diff --git a/doc/uuid.md b/doc/uuid.md
new file mode 100644
index 000000000..5b7bd0aa0
--- /dev/null
+++ b/doc/uuid.md
@@ -0,0 +1,19 @@
+## Mongodb ObjectId
+
+* 4-byte value representing the seconds since the Unix epoch,
+* 3-byte machine identifier,
+* 2-byte process id, and
+* 3-byte counter, starting with a random value.
+
+## Flake Id
+
+* 64bits Timestamp
+* 48bits WorkerId
+* 16bits Sequence
+
+## emqttd Id
+
+* 64bits Timestamp: erlang:now(), erlang:system_time
+* 48bits (node+pid): Node + Pid -> Integer 
+* 16bits Sequence: PktId
+
diff --git a/src/emqttd_guid.erl b/src/emqttd_guid.erl
new file mode 100644
index 000000000..ffdf4c646
--- /dev/null
+++ b/src/emqttd_guid.erl
@@ -0,0 +1,126 @@
+%%%-----------------------------------------------------------------------------
+%%% Copyright (c) 2012-2015 eMQTT.IO, All Rights Reserved.
+%%%
+%%% Permission is hereby granted, free of charge, to any person obtaining a copy
+%%% of this software and associated documentation files (the "Software"), to deal
+%%% in the Software without restriction, including without limitation the rights
+%%% to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+%%% copies of the Software, and to permit persons to whom the Software is
+%%% furnished to do so, subject to the following conditions:
+%%%
+%%% The above copyright notice and this permission notice shall be included in all
+%%% copies or substantial portions of the Software.
+%%%
+%%% THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+%%% IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+%%% FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+%%% AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+%%% LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+%%% OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+%%% SOFTWARE.
+%%%-----------------------------------------------------------------------------
+%%% @doc
+%%%
+%%% Generate global unique id for mqtt message.
+%%%
+%%% --------------------------------------------------------
+%%% |        Timestamp       |  NodeID + PID  |  Sequence  | 
+%%% |<------- 64bits ------->|<--- 48bits --->|<- 16bits ->|
+%%% --------------------------------------------------------
+%%% 
+%%% 1. Timestamp: erlang:system_time if Erlang >= R18, otherwise os:timestamp
+%%% 2. NodeId:    encode node() to 2 bytes integer
+%%% 3. Pid:       encode pid to 4 bytes integer
+%%% 4. Sequence:  2 bytes sequence no per pid
+%%%
+%%% @end
+%%%-----------------------------------------------------------------------------
+
+-module(emqttd_guid).
+
+-export([gen/0]).
+
+-define(MAX_SEQ, 16#FFFF).
+
+-type guid() :: <<_:128>>.
+
+-spec gen() -> guid().
+gen() ->
+    Guid = case get(guid) of
+        undefined        -> new();
+        {_Ts, NPid, Seq} -> next(NPid, Seq)
+    end,
+    put(guid, Guid), enc(Guid).
+
+new() ->
+    {ts(), npid(), 0}.
+
+next(NPid, Seq) when Seq >= ?MAX_SEQ ->
+    {ts(), NPid, 0};
+next(NPid, Seq) ->
+    {ts(), NPid, Seq + 1}.
+
+enc({Ts, NPid, Seq}) ->
+    <<Ts:64, NPid:48, Seq:16>>.
+
+ts() ->
+    case erlang:function_exported(erlang, system_time, 1) of
+        true -> %% R18
+            erlang:system_time(micro_seconds);
+        false ->
+            {MegaSeconds, Seconds, MicroSeconds} = erlang:now(),
+            (MegaSeconds * 1000000 + Seconds) * 1000000 + MicroSeconds
+    end.
+
+%% code copied from https://github.com/okeuday/uuid.git.
+npid() ->
+    <<NodeD01, NodeD02, NodeD03, NodeD04, NodeD05,
+      NodeD06, NodeD07, NodeD08, NodeD09, NodeD10,
+      NodeD11, NodeD12, NodeD13, NodeD14, NodeD15,
+      NodeD16, NodeD17, NodeD18, NodeD19, NodeD20>> =
+      crypto:hash(sha, erlang:list_to_binary(erlang:atom_to_list(node()))),
+
+    % later, when the pid format changes, handle the different format
+    ExternalTermFormatVersion = 131,
+    PidExtType = 103,
+    <<ExternalTermFormatVersion:8,
+      PidExtType:8,
+      PidBin/binary>> = erlang:term_to_binary(self()),
+    % 72 bits for the Erlang pid
+    <<PidID1:8, PidID2:8, PidID3:8, PidID4:8, % ID (Node specific, 15 bits)
+      PidSR1:8, PidSR2:8, PidSR3:8, PidSR4:8, % Serial (extra uniqueness)
+      PidCR1:8                       % Node Creation Count
+      >> = binary:part(PidBin, erlang:byte_size(PidBin), -9),
+
+    % reduce the 160 bit NodeData checksum to 16 bits
+    NodeByte1 = ((((((((NodeD01 bxor NodeD02)
+                       bxor NodeD03)
+                      bxor NodeD04)
+                     bxor NodeD05)
+                    bxor NodeD06)
+                   bxor NodeD07)
+                  bxor NodeD08)
+                 bxor NodeD09)
+                bxor NodeD10,
+    NodeByte2 = (((((((((NodeD11 bxor NodeD12)
+                        bxor NodeD13)
+                       bxor NodeD14)
+                      bxor NodeD15)
+                     bxor NodeD16)
+                    bxor NodeD17)
+                   bxor NodeD18)
+                  bxor NodeD19)
+                 bxor NodeD20)
+                bxor PidCR1,
+
+    % reduce the Erlang pid to 32 bits
+    PidByte1 = PidID1 bxor PidSR4,
+    PidByte2 = PidID2 bxor PidSR3,
+    PidByte3 = PidID3 bxor PidSR2,
+    PidByte4 = PidID4 bxor PidSR1,
+
+    <<NPid:48>> = <<NodeByte1:8, NodeByte2:8,
+                    PidByte1:8, PidByte2:8,
+                    PidByte3:8, PidByte4:8>>,
+    NPid.
+
diff --git a/src/emqttd_msg_store.erl b/src/emqttd_retained.erl
similarity index 100%
rename from src/emqttd_msg_store.erl
rename to src/emqttd_retained.erl