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a0cb3900 JH |
1 | |
2 | package Memoize::Expire; | |
3 | # require 5.00556; | |
4 | use Carp; | |
5 | $DEBUG = 0; | |
899dc88a | 6 | $VERSION = '0.65'; |
a0cb3900 JH |
7 | |
8 | # This package will implement expiration by prepending a fixed-length header | |
9 | # to the font of the cached data. The format of the header will be: | |
10 | # (4-byte number of last-access-time) (For LRU when I implement it) | |
11 | # (4-byte expiration time: unsigned seconds-since-unix-epoch) | |
12 | # (2-byte number-of-uses-before-expire) | |
13 | ||
14 | sub _header_fmt () { "N N n" } | |
15 | sub _header_size () { length(_header_fmt) } | |
16 | ||
17 | # Usage: memoize func | |
18 | # TIE => [Memoize::Expire, LIFETIME => sec, NUM_USES => n, | |
19 | # TIE => [...] ] | |
20 | ||
21 | sub TIEHASH { | |
22 | my ($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; | |
899dc88a JH |
125 | use Memoize::Expire; |
126 | tie my %cache => 'Memoize::Expire', | |
a0cb3900 | 127 | LIFETIME => $lifetime, # In seconds |
899dc88a JH |
128 | NUM_USES => $n_uses; |
129 | ||
130 | memoize 'function', SCALAR_CACHE => [HASH => \%cache ]; | |
a0cb3900 JH |
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 | |
899dc88a | 138 | to the HASH feature. |
a0cb3900 JH |
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. | |
899dc88a JH |
142 | The layer presents a hash variable whose values expire whenever they |
143 | get too old, have been used too often, or both. You tell C<Memoize> to | |
144 | use this forgetful hash as its cache instead of the default, which is | |
145 | an ordinary hash. | |
a0cb3900 | 146 | |
899dc88a | 147 | To specify a real-time timeout, supply the C<LIFETIME> option with a |
a0cb3900 JH |
148 | numeric value. Cached data will expire after this many seconds, and |
149 | will be looked up afresh when it expires. When a data item is looked | |
150 | up afresh, its lifetime is reset. | |
151 | ||
899dc88a | 152 | If you specify C<NUM_USES> with an argument of I<n>, then each cached |
a0cb3900 JH |
153 | data item will be discarded and looked up afresh after the I<n>th time |
154 | you access it. When a data item is looked up afresh, its number of | |
155 | uses is reset. | |
156 | ||
157 | If you specify both arguments, data will be discarded from the cache | |
899dc88a JH |
158 | when either expiration condition holds. |
159 | ||
160 | Memoize::Expire uses a real hash internally to store the cached data. | |
161 | You can use the C<HASH> option to Memoize::Expire to supply a tied | |
162 | hash in place of the ordinary hash that Memoize::Expire will normally | |
163 | use. You can use this feature to add Memoize::Expire as a layer in | |
164 | between a persistent disk hash and Memoize. If you do this, you get a | |
165 | persistent disk cache whose entries expire automatically. For | |
166 | example: | |
167 | ||
168 | # Memoize | |
169 | # | | |
170 | # Memoize::Expire enforces data expiration policy | |
171 | # | | |
172 | # DB_File implements persistence of data in a disk file | |
173 | # | | |
174 | # Disk file | |
a0cb3900 JH |
175 | |
176 | use Memoize; | |
899dc88a | 177 | use Memoize::Expire; |
a0cb3900 | 178 | use DB_File; |
a0cb3900 | 179 | |
899dc88a JH |
180 | # Set up persistence |
181 | tie my %disk_cache => 'DB_File', $filename, O_CREAT|O_RDWR, 0666]; | |
a0cb3900 | 182 | |
899dc88a JH |
183 | # Set up expiration policy, supplying persistent hash as a target |
184 | tie my %cache => 'Memoize::Expire', | |
185 | LIFETIME => $lifetime, # In seconds | |
186 | NUM_USES => $n_uses, | |
187 | HASH => \%disk_cache; | |
188 | ||
189 | # Set up memoization, supplying expiring persistent hash for cache | |
190 | memoize 'function', SCALAR_CACHE => [ HASH => \%cache ]; | |
a0cb3900 JH |
191 | |
192 | =head1 INTERFACE | |
193 | ||
194 | There is nothing special about Memoize::Expire. It is just an | |
195 | example. If you don't like the policy that it implements, you are | |
196 | free to write your own expiration policy module that implements | |
197 | whatever policy you desire. Here is how to do that. Let us suppose | |
198 | that your module will be named MyExpirePolicy. | |
199 | ||
200 | Short summary: You need to create a package that defines four methods: | |
201 | ||
202 | =over 4 | |
203 | ||
204 | =item | |
205 | TIEHASH | |
206 | ||
207 | Construct and return cache object. | |
208 | ||
209 | =item | |
210 | EXISTS | |
211 | ||
212 | Given a function argument, is the corresponding function value in the | |
213 | cache, and if so, is it fresh enough to use? | |
214 | ||
215 | =item | |
216 | FETCH | |
217 | ||
218 | Given a function argument, look up the corresponding function value in | |
219 | the cache and return it. | |
220 | ||
221 | =item | |
222 | STORE | |
223 | ||
224 | Given a function argument and the corresponding function value, store | |
225 | them into the cache. | |
226 | ||
227 | =back | |
228 | ||
229 | The user who wants the memoization cache to be expired according to | |
230 | your policy will say so by writing | |
231 | ||
899dc88a JH |
232 | tie my %cache => 'MyExpirePolicy', args...; |
233 | memoize 'function', SCALAR_CACHE => [HASH => \%cache]; | |
a0cb3900 | 234 | |
899dc88a | 235 | This will invoke C<< MyExpirePolicy->TIEHASH(args) >>. |
a0cb3900 | 236 | MyExpirePolicy::TIEHASH should do whatever is appropriate to set up |
899dc88a | 237 | the cache, and it should return the cache object to the caller. |
a0cb3900 JH |
238 | |
239 | For example, MyExpirePolicy::TIEHASH might create an object that | |
240 | contains a regular Perl hash (which it will to store the cached | |
241 | values) and some extra information about the arguments and how old the | |
242 | data is and things like that. Let us call this object `C'. | |
243 | ||
244 | When Memoize needs to check to see if an entry is in the cache | |
899dc88a | 245 | already, it will invoke C<< C->EXISTS(key) >>. C<key> is the normalized |
a0cb3900 JH |
246 | function argument. MyExpirePolicy::EXISTS should return 0 if the key |
247 | is not in the cache, or if it has expired, and 1 if an unexpired value | |
248 | is in the cache. It should I<not> return C<undef>, because there is a | |
249 | bug in some versions of Perl that will cause a spurious FETCH if the | |
250 | EXISTS method returns C<undef>. | |
251 | ||
252 | If your EXISTS function returns true, Memoize will try to fetch the | |
899dc88a | 253 | cached value by invoking C<< C->FETCH(key) >>. MyExpirePolicy::FETCH should |
a0cb3900 JH |
254 | return the cached value. Otherwise, Memoize will call the memoized |
255 | function to compute the appropriate value, and will store it into the | |
899dc88a | 256 | cache by calling C<< C->STORE(key, value) >>. |
a0cb3900 JH |
257 | |
258 | Here is a very brief example of a policy module that expires each | |
259 | cache item after ten seconds. | |
260 | ||
261 | package Memoize::TenSecondExpire; | |
262 | ||
263 | sub TIEHASH { | |
899dc88a JH |
264 | my ($package, %args) = @_; |
265 | my $cache = $args{$HASH} || {}; | |
266 | bless $cache => $package; | |
a0cb3900 JH |
267 | } |
268 | ||
269 | sub EXISTS { | |
270 | my ($cache, $key) = @_; | |
271 | if (exists $cache->{$key} && | |
272 | $cache->{$key}{EXPIRE_TIME} > time) { | |
273 | return 1 | |
274 | } else { | |
275 | return 0; # Do NOT return `undef' here. | |
276 | } | |
277 | } | |
278 | ||
279 | sub FETCH { | |
280 | my ($cache, $key) = @_; | |
281 | return $cache->{$key}{VALUE}; | |
282 | } | |
283 | ||
284 | sub STORE { | |
285 | my ($cache, $key, $newvalue) = @_; | |
286 | $cache->{$key}{VALUE} = $newvalue; | |
287 | $cache->{$key}{EXPIRE_TIME} = time + 10; | |
288 | } | |
289 | ||
290 | To use this expiration policy, the user would say | |
291 | ||
292 | use Memoize; | |
899dc88a JH |
293 | tie my %cache10sec => 'Memoize::TenSecondExpire'; |
294 | memoize 'function', SCALAR_CACHE => [HASH => \%cache10sec]; | |
a0cb3900 JH |
295 | |
296 | Memoize would then call C<function> whenever a cached value was | |
297 | entirely absent or was older than ten seconds. | |
298 | ||
899dc88a JH |
299 | You should always support a C<HASH> argument to C<TIEHASH> that ties |
300 | the underlying cache so that the user can specify that the cache is | |
301 | also persistent or that it has some other interesting semantics. The | |
302 | example above demonstrates how to do this, as does C<Memozie::Expire>. | |
a0cb3900 JH |
303 | |
304 | Another sample module, C<Memoize::Saves>, is included with this | |
305 | package. It implements a policy that allows you to specify that | |
306 | certain function values whould always be looked up afresh. See the | |
307 | documentation for details. | |
308 | ||
309 | =head1 ALTERNATIVES | |
310 | ||
899dc88a JH |
311 | Brent Powers has a C<Memoize::ExpireLRU> module that was designed to |
312 | wotk with Memoize and provides expiration of least-recently-used data. | |
313 | The cache is held at a fixed number of entries, and when new data | |
314 | comes in, the least-recently used data is expired. See | |
315 | L<http://search.cpan.org/search?mode=module&query=ExpireLRU>. | |
316 | ||
a0cb3900 JH |
317 | Joshua Chamas's Tie::Cache module may be useful as an expiration |
318 | manager. (If you try this, let me know how it works out.) | |
319 | ||
320 | If you develop any useful expiration managers that you think should be | |
321 | distributed with Memoize, please let me know. | |
322 | ||
323 | =head1 CAVEATS | |
324 | ||
325 | This module is experimental, and may contain bugs. Please report bugs | |
326 | to the address below. | |
327 | ||
328 | Number-of-uses is stored as a 16-bit unsigned integer, so can't exceed | |
329 | 65535. | |
330 | ||
331 | Because of clock granularity, expiration times may occur up to one | |
332 | second sooner than you expect. For example, suppose you store a value | |
333 | with a lifetime of ten seconds, and you store it at 12:00:00.998 on a | |
334 | certain day. Memoize will look at the clock and see 12:00:00. Then | |
335 | 9.01 seconds later, at 12:00:10.008 you try to read it back. Memoize | |
336 | will look at the clock and see 12:00:10 and conclude that the value | |
337 | has expired. Solution: Build an expiration policy module that uses | |
338 | Time::HiRes to examine a clock with better granularity. Contributions | |
339 | are welcome. Send them to: | |
340 | ||
341 | =head1 AUTHOR | |
342 | ||
343 | Mark-Jason Dominus (mjd-perl-memoize+@plover.com) | |
344 | ||
345 | Mike Cariaso provided valuable insight into the best way to solve this | |
899dc88a | 346 | problem. |
a0cb3900 JH |
347 | |
348 | =head1 SEE ALSO | |
349 | ||
350 | perl(1) | |
351 | ||
352 | The Memoize man page. | |
353 | ||
354 | http://www.plover.com/~mjd/perl/Memoize/ (for news and updates) | |
355 | ||
356 | I maintain a mailing list on which I occasionally announce new | |
357 | versions of Memoize. The list is for announcements only, not | |
358 | discussion. To join, send an empty message to | |
359 | mjd-perl-memoize-request@Plover.com. | |
360 | ||
361 | =cut |