使用ets和dets，dict和dets

编写一种键->值存储，让它把较小的值放入内存，把较大的值放入磁盘。制作一个适配器模块来实现它，并让这个模块与本章前面的适配器具有相同的接口。
1 使用dict存数据

-module(adapter_db3).
-export([new/0, store/3, lookup/2, close/1]).
new() ->
	{?MODULE, dict:new(), []}.
store(Key, Val, {?MODULE, MemDict, DiskList}) when Val < 5000 ->
	MemDict1 = dict:store(Key, Val, MemDict),
	{?MODULE, MemDict1, DiskList};
store(Key, Val, {?MODULE, MemDict, DiskList}) ->
	DiskList1 = lists:keystore(Key, 1, DiskList, {Key, Val}),
	{?MODULE, MemDict, DiskList1}.
lookup(Key, {?MODULE, MemDict, DiskList}) ->
	case dict:find(Key, MemDict) of
		{ok, Val} ->
			{ok, Val};
		error ->
			case lists:keysearch(Key, 1, DiskList) of
				{value, {Key, Val}} ->
					{ok, Val};
				false ->
					error
			end
	end.
close({?MODULE, _MemDict, DiskList}) ->
	case dets:open_file(data_db, [{keypos, 1}, {auto_save, 5000}]) of
		{ok, Tab} ->
			[dets:insert(Tab, {K, V}) || {K, V} <- DiskList],
			dets:sync(Tab),
			dets:close(Tab),
			ok;
		{error, Reason} ->
			{error, Reason}
	end.

2 使用ets存数据

-module(adapter_db3).
-export([new/0, store/3, lookup/2, close/1]).
new() ->
	{?MODULE, ets:new(?MODULE, [named_table, public]), []}.
store(Key, Val, {?MODULE, MemTable, DiskList}) when Val < 5000 ->
	MemTable1 = ets:insert(MemTable, {Key, Val}),
	{?MODULE, MemTable1, DiskList};
store(Key, Val, {?MODULE, MemTable, DiskList}) ->
	DiskList1 = lists:keystore(Key, 1, DiskList, {Key, Val}),
	{?MODULE, MemTable, DiskList1}.

lookup(Key, {?MODULE, MemTable, DiskList}) ->
	case ets:lookup(MemTable, Key) of
		[{Key, Val}] ->
			{ok, Val};
		[] ->
			case lists:keysearch(Key, 1, DiskList) of
				{value, {Key, Val}} ->
					{ok, Val};
				false ->
					error
			end
	end.
close({?MODULE, _MemTable, DiskList}) ->
	case dets:open_file(data_db, [{keypos, 1}, {auto_save, 5000}]) of
		{ok, Tab} ->
			[dets:insert(Tab, {K, V}) || {K, V} <- DiskList],
			dets:close(Tab),
			ok;
		{error, Reason} ->
			{error, Reason}
	end.