diff --git a/lib/futu/component/api.ex b/lib/futu/component/api.ex
index 7bf8b59..4ae8a34 100644
--- a/lib/futu/component/api.ex
+++ b/lib/futu/component/api.ex
@@ -16,8 +16,6 @@ defmodule Futu.Component.Api do
 
       @before_compile unquote(__MODULE__)
 
-      @mapper_module Application.compile_env(:futu, :mapper_module, nil)
-
       @spec map_c2s(any()) :: list()
       def map_c2s(opts \\ [])
       def map_c2s(opts), do: opts
@@ -25,15 +23,6 @@ defmodule Futu.Component.Api do
       @spec map_s2c(any()) :: any()
       def map_s2c(opts), do: opts
 
-      defp post_process_s2c(s2c, opts) do
-        mapped_s2c = map_s2c(s2c)
-
-        case @mapper_module do
-          nil -> mapped_s2c
-          _ -> apply(@mapper_module, :map, [mapped_s2c, opts])
-        end
-      end
-
       defoverridable map_c2s: 1,
                      map_s2c: 1
     end
@@ -81,7 +70,7 @@ defmodule Futu.Component.Api do
 
         case response do
           %{retType: 0, s2c: s2c} ->
-            {:ok, post_process_s2c(s2c, opts)}
+            {:ok, map_s2c(s2c)}
 
           %{retType: _ret_type, retMsg: ret_msg, errCode: _err_code} ->
             {:error, ret_msg}
diff --git a/lib/futu/component/mapper/close_only.ex b/lib/futu/component/mapper/close_only.ex
deleted file mode 100644
index ed25e9f..0000000
--- a/lib/futu/component/mapper/close_only.ex
+++ /dev/null
@@ -1,19 +0,0 @@
-defmodule Futu.Component.Mapper.CloseOnly do
-  @moduledoc false
-  @behaviour Futu.Component.Mapper
-
-  def map(data, opts) when is_list(data) do
-    Enum.map(data, fn item ->
-      map(item, opts)
-    end)
-  end
-
-  def map(data = %Qot_Common.KLine{}, opts) do
-    %{
-      code: opts[:code],
-      close: data.highPrice
-    }
-  end
-
-  def map(data, _opts), do: data
-end
diff --git a/lib/futu/component/mapper/mapper.ex b/lib/futu/component/mapper/mapper.ex
deleted file mode 100644
index bd2804f..0000000
--- a/lib/futu/component/mapper/mapper.ex
+++ /dev/null
@@ -1,5 +0,0 @@
-defmodule Futu.Component.Mapper do
-  @moduledoc false
-
-  @callback map(any(), list()) :: any()
-end
diff --git a/lib/futu/component/mapper/nothing.ex b/lib/futu/component/mapper/nothing.ex
deleted file mode 100644
index 5d5a0d2..0000000
--- a/lib/futu/component/mapper/nothing.ex
+++ /dev/null
@@ -1,7 +0,0 @@
-defmodule Futu.Component.Mapper.Nothing do
-  @moduledoc false
-  @behaviour Futu.Component.Mapper
-
-  @spec map(any(), list()) :: any()
-  def map(data, _opts), do: data
-end
diff --git a/lib/futu/gen_server/serial_number.ex b/lib/futu/gen_server/serial_number.ex
new file mode 100644
index 0000000..0fbf4fb
--- /dev/null
+++ b/lib/futu/gen_server/serial_number.ex
@@ -0,0 +1,26 @@
+defmodule Futu.GenServer.SerialNumber do
+  @moduledoc """
+  Generate incremental numbers
+  """
+  use GenServer
+
+  @spec start_link(list()) :: {:ok, pid()}
+  def start_link(_opts) do
+    GenServer.start_link(__MODULE__, nil)
+  end
+
+  @spec init(nil) :: {:ok, map()}
+  def init(nil) do
+    {:ok, %{sn: os_time()}}
+  end
+
+  def handle_call(:generate, _from, %{sn: sn} = state) do
+    {:reply, sn + 1, %{state | sn: sn + 1}}
+  end
+
+  def handle_call(:get_current_state, _from, %{sn: sn} = state) do
+    {:reply, sn, state}
+  end
+
+  defp os_time(), do: :os.system_time(:millisecond)
+end
diff --git a/test/futu/genserver/serial_number_test.ex b/test/futu/genserver/serial_number_test.ex
new file mode 100644
index 0000000..e184468
--- /dev/null
+++ b/test/futu/genserver/serial_number_test.ex
@@ -0,0 +1,20 @@
+defmodule Futu.GenServer.SerialNumberTest do
+  use ExUnit.Case
+  alias Futu.GenServer.SerialNumber
+
+  setup_all do
+    {:ok, pid} = SerialNumber.start_link([])
+    {:ok, pid: pid}
+  end
+
+  describe "generate" do
+    test "increments", %{pid: pid} do
+      sn_1 = GenServer.call(pid, :generate)
+      sn_2 = GenServer.call(pid, :generate)
+      sn_3 = GenServer.call(pid, :generate)
+
+      assert sn_3 > sn_2
+      assert sn_2 > sn_1
+    end
+  end
+end