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a0cb3900 JH |
1 | |
2 | package Memoize::Expire; | |
3 | # require 5.00556; | |
4 | use Carp; | |
5 | $DEBUG = 0; | |
6 | $VERSION = '0.51'; | |
7 | ||
8 | # This package will implement expiration by prepending a fixed-length header | |
9 | # to the font of the cached data. The format of the header will be: | |
10 | # (4-byte number of last-access-time) (For LRU when I implement it) | |
11 | # (4-byte expiration time: unsigned seconds-since-unix-epoch) | |
12 | # (2-byte number-of-uses-before-expire) | |
13 | ||
14 | sub _header_fmt () { "N N n" } | |
15 | sub _header_size () { length(_header_fmt) } | |
16 | ||
17 | # Usage: memoize func | |
18 | # TIE => [Memoize::Expire, LIFETIME => sec, NUM_USES => n, | |
19 | # TIE => [...] ] | |
20 | ||
21 | sub TIEHASH { | |
22 | my ($package, %args) = @_; | |
23 | my %cache; | |
24 | if ($args{TIE}) { | |
25 | my ($module, @opts) = @{$args{TIE}}; | |
26 | my $modulefile = $module . '.pm'; | |
27 | $modulefile =~ s{::}{/}g; | |
28 | eval { require $modulefile }; | |
29 | if ($@) { | |
30 | croak "Memoize::Expire: Couldn't load hash tie module `$module': $@; aborting"; | |
31 | } | |
32 | my $rc = (tie %cache => $module, @opts); | |
33 | unless ($rc) { | |
34 | croak "Memoize::Expire: Couldn't tie hash to `$module': $@; aborting"; | |
35 | } | |
36 | } | |
37 | $args{LIFETIME} ||= 0; | |
38 | $args{NUM_USES} ||= 0; | |
39 | $args{C} = \%cache; | |
40 | bless \%args => $package; | |
41 | } | |
42 | ||
43 | sub STORE { | |
44 | $DEBUG and print STDERR " >> Store $_[1] $_[2]\n"; | |
45 | my ($self, $key, $value) = @_; | |
46 | my $expire_time = $self->{LIFETIME} > 0 ? $self->{LIFETIME} + time : 0; | |
47 | # The call that results in a value to store into the cache is the | |
48 | # first of the NUM_USES allowed calls. | |
49 | my $header = _make_header(time, $expire_time, $self->{NUM_USES}-1); | |
50 | $self->{C}{$key} = $header . $value; | |
51 | $value; | |
52 | } | |
53 | ||
54 | sub FETCH { | |
55 | $DEBUG and print STDERR " >> Fetch cached value for $_[1]\n"; | |
56 | my ($data, $last_access, $expire_time, $num_uses_left) = _get_item($_[0]{C}{$_[1]}); | |
57 | $DEBUG and print STDERR " >> (ttl: ", ($expire_time-time), ", nuses: $num_uses_left)\n"; | |
58 | $num_uses_left--; | |
59 | $last_access = time; | |
60 | _set_header(@_, $data, $last_access, $expire_time, $num_uses_left); | |
61 | $data; | |
62 | } | |
63 | ||
64 | sub EXISTS { | |
65 | $DEBUG and print STDERR " >> Exists $_[1]\n"; | |
66 | unless (exists $_[0]{C}{$_[1]}) { | |
67 | $DEBUG and print STDERR " Not in underlying hash at all.\n"; | |
68 | return 0; | |
69 | } | |
70 | my $item = $_[0]{C}{$_[1]}; | |
71 | my ($last_access, $expire_time, $num_uses_left) = _get_header($item); | |
72 | my $ttl = $expire_time - time; | |
73 | if ($DEBUG) { | |
74 | $_[0]{LIFETIME} and print STDERR " Time to live for this item: $ttl\n"; | |
75 | $_[0]{NUM_USES} and print STDERR " Uses remaining: $num_uses_left\n"; | |
76 | } | |
77 | if ( (! $_[0]{LIFETIME} || $expire_time > time) | |
78 | && (! $_[0]{NUM_USES} || $num_uses_left > 0 )) { | |
79 | $DEBUG and print STDERR " (Still good)\n"; | |
80 | return 1; | |
81 | } else { | |
82 | $DEBUG and print STDERR " (Expired)\n"; | |
83 | return 0; | |
84 | } | |
85 | } | |
86 | ||
87 | # Arguments: last access time, expire time, number of uses remaining | |
88 | sub _make_header { | |
89 | pack "N N n", @_; | |
90 | } | |
91 | ||
92 | sub _strip_header { | |
93 | substr($_[0], 10); | |
94 | } | |
95 | ||
96 | # Arguments: last access time, expire time, number of uses remaining | |
97 | sub _set_header { | |
98 | my ($self, $key, $data, @header) = @_; | |
99 | $self->{C}{$key} = _make_header(@header) . $data; | |
100 | } | |
101 | ||
102 | sub _get_item { | |
103 | my $data = substr($_[0], 10); | |
104 | my @header = unpack "N N n", substr($_[0], 0, 10); | |
105 | # print STDERR " >> _get_item: $data => $data @header\n"; | |
106 | ($data, @header); | |
107 | } | |
108 | ||
109 | # Return last access time, expire time, number of uses remaining | |
110 | sub _get_header { | |
111 | unpack "N N n", substr($_[0], 0, 10); | |
112 | } | |
113 | ||
114 | 1; | |
115 | ||
116 | # Below is the stub of documentation for your module. You better edit it! | |
117 | ||
118 | =head1 NAME | |
119 | ||
120 | Memoize::Expire - Plug-in module for automatic expiration of memoized values | |
121 | ||
122 | =head1 SYNOPSIS | |
123 | ||
124 | use Memoize; | |
125 | memoize 'function', | |
126 | SCALAR_CACHE => [TIE, Memoize::Expire, | |
127 | LIFETIME => $lifetime, # In seconds | |
128 | NUM_USES => $n_uses, | |
129 | TIE => [Module, args...], | |
130 | ], | |
131 | ||
132 | =head1 DESCRIPTION | |
133 | ||
134 | Memoize::Expire is a plug-in module for Memoize. It allows the cached | |
135 | values for memoized functions to expire automatically. This manual | |
136 | assumes you are already familiar with the Memoize module. If not, you | |
137 | should study that manual carefully first, paying particular attention | |
138 | to the TIE feature. | |
139 | ||
140 | Memoize::Expire is a layer of software that you can insert in between | |
141 | Memoize itself and whatever underlying package implements the cache. | |
142 | (By default, plain hash variables implement the cache.) The layer | |
143 | expires cached values whenever they get too old, have been used too | |
144 | often, or both. | |
145 | ||
146 | To specify a real-time timeout, supply the LIFETIME option with a | |
147 | numeric value. Cached data will expire after this many seconds, and | |
148 | will be looked up afresh when it expires. When a data item is looked | |
149 | up afresh, its lifetime is reset. | |
150 | ||
151 | If you specify NUM_USES with an argument of I<n>, then each cached | |
152 | data item will be discarded and looked up afresh after the I<n>th time | |
153 | you access it. When a data item is looked up afresh, its number of | |
154 | uses is reset. | |
155 | ||
156 | If you specify both arguments, data will be discarded from the cache | |
157 | when either expiration condition holds. | |
158 | ||
159 | If you want the cache to persist between invocations of your program, | |
160 | supply a TIE option to specify the package name and arguments for a | |
161 | the tied hash that will implement the persistence. For example: | |
162 | ||
163 | use Memoize; | |
164 | use DB_File; | |
165 | memoize 'function', | |
166 | SCALAR_CACHE => [TIE, Memoize::Expire, | |
167 | LIFETIME => $lifetime, # In seconds | |
168 | NUM_USES => $n_uses, | |
169 | TIE => [DB_File, $filename, O_CREAT|O_RDWR, 0666], | |
170 | ], ...; | |
171 | ||
172 | ||
173 | ||
174 | =head1 INTERFACE | |
175 | ||
176 | There is nothing special about Memoize::Expire. It is just an | |
177 | example. If you don't like the policy that it implements, you are | |
178 | free to write your own expiration policy module that implements | |
179 | whatever policy you desire. Here is how to do that. Let us suppose | |
180 | that your module will be named MyExpirePolicy. | |
181 | ||
182 | Short summary: You need to create a package that defines four methods: | |
183 | ||
184 | =over 4 | |
185 | ||
186 | =item | |
187 | TIEHASH | |
188 | ||
189 | Construct and return cache object. | |
190 | ||
191 | =item | |
192 | EXISTS | |
193 | ||
194 | Given a function argument, is the corresponding function value in the | |
195 | cache, and if so, is it fresh enough to use? | |
196 | ||
197 | =item | |
198 | FETCH | |
199 | ||
200 | Given a function argument, look up the corresponding function value in | |
201 | the cache and return it. | |
202 | ||
203 | =item | |
204 | STORE | |
205 | ||
206 | Given a function argument and the corresponding function value, store | |
207 | them into the cache. | |
208 | ||
209 | =back | |
210 | ||
211 | The user who wants the memoization cache to be expired according to | |
212 | your policy will say so by writing | |
213 | ||
214 | memoize 'function', | |
215 | SCALAR_CACHE => [TIE, MyExpirePolicy, args...]; | |
216 | ||
217 | This will invoke MyExpirePolicy->TIEHASH(args). | |
218 | MyExpirePolicy::TIEHASH should do whatever is appropriate to set up | |
219 | the cache, and it should return the cache object to the caller. | |
220 | ||
221 | For example, MyExpirePolicy::TIEHASH might create an object that | |
222 | contains a regular Perl hash (which it will to store the cached | |
223 | values) and some extra information about the arguments and how old the | |
224 | data is and things like that. Let us call this object `C'. | |
225 | ||
226 | When Memoize needs to check to see if an entry is in the cache | |
227 | already, it will invoke C->EXISTS(key). C<key> is the normalized | |
228 | function argument. MyExpirePolicy::EXISTS should return 0 if the key | |
229 | is not in the cache, or if it has expired, and 1 if an unexpired value | |
230 | is in the cache. It should I<not> return C<undef>, because there is a | |
231 | bug in some versions of Perl that will cause a spurious FETCH if the | |
232 | EXISTS method returns C<undef>. | |
233 | ||
234 | If your EXISTS function returns true, Memoize will try to fetch the | |
235 | cached value by invoking C->FETCH(key). MyExpirePolicy::FETCH should | |
236 | return the cached value. Otherwise, Memoize will call the memoized | |
237 | function to compute the appropriate value, and will store it into the | |
238 | cache by calling C->STORE(key, value). | |
239 | ||
240 | Here is a very brief example of a policy module that expires each | |
241 | cache item after ten seconds. | |
242 | ||
243 | package Memoize::TenSecondExpire; | |
244 | ||
245 | sub TIEHASH { | |
246 | my ($package) = @_; | |
247 | my %cache; | |
248 | bless \%cache => $package; | |
249 | } | |
250 | ||
251 | sub EXISTS { | |
252 | my ($cache, $key) = @_; | |
253 | if (exists $cache->{$key} && | |
254 | $cache->{$key}{EXPIRE_TIME} > time) { | |
255 | return 1 | |
256 | } else { | |
257 | return 0; # Do NOT return `undef' here. | |
258 | } | |
259 | } | |
260 | ||
261 | sub FETCH { | |
262 | my ($cache, $key) = @_; | |
263 | return $cache->{$key}{VALUE}; | |
264 | } | |
265 | ||
266 | sub STORE { | |
267 | my ($cache, $key, $newvalue) = @_; | |
268 | $cache->{$key}{VALUE} = $newvalue; | |
269 | $cache->{$key}{EXPIRE_TIME} = time + 10; | |
270 | } | |
271 | ||
272 | To use this expiration policy, the user would say | |
273 | ||
274 | use Memoize; | |
275 | memoize 'function', | |
276 | SCALAR_CACHE => [TIE, Memoize::TenSecondExpire]; | |
277 | ||
278 | Memoize would then call C<function> whenever a cached value was | |
279 | entirely absent or was older than ten seconds. | |
280 | ||
281 | It's nice if you allow a C<TIE> argument to C<TIEHASH> that ties the | |
282 | underlying cache so that the user can specify that the cache is | |
283 | persistent or that it has some other interesting semantics. The | |
284 | sample C<Memoize::Expire> module demonstrates how to do this. It | |
285 | implements a policy that expires cache items when they get too old or | |
286 | when they have been accessed too many times. | |
287 | ||
288 | Another sample module, C<Memoize::Saves>, is included with this | |
289 | package. It implements a policy that allows you to specify that | |
290 | certain function values whould always be looked up afresh. See the | |
291 | documentation for details. | |
292 | ||
293 | =head1 ALTERNATIVES | |
294 | ||
295 | Joshua Chamas's Tie::Cache module may be useful as an expiration | |
296 | manager. (If you try this, let me know how it works out.) | |
297 | ||
298 | If you develop any useful expiration managers that you think should be | |
299 | distributed with Memoize, please let me know. | |
300 | ||
301 | =head1 CAVEATS | |
302 | ||
303 | This module is experimental, and may contain bugs. Please report bugs | |
304 | to the address below. | |
305 | ||
306 | Number-of-uses is stored as a 16-bit unsigned integer, so can't exceed | |
307 | 65535. | |
308 | ||
309 | Because of clock granularity, expiration times may occur up to one | |
310 | second sooner than you expect. For example, suppose you store a value | |
311 | with a lifetime of ten seconds, and you store it at 12:00:00.998 on a | |
312 | certain day. Memoize will look at the clock and see 12:00:00. Then | |
313 | 9.01 seconds later, at 12:00:10.008 you try to read it back. Memoize | |
314 | will look at the clock and see 12:00:10 and conclude that the value | |
315 | has expired. Solution: Build an expiration policy module that uses | |
316 | Time::HiRes to examine a clock with better granularity. Contributions | |
317 | are welcome. Send them to: | |
318 | ||
319 | =head1 AUTHOR | |
320 | ||
321 | Mark-Jason Dominus (mjd-perl-memoize+@plover.com) | |
322 | ||
323 | Mike Cariaso provided valuable insight into the best way to solve this | |
324 | problem. | |
325 | ||
326 | =head1 SEE ALSO | |
327 | ||
328 | perl(1) | |
329 | ||
330 | The Memoize man page. | |
331 | ||
332 | http://www.plover.com/~mjd/perl/Memoize/ (for news and updates) | |
333 | ||
334 | I maintain a mailing list on which I occasionally announce new | |
335 | versions of Memoize. The list is for announcements only, not | |
336 | discussion. To join, send an empty message to | |
337 | mjd-perl-memoize-request@Plover.com. | |
338 | ||
339 | =cut |