -40,7 +40,7 @@ start_link() ->
 %% API
 %%--------------------------------------------------------------------
 
--spec(start_link(atom(), pos_integer()) -> {ok, pid()} | ignore | {error, any()}).
+-spec(start_link(atom(), pos_integer()) -> {ok, pid()} | ignore | {error, term()}).
 start_link(Pool, Id) ->
     gen_server:start_link({local, ?PROC_NAME(?MODULE, Id)}, ?MODULE, [Pool, Id], []).
 
diff --git a/src/emqttd_protocol.erl b/src/emqttd_protocol.erl
index 31354dd84..35c914b14 100644
--- a/src/emqttd_protocol.erl
+++ b/src/emqttd_protocol.erl
@@ -124,7 +124,7 @@ session(#proto_state{session = Session}) ->
 %% CONNECT – Client requests a connection to a Server
 
 %% A Client can only send the CONNECT Packet once over a Network Connection. 
--spec(received(mqtt_packet(), proto_state()) -> {ok, proto_state()} | {error, any()}).
+-spec(received(mqtt_packet(), proto_state()) -> {ok, proto_state()} | {error, term()}).
 received(Packet = ?PACKET(?CONNECT),
          State = #proto_state{connected = false, stats_data = Stats}) ->
     trace(recv, Packet, State), Stats1 = inc_stats(recv, ?CONNECT, Stats),
diff --git a/src/emqttd_session.erl b/src/emqttd_session.erl
index 506743d03..aa08d746c 100644
--- a/src/emqttd_session.erl
+++ b/src/emqttd_session.erl
@@ -172,7 +172,7 @@
                         "Session(~s): " ++ Format, [State#state.client_id | Args])).
 
 %% @doc Start a Session
--spec(start_link(boolean(), {mqtt_client_id(), mqtt_username()}, pid()) -> {ok, pid()} | {error, any()}).
+-spec(start_link(boolean(), {mqtt_client_id(), mqtt_username()}, pid()) -> {ok, pid()} | {error, term()}).
 start_link(CleanSess, {ClientId, Username}, ClientPid) ->
     gen_server2:start_link(?MODULE, [CleanSess, {ClientId, Username}, ClientPid], []).
 
@@ -192,7 +192,7 @@ subscribe(Session, PacketId, TopicTable) -> %%TODO: the ack function??...
     gen_server2:cast(Session, {subscribe, From, TopicTable, AckFun}).
 
 %% @doc Publish Message
--spec(publish(pid(), mqtt_message()) -> ok | {error, any()}).
+-spec(publish(pid(), mqtt_message()) -> ok | {error, term()}).
 publish(_Session, Msg = #mqtt_message{qos = ?QOS_0}) ->
     %% Publish QoS0 Directly
     emqttd_server:publish(Msg), ok;
@@ -582,9 +582,9 @@ handle_info(Info, Session) ->
     ?UNEXPECTED_INFO(Info, Session).
 
 terminate(Reason, #state{client_id = ClientId, username = Username}) ->
-    emqttd_stats:del_session_stats(ClientId),
+    %% Move to emqttd_sm to avoid race condition
+    %%emqttd_stats:del_session_stats(ClientId),
     emqttd_hooks:run('session.terminated', [ClientId, Username, Reason]),
-    emqttd_server:subscriber_down(ClientId),
     emqttd_sm:unregister_session(ClientId).
 
 code_change(_OldVsn, Session, _Extra) ->
diff --git a/src/emqttd_sm.erl b/src/emqttd_sm.erl
index dacb0b6a1..8d4c220b2 100644
--- a/src/emqttd_sm.erl
+++ b/src/emqttd_sm.erl
@@ -76,12 +76,12 @@ mnesia(copy) ->
 %%--------------------------------------------------------------------
 
 %% @doc Start a session manager
--spec(start_link(atom(), pos_integer()) -> {ok, pid()} | ignore | {error, any()}).
+-spec(start_link(atom(), pos_integer()) -> {ok, pid()} | ignore | {error, term()}).
 start_link(Pool, Id) ->
     gen_server2:start_link({local, ?PROC_NAME(?MODULE, Id)}, ?MODULE, [Pool, Id], []).
 
 %% @doc Start a session
--spec(start_session(boolean(), {binary(), binary() | undefined}) -> {ok, pid(), boolean()} | {error, any()}).
+-spec(start_session(boolean(), {binary(), binary() | undefined}) -> {ok, pid(), boolean()} | {error, term()}).
 start_session(CleanSess, {ClientId, Username}) ->
     SM = gproc_pool:pick_worker(?POOL, ClientId),
     call(SM, {start_session, CleanSess, {ClientId, Username}, self()}).
@@ -107,6 +107,7 @@ unregister_session(ClientId) ->
 unregister_session(ClientId, Pid) ->
     case ets:lookup(mqtt_local_session, ClientId) of
         [LocalSess = {_, Pid, _, _}] ->
+            emqttd_stats:del_session_stats(ClientId),
             ets:delete_object(mqtt_local_session, LocalSess);
         _ ->
             false
@@ -187,7 +188,6 @@ handle_info({'DOWN', MRef, process, DownPid, _Reason}, State) ->
                     [] ->
                         ok;
                     [Sess = #mqtt_session{sess_pid = DownPid}] ->
-                        emqttd_stats:del_session_stats(ClientId),
                         mnesia:delete_object(mqtt_session, Sess, write);
                     [_Sess] ->
                         ok
diff --git a/src/emqttd_sm_helper.erl b/src/emqttd_sm_helper.erl
index 2e8e9a749..0721339fd 100644
--- a/src/emqttd_sm_helper.erl
+++ b/src/emqttd_sm_helper.erl
@@ -39,7 +39,7 @@
 -define(LOCK, {?MODULE, clean_sessions}).
 
 %% @doc Start a session helper
--spec(start_link(fun()) -> {ok, pid()} | ignore | {error, any()}).
+-spec(start_link(fun()) -> {ok, pid()} | ignore | {error, term()}).
 start_link(StatsFun) ->
     gen_server:start_link({local, ?MODULE}, ?MODULE, [StatsFun], []).
 
diff --git a/src/emqttd_trace.erl b/src/emqttd_trace.erl
index eda84bcb1..05734c2d2 100644
--- a/src/emqttd_trace.erl
+++ b/src/emqttd_trace.erl
@@ -47,7 +47,7 @@ start_link() ->
     gen_server:start_link({local, ?MODULE}, ?MODULE, [], []).
 
 %% @doc Start to trace client or topic.
--spec(start_trace(trace_who(), string()) -> ok | {error, any()}).
+-spec(start_trace(trace_who(), string()) -> ok | {error, term()}).
 start_trace({client, ClientId}, LogFile) ->
     start_trace({start_trace, {client, ClientId}, LogFile});
 
@@ -57,7 +57,7 @@ start_trace({topic, Topic}, LogFile) ->
 start_trace(Req) -> gen_server:call(?MODULE, Req, infinity).
 
 %% @doc Stop tracing client or topic.
--spec(stop_trace(trace_who()) -> ok | {error, any()}).
+-spec(stop_trace(trace_who()) -> ok | {error, term()}).
 stop_trace({client, ClientId}) ->
     gen_server:call(?MODULE, {stop_trace, {client, ClientId}});
 stop_trace({topic, Topic}) ->