botlistprojects/botsocial/lib/erlang/www_tools/disk_cache.erl

   1 -module(disk_cache).
   2
   3 %IA Joe Armstrong
   4 %ID 970212
   5 %IK [disk,cache,key,value]
   6 %IH A disk Key-Value cache which uses dets
   7 %IT <p><b>disk_cache:start(File)</b> starts a disk cache with the
   8 % dets file <b>File</b>. If the file does not exist a new cache is
   9 % created. Note Key is single valued.
  10 % <p><b>store(Key, Value)</b> inserts a value in the cache.
  11 % <p><b>fetch(Key) -> ok{Val} | not_found</b> retrieves a value
  12 % <p><b>set_max(int())</b> sets the maximum size of the cache.
  13 % <p><b>flush(Key)</b> clears an individual cache item.
  14 % <p><b>clear()</b> clers the entire cache.
  15 % <p><b>info() -> {Max, [Key]}</b> provides information aboiut the cache.
  16 % <p><b>stop()</b> stops the cache.
  17
  18 -behaviour(gen_server).
  19
  20 %% This is the interface
  21
  22 -export([start/1, store/2, fetch/1, flush/1, clear/0, set_max/1, info/0,
  23          stop/0]).
  24
  25 %% And these are cos we use gen_server
  26 -export([init/1, handle_call/3, terminate/2]).
  27
  28 -import(lists, [member/2, reverse/1]).
  29
  30 -define(SERVER, disk_cache_server).
  31 -define(RPC(X), gen_server:call(?SERVER, X, 10000)).
  32 -define(DEFAULT_CACHE_ENTRIES, 100).
  33
  34 %% start(File) -> ok (creates file if non existent)
  35 %% store(Key, Val) -> ok
  36 %% fetch(Key) -> {ok, Val} | not_found
  37 %% flush(Key) -> ok
  38 %% clear()    -> ok
  39 %% set_max(N) -> ok
  40 %% info()     -> {File, Max, [Key]}
  41 %% stop()     -> ok
  42
  43 start(F)             -> gen_server:start({local,?SERVER},?MODULE,F,[]).
  44 store(Key, Val)      -> ?RPC({store, Key, Val}).
  45 fetch(Key)           -> ?RPC({fetch, Key}).
  46 flush(Key)           -> ?RPC(flush).
  47 clear()              -> ?RPC(clear).
  48 set_max(Max) when integer(Max), Max > 0  -> ?RPC({set_max, Max}).
  49 info()               -> ?RPC(info).
  50 stop()               -> ?RPC(stop).
  51
  52 %% End Interface
  53
  54
  55 init(File) ->
  56     io:format("Starting disk cache in file:~p\n", [File]),
  57     case dets:open_file(myFile, [{type,set}, {file, File}]) of
  58         {ok, Tab} ->
  59             Max = get_key_or_default(Tab, max, ?DEFAULT_CACHE_ENTRIES),
  60             Keys = get_key_or_default(Tab, keys, []),
  61             {ok, {Tab, Max, Keys}};
  62         _ ->
  63             io:format("Cannot open cache - run make_cache\n",[]),
  64             {stop, cannot_open_cache}
  65     end.
  66 handle_call({store, Key, Val}, _, {Tab, Max, Keys}) ->
  67     case member(Key, Keys) of
  68         true ->
  69             %% remove the old value
  70             dets:delete(Tab, {key, Key}),
  71             Keys1 = move_to_front(Key, Keys),
  72             dets:insert(Tab, {{item,Key},Val}),
  73             {reply, ok, {Tab, Max, Keys1}};
  74         false ->
  75             dets:insert(Tab, {{item,Key},Val}),
  76             Keys1 = trim(Tab, [Key|Keys], Max),
  77             {reply, ok, {Tab, Max, Keys1}}
  78     end;
  79 handle_call({fetch, Key}, _, {Tab, Max, Keys}) ->
  80     case member(Key, Keys) of
  81         true ->
  82             Keys1 = move_to_front(Key, Keys),
  83             [{_, Val}] = dets:lookup(Tab, {item, Key}),
  84             {reply, {ok, Val}, {Tab, Max, Keys1}};
  85         false ->
  86             {reply, not_found, {Tab, Max, Keys}}
  87     end;
  88 handle_call({flush, Key}, _, {Tab, Max, Keys}) ->
  89     case member(Key, Keys) of
  90         true ->
  91             Keys1 = remove(Key, Keys),
  92             dets:delete(Tab, {item, Key}),
  93             {reply, ok, {Tab, Max, Keys1}};
  94         false ->
  95             {reply, ok, {Tab, Max, Keys}}
  96     end;
  97 handle_call(clear, _, {Tab, Max, Keys}) ->
  98     Keys1 = trim(Tab, Keys, 0),
  99     {reply, ok, {Tab, Max, Keys}};
 100 handle_call({set_max,N}, _, {Tab, Max, Keys}) ->
 101     Keys1 = trim(Tab, Keys, N),
 102     {reply, ok, {Tab, N, Keys1}};
 103 handle_call(info, _, {Tab, Max, Keys}) ->
 104     {reply, {info, Max, Keys}, {Tab, Max, Keys}};
 105 handle_call(stop, _, {Tab, Max, Keys}) ->
 106     dets:insert(Tab, {max, Max}),
 107     dets:insert(Tab, {keys, Keys}),
 108     {stop, normal, ok, []}.
 109
 110 terminate(normal, _) ->
 111     true.
 112
 113
 114 get_key_or_default(Tab, Key, Default) ->
 115     case dets:lookup(Tab, Key) of
 116         [] ->
 117             Default;
 118         [{_,Val}] ->
 119             Val
 120     end.
 121
 122 %% trim(Tab, Keys, Max)
 123 %% Keys is an ordered list of keys
 124 %% If the length of Keys is > max then the list
 125 %% is trimmed to Max elements
 126 %% and the last elements are removed from the data base
 127
 128 trim(Tab, Keys, Max) ->
 129     case length(Keys) of
 130         N when N > Max ->
 131             trim1(N-Max, Tab, reverse(Keys));
 132         _ ->
 133             Keys
 134     end.
 135
 136 trim1(0, Tab, Keys) ->
 137     reverse(Keys);
 138 trim1(N, Tab, [H|T]) ->
 139     dets:delete(Tab, {item, H}),
 140     trim1(N-1, Tab, T).
 141
 142 %% moves H which is *known* to be a member of L to the front of L
 143
 144 %% -type move_to_front(A, [A]) -> [A].
 145
 146 move_to_front(H, L) -> [H|remove(H,L)].
 147
 148 remove(X, [X|T]) -> T;
 149 remove(X, [H|T]) -> [H|remove(X, T)].
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