diff --git a/.gitignore b/.gitignore
index 6784d9c..8b3c449 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,2 +1,3 @@
+*.db
 *.beam
 .*
diff --git a/chapter_10/UsrTabFile b/chapter_10/UsrTabFile
deleted file mode 100644
index de37b4e..0000000
Binary files a/chapter_10/UsrTabFile and /dev/null differ
diff --git a/chapter_12/gensrv.erl b/chapter_12/gensrv.erl
new file mode 100644
index 0000000..f94d9ae
--- /dev/null
+++ b/chapter_12/gensrv.erl
@@ -0,0 +1,34 @@
+-module(sample_gensrv).
+-export([start/0, stop/0]).
+-export([init/1, handle_cast/2, handle_call/3, terminate/2]).
+-export([say_hello/1]).
+
+-behavior(gen_server).
+
+start() ->
+  Name = "Jack",
+  gen_server:start({local, ?MODULE}, ?MODULE, Name, []).
+
+init(Name) ->
+  io:format("Init OPT ~p~n", [Name]),
+  {ok, null}.
+
+say_hello(Name) ->
+  NewName = gen_server:call(?MODULE, {say_hello, Name}),
+  io:format("Hello ~p ~n", [NewName]).
+
+stop() ->
+  gen_server:cast(?MODULE, stop).
+
+handle_cast(stop, LoopData) ->
+  io:format("Stop OPT~n"),
+  {stop, normal, LoopData}.
+
+terminate(Reason, LoopData) ->
+  io:format("Terminate OPT ~p ~p ~n", [Reason, LoopData]).
+
+handle_call({say_hello, Name}, _From, LoopData) ->
+  io:format("Hello ~p ~n", [Name]),
+  {reply, "Tommy", LoopData}.
+
+
diff --git a/chapter_12/usr-1.0/ebin/usr.app b/chapter_12/usr-1.0/ebin/usr.app
new file mode 100644
index 0000000..5fadc61
--- /dev/null
+++ b/chapter_12/usr-1.0/ebin/usr.app
@@ -0,0 +1,9 @@
+{application, usr, [
+    {description, "Mobile Services Database"},
+    {vsn, "1.0"},
+    {modules, [usr, usr_db, usr_sup, usr_app]},
+    {registered, [usr, usr_sup]},
+    {applications, [kernel, stdlib]},
+    {env, [{dets_name, "usrDb.db"}]},
+    {mod, {usr_app, []}}]}.
+    
diff --git a/chapter_12/usr-1.0/include/usr.hrl b/chapter_12/usr-1.0/include/usr.hrl
new file mode 100644
index 0000000..e86b6df
--- /dev/null
+++ b/chapter_12/usr-1.0/include/usr.hrl
@@ -0,0 +1,16 @@
+%%% File: usr.hrl
+%%% Description: Include file for user db
+
+-type(plan()    :: prepay | postpay).
+-type(status()  :: enabled | disabled).
+-type(service() :: atom()).
+
+-record(usr, 
+  {
+    msisdn              ::integer(),
+    id                  ::integer(),
+    status = enabled    ::status(),
+    plan                ::plan(),
+    services = []       ::[service()]
+  }
+).
diff --git a/chapter_12/usr-1.0/src/usr.erl b/chapter_12/usr-1.0/src/usr.erl
new file mode 100644
index 0000000..24848d5
--- /dev/null
+++ b/chapter_12/usr-1.0/src/usr.erl
@@ -0,0 +1,107 @@
+-module(usr).
+-export([start_link/0, start_link/1, stop/0]).
+-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2]).
+-export([add_usr/3, delete_usr/1, set_service/3, set_status/2, delete_disabled/0, lookup_id/1]).
+-export([lookup_msisdn/1, service_flag/2]).
+
+-behavior(gen_server).
+
+-include("usr.hrl").
+-define(TIMEOUT, 30000).
+
+start_link() ->
+  {ok, FileName} = application:get_env(dets_name),
+  start_link(FileName).
+
+start_link(FileName) ->
+  gen_server:start_link({local, ?MODULE}, ?MODULE, FileName, []).
+
+stop() ->
+  gen_server:cast(?MODULE, stop).
+
+init(FileName) ->
+  usr_db:create_tables(FileName),
+  usr_db:restore_backup(),
+  {ok, null}.
+
+terminate(Reason, LoopData) ->
+  usr_db:close_tables().
+
+add_usr(PhoneNum, CustId, Plan) when Plan == prepay; Plan == postpay ->
+  gen_server:call(?MODULE, {add_usr, PhoneNum, CustId, Plan}).
+
+delete_usr(CustId) ->
+  gen_server:call(?MODULE, {delete_usr, CustId}).
+
+set_service(CustId, Service, Flag) when Flag == true; Flag == false ->
+  gen_server:call(?MODULE, {set_service, CustId, Service, Flag}).
+
+set_status(CustId, Status) when Status == enabled; Status == disabled ->
+  gen_server:call(?MODULE, {set_status, CustId, Status}).
+
+delete_disabled() ->
+  gen_server:call(?MODULE, delete_disabled).
+
+lookup_id(CustId) ->
+  usr_db:lookup_id(CustId).
+
+lookup_msisdn(PhoneNo) ->
+  usr_db:lookup_msisdn(PhoneNo).
+
+service_flag(PhoneNo, Service) ->
+  case usr_db:lookup_msisdn(PhoneNo) of
+    {ok, #usr{services = Services, status = enabled}} ->
+      lists:member(Service, Services);
+    {ok, #usr{status=disabled}} ->
+      {error, disabled};
+    {error, Reason} ->
+      {error, Reason}
+  end.
+
+handle_call({add_usr, PhoneNo, CustId, Plan}, _From, LoopData) ->
+  Reply = usr_db:add_usr(
+    #usr{
+      msisdn=PhoneNo, 
+      id=CustId, 
+      plan=Plan
+    }
+  ),
+  {reply, Reply, LoopData};
+
+handle_call({delete_usr, CustId}, _From, LoopData) ->
+  Reply = usr_db:delete_usr(CustId),
+  {reply, Reply, LoopData};
+
+handle_call({set_service, CustId, Service, Flag}, _From, LoopData) ->
+  Reply = case usr_db:lookup_id(CustId) of
+    {ok, Usr} ->
+      Services = lists:delete(Service, Usr#usr.services),
+      NewServices = case Flag of
+        true -> [Service | Services];
+        false -> Services
+      end,
+      usr_db:update_usr(Usr#usr{services = NewServices});
+    {error, instance} ->
+      {error, instance}
+  end,
+  {reply, Reply, LoopData};
+
+handle_call({set_status, CustId, Status}, _From, LoopData) ->
+  Reply = case usr_db:lookup_id(CustId) of
+    {ok, Usr} ->
+      usr_db:update_usr(Usr#usr{status = Status});
+    {error, instance} ->
+      {error, instance}
+  end,
+  {reply, Reply, LoopData};
+
+handle_call({delete_disabled}, _From, LoopData) ->
+  Reply = usr_db:delete_disabled(),
+  {reply, Reply, LoopData}.
+
+handle_cast(stop, LoopData) ->
+  {stop, normal, LoopData}.
+
+handle_info(Msg, LoopData) ->
+  io:format("Recive Not OPT Message ~p~n", [Msg]),
+  {noreply, LoopData}.
diff --git a/chapter_12/usr-1.0/src/usr_app.erl b/chapter_12/usr-1.0/src/usr_app.erl
new file mode 100644
index 0000000..53e51c8
--- /dev/null
+++ b/chapter_12/usr-1.0/src/usr_app.erl
@@ -0,0 +1,10 @@
+-module(usr_app).
+-behavior(application).
+
+-export([start/2, stop/1]).
+
+start(_Type, StartArgs) ->
+  usr_sup:start_link().
+
+stop(_State) ->
+  ok.
diff --git a/chapter_12/usr-1.0/src/usr_db.erl b/chapter_12/usr-1.0/src/usr_db.erl
new file mode 100644
index 0000000..0547459
--- /dev/null
+++ b/chapter_12/usr-1.0/src/usr_db.erl
@@ -0,0 +1,89 @@
+%%% File: usr_db.erl
+%%% Description: Database API for subscriber DB
+
+-module(usr_db).
+-include("usr.hrl").
+
+-export([create_tables/1, close_tables/0]).
+-export([add_usr/1, update_usr/1]).
+-export([lookup_id/1, lookup_msisdn/1]).
+-export([restore_backup/0]).
+
+-export([delete_disabled/0, delete_usr/1]).
+
+create_tables(FileName) ->
+  ets:new(usrRam, [named_table, {keypos, #usr.msisdn}]),
+  ets:new(usrIndex, [named_table]),
+  dets:open_file(usrDisk, [{file, FileName}, {keypos, #usr.msisdn}]).
+
+close_tables() ->
+  ets:delete(usrRam),
+  ets:delete(usrIndex),
+  dets:close(usrDisk).
+
+add_usr(#usr{msisdn=PhoneNo, id=CustId} = Usr) ->
+  ets:insert(usrIndex, {CustId, PhoneNo}),
+  update_usr(Usr).
+
+update_usr(Usr) ->
+  ets:insert(usrRam, Usr),
+  dets:insert(usrDisk, Usr),
+  ok.
+
+lookup_id(CustId) ->
+  case get_index(CustId) of
+    {ok, PhoneNo} -> lookup_msisdn(PhoneNo);
+    {error, instance} -> {error, instance}
+  end.
+
+lookup_msisdn(PhoneNo) ->
+  case ets:lookup(usrRam, PhoneNo) of
+    [Usr] -> {ok, Usr};
+    [] -> {error, instance}
+  end.
+
+get_index(CustId) ->
+  case ets:lookup(usrIndex, CustId) of
+    [{CustId, PhoneNo}] -> {ok, PhoneNo};
+    [] -> {error, instance}
+  end.
+
+restore_backup() ->
+  Insert = fun(#usr{msisdn=PhoneNo, id=Id} = Usr) ->
+             ets:insert(usrRam, Usr),
+             ets:insert(usrIndex, {Id, PhoneNo}),
+             continue
+           end,
+  dets:traverse(usrDisk, Insert).
+
+delete_disabled() ->
+  ets:safe_fixtable(usrRam, true),
+  catch loop_delete_disabled(ets:first(usrRam)),
+  ets:safe_fixtable(usrRam, false),
+  ok.
+
+delete_usr(CustId) ->
+  case get_index(CustId) of
+    {ok, PhoneNo} -> 
+      ets:safe_fixtable(usrRam, true),
+      ets:delete(usrRam, PhoneNo),
+      ets:safe_fixtable(usrRam, false),
+      ets:safe_fixtable(usrIndex, true),
+      ets:delete(usrIndex, CustId),
+      ets:safe_fixtable(usrIndex, false),
+
+      dets:delete(usrDisk, PhoneNo);
+    {error, instance} -> {error, instance}
+  end.
+  
+loop_delete_disabled('$end_of_table') ->
+  ok;
+
+loop_delete_disabled(PhoneNo) ->
+  case ets:lookup(usrRam, PhoneNo) of
+    [#usr{status=disabled, id=CustId}] ->
+      delete_usr(CustId);
+    _ ->
+      ok
+  end,
+  loop_delete_disabled(ets:next(usrRam, PhoneNo)).
diff --git a/chapter_12/usr-1.0/src/usr_sup.erl b/chapter_12/usr-1.0/src/usr_sup.erl
new file mode 100644
index 0000000..83868fb
--- /dev/null
+++ b/chapter_12/usr-1.0/src/usr_sup.erl
@@ -0,0 +1,16 @@
+-module(usr_sup).
+-behavior(supervisor).
+
+-export([start_link/0]).
+-export([init/1]).
+
+start_link() ->
+  supervisor:start_link({local, ?MODULE}, ?MODULE, null).
+
+init(_) ->
+  UsrChild = {
+    usr, {usr, start_link, []},
+    permanent, 2000, worker, [usr, usr_db]},
+
+  {ok, {{one_for_all, 2, 2}, [UsrChild]}}.
+
diff --git a/chapter_12/usr.erl b/chapter_12/usr.erl
new file mode 100644
index 0000000..24848d5
--- /dev/null
+++ b/chapter_12/usr.erl
@@ -0,0 +1,107 @@
+-module(usr).
+-export([start_link/0, start_link/1, stop/0]).
+-export([init/1, handle_call/3, handle_cast/2, handle_info/2, terminate/2]).
+-export([add_usr/3, delete_usr/1, set_service/3, set_status/2, delete_disabled/0, lookup_id/1]).
+-export([lookup_msisdn/1, service_flag/2]).
+
+-behavior(gen_server).
+
+-include("usr.hrl").
+-define(TIMEOUT, 30000).
+
+start_link() ->
+  {ok, FileName} = application:get_env(dets_name),
+  start_link(FileName).
+
+start_link(FileName) ->
+  gen_server:start_link({local, ?MODULE}, ?MODULE, FileName, []).
+
+stop() ->
+  gen_server:cast(?MODULE, stop).
+
+init(FileName) ->
+  usr_db:create_tables(FileName),
+  usr_db:restore_backup(),
+  {ok, null}.
+
+terminate(Reason, LoopData) ->
+  usr_db:close_tables().
+
+add_usr(PhoneNum, CustId, Plan) when Plan == prepay; Plan == postpay ->
+  gen_server:call(?MODULE, {add_usr, PhoneNum, CustId, Plan}).
+
+delete_usr(CustId) ->
+  gen_server:call(?MODULE, {delete_usr, CustId}).
+
+set_service(CustId, Service, Flag) when Flag == true; Flag == false ->
+  gen_server:call(?MODULE, {set_service, CustId, Service, Flag}).
+
+set_status(CustId, Status) when Status == enabled; Status == disabled ->
+  gen_server:call(?MODULE, {set_status, CustId, Status}).
+
+delete_disabled() ->
+  gen_server:call(?MODULE, delete_disabled).
+
+lookup_id(CustId) ->
+  usr_db:lookup_id(CustId).
+
+lookup_msisdn(PhoneNo) ->
+  usr_db:lookup_msisdn(PhoneNo).
+
+service_flag(PhoneNo, Service) ->
+  case usr_db:lookup_msisdn(PhoneNo) of
+    {ok, #usr{services = Services, status = enabled}} ->
+      lists:member(Service, Services);
+    {ok, #usr{status=disabled}} ->
+      {error, disabled};
+    {error, Reason} ->
+      {error, Reason}
+  end.
+
+handle_call({add_usr, PhoneNo, CustId, Plan}, _From, LoopData) ->
+  Reply = usr_db:add_usr(
+    #usr{
+      msisdn=PhoneNo, 
+      id=CustId, 
+      plan=Plan
+    }
+  ),
+  {reply, Reply, LoopData};
+
+handle_call({delete_usr, CustId}, _From, LoopData) ->
+  Reply = usr_db:delete_usr(CustId),
+  {reply, Reply, LoopData};
+
+handle_call({set_service, CustId, Service, Flag}, _From, LoopData) ->
+  Reply = case usr_db:lookup_id(CustId) of
+    {ok, Usr} ->
+      Services = lists:delete(Service, Usr#usr.services),
+      NewServices = case Flag of
+        true -> [Service | Services];
+        false -> Services
+      end,
+      usr_db:update_usr(Usr#usr{services = NewServices});
+    {error, instance} ->
+      {error, instance}
+  end,
+  {reply, Reply, LoopData};
+
+handle_call({set_status, CustId, Status}, _From, LoopData) ->
+  Reply = case usr_db:lookup_id(CustId) of
+    {ok, Usr} ->
+      usr_db:update_usr(Usr#usr{status = Status});
+    {error, instance} ->
+      {error, instance}
+  end,
+  {reply, Reply, LoopData};
+
+handle_call({delete_disabled}, _From, LoopData) ->
+  Reply = usr_db:delete_disabled(),
+  {reply, Reply, LoopData}.
+
+handle_cast(stop, LoopData) ->
+  {stop, normal, LoopData}.
+
+handle_info(Msg, LoopData) ->
+  io:format("Recive Not OPT Message ~p~n", [Msg]),
+  {noreply, LoopData}.
diff --git a/chapter_12/usr.hrl b/chapter_12/usr.hrl
new file mode 100644
index 0000000..e86b6df
--- /dev/null
+++ b/chapter_12/usr.hrl
@@ -0,0 +1,16 @@
+%%% File: usr.hrl
+%%% Description: Include file for user db
+
+-type(plan()    :: prepay | postpay).
+-type(status()  :: enabled | disabled).
+-type(service() :: atom()).
+
+-record(usr, 
+  {
+    msisdn              ::integer(),
+    id                  ::integer(),
+    status = enabled    ::status(),
+    plan                ::plan(),
+    services = []       ::[service()]
+  }
+).
diff --git a/chapter_12/usr_app.erl b/chapter_12/usr_app.erl
new file mode 100644
index 0000000..53e51c8
--- /dev/null
+++ b/chapter_12/usr_app.erl
@@ -0,0 +1,10 @@
+-module(usr_app).
+-behavior(application).
+
+-export([start/2, stop/1]).
+
+start(_Type, StartArgs) ->
+  usr_sup:start_link().
+
+stop(_State) ->
+  ok.
diff --git a/chapter_12/usr_db.erl b/chapter_12/usr_db.erl
new file mode 100644
index 0000000..0547459
--- /dev/null
+++ b/chapter_12/usr_db.erl
@@ -0,0 +1,89 @@
+%%% File: usr_db.erl
+%%% Description: Database API for subscriber DB
+
+-module(usr_db).
+-include("usr.hrl").
+
+-export([create_tables/1, close_tables/0]).
+-export([add_usr/1, update_usr/1]).
+-export([lookup_id/1, lookup_msisdn/1]).
+-export([restore_backup/0]).
+
+-export([delete_disabled/0, delete_usr/1]).
+
+create_tables(FileName) ->
+  ets:new(usrRam, [named_table, {keypos, #usr.msisdn}]),
+  ets:new(usrIndex, [named_table]),
+  dets:open_file(usrDisk, [{file, FileName}, {keypos, #usr.msisdn}]).
+
+close_tables() ->
+  ets:delete(usrRam),
+  ets:delete(usrIndex),
+  dets:close(usrDisk).
+
+add_usr(#usr{msisdn=PhoneNo, id=CustId} = Usr) ->
+  ets:insert(usrIndex, {CustId, PhoneNo}),
+  update_usr(Usr).
+
+update_usr(Usr) ->
+  ets:insert(usrRam, Usr),
+  dets:insert(usrDisk, Usr),
+  ok.
+
+lookup_id(CustId) ->
+  case get_index(CustId) of
+    {ok, PhoneNo} -> lookup_msisdn(PhoneNo);
+    {error, instance} -> {error, instance}
+  end.
+
+lookup_msisdn(PhoneNo) ->
+  case ets:lookup(usrRam, PhoneNo) of
+    [Usr] -> {ok, Usr};
+    [] -> {error, instance}
+  end.
+
+get_index(CustId) ->
+  case ets:lookup(usrIndex, CustId) of
+    [{CustId, PhoneNo}] -> {ok, PhoneNo};
+    [] -> {error, instance}
+  end.
+
+restore_backup() ->
+  Insert = fun(#usr{msisdn=PhoneNo, id=Id} = Usr) ->
+             ets:insert(usrRam, Usr),
+             ets:insert(usrIndex, {Id, PhoneNo}),
+             continue
+           end,
+  dets:traverse(usrDisk, Insert).
+
+delete_disabled() ->
+  ets:safe_fixtable(usrRam, true),
+  catch loop_delete_disabled(ets:first(usrRam)),
+  ets:safe_fixtable(usrRam, false),
+  ok.
+
+delete_usr(CustId) ->
+  case get_index(CustId) of
+    {ok, PhoneNo} -> 
+      ets:safe_fixtable(usrRam, true),
+      ets:delete(usrRam, PhoneNo),
+      ets:safe_fixtable(usrRam, false),
+      ets:safe_fixtable(usrIndex, true),
+      ets:delete(usrIndex, CustId),
+      ets:safe_fixtable(usrIndex, false),
+
+      dets:delete(usrDisk, PhoneNo);
+    {error, instance} -> {error, instance}
+  end.
+  
+loop_delete_disabled('$end_of_table') ->
+  ok;
+
+loop_delete_disabled(PhoneNo) ->
+  case ets:lookup(usrRam, PhoneNo) of
+    [#usr{status=disabled, id=CustId}] ->
+      delete_usr(CustId);
+    _ ->
+      ok
+  end,
+  loop_delete_disabled(ets:next(usrRam, PhoneNo)).
diff --git a/chapter_12/usr_sup.erl b/chapter_12/usr_sup.erl
new file mode 100644
index 0000000..83868fb
--- /dev/null
+++ b/chapter_12/usr_sup.erl
@@ -0,0 +1,16 @@
+-module(usr_sup).
+-behavior(supervisor).
+
+-export([start_link/0]).
+-export([init/1]).
+
+start_link() ->
+  supervisor:start_link({local, ?MODULE}, ?MODULE, null).
+
+init(_) ->
+  UsrChild = {
+    usr, {usr, start_link, []},
+    permanent, 2000, worker, [usr, usr_db]},
+
+  {ok, {{one_for_all, 2, 2}, [UsrChild]}}.
+