1SharedCache(3) User Contributed Perl Documentation SharedCache(3)
2
6 IPC::SharedCache - a Perl module to manage a cache in SysV IPC shared
7 memory.
8
10 use IPC::SharedCache;
11 # the cache is accessed using a tied hash.
13 tie %cache, 'IPC::SharedCache', ipc_key => 'AKEY',
14 load_callback => \&load,
15 validate_callback => \&validate;
16 # get an item from the cache
18 $config_file = $cache{'/some/path/to/some.config'};
19
21 This module provides a shared memory cache accessed as a tied hash.
22 Shared memory is an area of memory that is available to all processes.
24 It is accessed by choosing a key, the ipc_key arguement to tie. Every
25 process that accesses shared memory with the same key gets access to
26 the same region of memory. In some ways it resembles a file system,
27 but it is not hierarchical and it is resident in memory. This makes it
28 harder to use than a filesystem but much faster. The data in shared
29 memory persists until the machine is rebooted or it is explicitely
30 deleted.
31 This module attempts to make shared memory easy to use for one specific
33 application - a shared memory cache. For other uses of shared memory
34 see the documentation to the excelent module I use, IPC::ShareLite
35 (IPC::ShareLite).
36 A cache is a place where processes can store the results of their
38 computations for use at a later time, possibly by other instances of
39 the application. A good example of the use of a cache is a web server.
40 When a web server receieves a request for an html page it goes to the
41 file system to read it. This is pretty slow, so the web server will
42 probably save the file in memory and use the in memory copy the next
43 time a request for that file comes in, as long as the file hasn't
44 changed on disk. This certainly speeds things up but web servers have
45 to serve multiple clients at once, and that means multiple copies of
46 the in-memory data. If the web server uses a shared memory cache, like
47 the one this module provides, then all the servers can use the same
48 cache and much less memory is consumed.
49 This module handles all shared memory interaction using the
51 IPC::ShareLite module (version 0.06 and higher) and all data
52 serialization using Storable. See IPC::ShareLite and Storable for
53 details.
54
56 This module began its life as an internal piece of HTML::Template (see
57 HTML::Template). HTML::Template has the ability to maintain a cache of
58 parsed template structures when running in a persistent environment
59 like Apache/mod_perl. Since parsing a template from disk takes a fair
60 ammount of time this can provide a big performance gain. Unfortunately
61 it can also consume large ammounts of memory since each web server
62 maintains its own cache in its own memory space.
63 By using IPC::ShareLite and Storable (IPC::ShareLite and Storable),
65 HTML::Template was able to maintain a single shared cache of templates.
66 The downside was that HTML::Template's cache routines became
67 complicated by a lot of IPC code. My solution is to break out the IPC
68 cache mechanisms into their own module, IPC::SharedCache. Hopefully
69 over time it can become general enough to be usable by more than just
70 HTML::Template.
71
73 This module allows you to store data in shared memory and have it load
74 automatically when needed. You can also define a test to screen cached
75 data for vailidty - if the test fails the data will be reloaded. This
76 is useful for defining a max-age for cached data or keeping cached data
77 in sync with other resources. In the web server example above the
78 validation test would look to see wether the file had changed on disk.
79 To initialize this module you provide two callback subroutines. The
81 first is the "load_callback". This gets called when a user of the
82 cache requests an item from that is not yet present or is stale. It
83 must return a reference to the data structure that will be stored in
84 the cache. The second is the "validate_callback". This gets called on
85 every cache access - its job is to check the cached object for
86 freshness (and/or some other validity, of course). It must return true
87 or false. When it returns true, the cached object is valid and is
88 retained in the cache. When it returns false, the object is re-loaded
89 using the "load_callback" and the result is stored in the cache.
90 To use the module you just request entries for the objects you need.
92 If the object is present in the cache and the "validate_callback"
93 returns true, then you get the object from the cache. If not, the
94 object is loaded into the cache with the "load_callback" and returned
95 to you.
96 The cache can be used to store any perl data structures that can be
98 serialized by the Storable module. See Storable for details.
99
101 In this example a shared cache of files is maintained. The
102 "load_callback" reads the file from disk into the cache and the
103 "validate_callback" checks its modification time using stat(). Note
104 that the "load_callback" stores information into the cached object that
105 "validate_callback" uses to check the freshness of the cache.
106 # the "load_callback", loads the file from disk, storing its stat()
108 # information along with the file into the cache. The key in this
109 # case is the filename to load.
110 sub load_file {
111 my $key = shift;
112 open(FILE, $key) or die "Unable to open file named $key : $!");
114 # note the modification time of this file - the 9th element of a
116 # stat() is the modification time of the file.
117 my $mtime = (stat($key))[9];
118 # read the file into the variable $contents in 1k chunks
120 my ($buffer, $contents);
121 while(read(FILE, $buffer, 1024)) { $contents .= $buffer }
122 close(FILE);
123 # prepare the record to store in the cache
125 my %record = ( mtime => $mtime, contents => $contents );
126 # this record goes into the cache associated with $key, which is
128 # the filename. Notice that we're returning a reference to the
129 # data structure here.
130 return \%record;
131 }
132 # the "validate" callback, checks the mtime of the file on disk and
134 # compares it to the cache value. The $record is a reference to the
135 # cached values array returned from load_file above.
136 sub validate_file {
137 my ($key, $record) = @_;
138 # get the modification time out of the record
140 my $stored_mtime = $record->{mtime};
141 # get the current modification time from the filesystem - the 9th
143 # element of a stat() is the modification time of the file.
144 my $current_mtime = (stat($key))[9];
145 # compare and return the appropriate result.
147 if ($stored_mtime == $current_mtime) {
148 # the cached object is valid, return true
149 return 1;
150 } else {
151 # the cached object is stale, return false - load_callback will
152 # be called to load it afresh from disk.
153 return 0;
154 }
155 }
156 # now we can construct the IPC::SharedCache object, using as a root
158 # key 'SAMS'.
159 tie %cache 'IPC::SharedCache' ipc_key => 'SAMS',
161 load_callback => \&load_file,
162 validate_callback => \&validate_file;
163 # fetch an object from the cache - if it's already in the cache and
165 # validate_file() returns 1, then we'll get the cached file. If it's
166 # not in the cache, or validate_file returns 0, then load_file is
167 # called to load the file into the cache.
168 $config_file = $cache{'/some/path/to/some.config'};
170
172 The module implements a full tied hash interface, meaning that you can
173 use exists(), delete(), keys() and each(). However, in normal usage
174 all you'll need to do is to fetch values from the cache and possible
175 delete keys. Just in case you were wondering, exists() doesn't trigger
176 a cache load - it returns 1 if the given key is already in the cache
177 and 0 if it isn't. Similarily, keys() and each() operate on key/value
178 pairs already loaded into the cache.
179 The most important thing to realize is that there is no need to
181 explicitely store into the cache since the load_callback is called
182 automatically when it is necessary to load new data. If you find
183 yourself using more than just ""$data = $cache{'key'};"" you need to
184 make sure you really know what you're doing!
185 OPTIONS
187 There are a number parameters to tie that can be used to control the
188 behavior of IPC::SharedCache. Some of them are required, and some art
189 optional. Here's a preview:
190 tie %cache, 'IPC::SharedCache',
192 # required parameters
194 ipc_key => 'MYKI',
195 load_callback => \&load,
196 validate_callback => \&validate,
197 # optional parameters
199 ipc_mode => 0666,
200 ipc_segment_size => 1_000_000,
201 max_size => 50_000_000,
202 debug => 1;
203 ipc_key (required)
205 This is the unique identifier for the particular cache. It can be
206 specified as either a four-character string or an integer value. Any
207 script that wishes to access the cache must use the same ipc_key value.
208 You can use the ftok() function from IPC::SysV to generate this value,
209 see IPC::SysV for details. Using an ipc_key value that's already in
210 use by a non-IPC::SharedCache application will cause an error. Many
211 systems provide a utility called 'ipcs' to examine shared memory; you
212 can use it to check for existing shared memory usage before choosing
213 your ipc_key.
214 load_callback and validate_callback (required)
216 These parameters both specify callbacks for IPC::SharedCache to use
217 when the cache gets a request for a key. When you access the cache
218 ("$data = $cache{$key}"), the cache first looks to see if it already
219 has an object for the given key. If it doesn't, it calls the
220 load_callback and returns the result which is also stored in the cache.
221 Alternately, if it does have the object in the cache it calls the
222 validate_callback to check if the object is still good. If the
223 validate_callback returns true then object is good and is returned. If
224 the validate_callback returns false then the object is discarded and
225 the load_callback is called.
226 The load_callback recieves a single parameter - the requested key. It
228 must return a reference to the data object be stored in the cache.
229 Returning something that is not a reference results in an error.
230 The validate_callback recieves two parameters - the key and the
232 reference to the stored object. It must return true or false.
233 There are two ways to specify the callbacks. The first is simply to
235 specify a subroutine reference. This can be an anonymous subroutine or
236 a named one. Example:
237 tie %cache, 'IPC::SharedCache',
239 ipc_key => 'TEST',
240 load_callback => sub { ... },
241 validate_callback => \&validate;
242 The second method allows parameters to be passed to the subroutine when
244 it is called. This is done by specifying a reference to an array of
245 values, the first being the subroutine reference and the rest are
246 parameters for the subroutine. The extra parameters are passed in
247 before the IPC::SharedCache provided parameters. Example:
248 tie %cache, 'IPC::SharedCache',
250 ipc_key => 'TEST',
251 load_callback => [\&load, $arg1, $arg2, $arg3]
252 validate_callback => [\&validate, $self];
253 ipc_mode (optional)
255 This option specifies the access mode of the IPC cache. It defaults to
256 0666. See IPC::ShareLite for more information on IPC access modes.
257 The default should be fine for most applications.
258 ipc_segment_size (optional)
260 This option allows you to specify the "chunk size" of the IPC shared
261 memory segments. The default is 65,536, which is 64K. This is a good
262 default and is very portable. If you know that your system supports
263 larger IPC segment sizes and you know that your cache will be storing
264 large data items you might get better performance by increasing this
265 value.
266 This value places no limit on the size of an object stored in the cache
268 - IPC::SharedCache automatically spreads large objects across multiple
269 IPC segments.
270 WARNING: setting this value too low (below 1024 in my experience) can
272 cause errors.
273 max_size (optional)
275 By setting this parameter you are setting a logical maximum to the
276 ammount of data stored in the cache. When an item is stored in the
277 cache and this limit is exceded the oldest item (or items, as
278 necessary) in the cache will be deleted to make room. This value is
279 specified in bytes. It defaults to 0, which specifies no limit on the
280 size of the cache.
281 Turning this feature on costs a fair ammount of performance - how much
283 depends largely on home much data is being stored into the cache versus
284 the size of max_cache. In the worst case (where the max_size is set
285 much too low) this option can cause severe "thrashing" and negate the
286 benefit of maintaining a cache entirely.
287 NOTE: The size of the cache may in fact exceed this value - the book-
289 keeping data stored in the root segment is not counted towards the
290 total. Also, extra padding imposed by the ipc_segment_size is not
291 counted. This may change in the future if I learn that it would be
292 appropriate to count this padding as used memory. It is not clear to
293 me that all IPC implementations will really waste this memory.
294 debug (optional)
296 Set this option to 1 to see a whole bunch of text on STDERR about what
297 IPC::SharedCache is doing.
298
300 Two static functions are included in this package that are meant to be
301 used from the command-line.
302 walk
304 Walk prints out a detailed listing of the contents of a shared cache at
305 a given ipc_key. It provides information the current keys stored and a
306 dump of the objects stored in each key. Be warned, this can be quite a
307 lot of data! Also, you'll need the Data::Dumper module installed to
308 use 'walk'. You can get it on CPAN.
309 You can call walk like:
311 perl -MIPC::SharedCache -e 'IPC::SharedCache::walk AKEY'"
313 Example:
315 $ perl -MIPC::SharedCache -e 'IPC::SharedCache::walk MYKI'"
317 *===================*
318 IPC::SharedCache Root
319 *===================*
320 IPC_KEY: MYKI
321 ELEMENTS: 3
322 TOTAL SIZE: 99 bytes
323 KEYS: a, b, c
324 *=======*
326 Data List
327 *=======*
328 KEY: a
330 $CONTENTS = [
331 950760892,
332 950760892,
333 950760892
334 ];
335 KEY: b
338 $CONTENTS = [
339 950760892,
340 950760892,
341 950760892
342 ];
343 KEY: c
346 $CONTENTS = [
347 950760892,
348 950760892,
349 950760892
350 ];
351 remove
353 This function totally removes an entire cache given an ipc_key value.
354 This should not be done to a running system! Still, it's an invaluable
355 tool during development when flawed data may become 'stuck' in the
356 cache.
357 $ perl -MIPC::SharedCache -e 'IPC::SharedCache::remove MYKI'
359 This function is silent and thus may be usefully called from within a
361 script if desired.
362
364 I am aware of no bugs - if you find one please email me at
365 sam@tregar.com. When submitting bug reports, be sure to include full
366 details, including the VERSION of the module and a test script
367 demonstrating the problem.
368
370 I would like to thank Maurice Aubrey for making this module possible by
371 producing the excelent IPC::ShareLite.
372 The following people have contributed patches, ideas or new features:
374 Tim Bunce
376 Roland Mas
377 Drew Taylor
378 Ed Loehr
379 Maverick
380 Thanks everyone!
382
384 Sam Tregar, sam@tregar.com (you can also find me on the mailing list
385 for HTML::Template at htmltmpl@lists.vm.com - join it by sending a
386 blank message to htmltmpl-subscribe@lists.vm.com).
387
389 IPC::SharedCache - a Perl module to manage a SysV IPC shared cache.
390 Copyright (C) 2000 Sam Tregar (sam@tregar.com)
391 This program is free software; you can redistribute it and/or modify it
393 under the terms of the GNU General Public License as published by the
394 Free Software Foundation; either version 2 of the License, or (at your
395 option) any later version.
396 This program is distributed in the hope that it will be useful, but
398 WITHOUT ANY WARRANTY; without even the implied warranty of
399 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
400 General Public License for more details.
401 You should have received a copy of the GNU General Public License along
403 with this program; if not, write to the Free Software Foundation, Inc.,
404 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
405
407 Sam Tregar, sam@tregar.com
408
410 perl(1).
411perl v5.12.0 2000-03-23 SharedCache(3)