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 com‐
38 putations for use at a later time, possibly by other instances of the
39 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 IPC::Share‐
51 Lite module (version 0.06 and higher) and all data serialization using
52 Storable. See IPC::ShareLite and Storable for details.
53
55 This module began its life as an internal piece of HTML::Template (see
56 HTML::Template). HTML::Template has the ability to maintain a cache of
57 parsed template structures when running in a persistent environment
58 like Apache/mod_perl. Since parsing a template from disk takes a fair
59 ammount of time this can provide a big performance gain. Unfortunately
60 it can also consume large ammounts of memory since each web server
61 maintains its own cache in its own memory space.
62 By using IPC::ShareLite and Storable (IPC::ShareLite and Storable),
64 HTML::Template was able to maintain a single shared cache of templates.
65 The downside was that HTML::Template's cache routines became compli‐
66 cated by a lot of IPC code. My solution is to break out the IPC cache
67 mechanisms into their own module, IPC::SharedCache. Hopefully over
68 time it can become general enough to be usable by more than just
69 HTML::Template.
70
72 This module allows you to store data in shared memory and have it load
73 automatically when needed. You can also define a test to screen cached
74 data for vailidty - if the test fails the data will be reloaded. This
75 is useful for defining a max-age for cached data or keeping cached data
76 in sync with other resources. In the web server example above the val‐
77 idation test would look to see wether the file had changed on disk.
78 To initialize this module you provide two callback subroutines. The
80 first is the "load_callback". This gets called when a user of the
81 cache requests an item from that is not yet present or is stale. It
82 must return a reference to the data structure that will be stored in
83 the cache. The second is the "validate_callback". This gets called on
84 every cache access - its job is to check the cached object for fresh‐
85 ness (and/or some other validity, of course). It must return true or
86 false. When it returns true, the cached object is valid and is
87 retained in the cache. When it returns false, the object is re-loaded
88 using the "load_callback" and the result is stored in the cache.
89 To use the module you just request entries for the objects you need.
91 If the object is present in the cache and the "validate_callback"
92 returns true, then you get the object from the cache. If not, the
93 object is loaded into the cache with the "load_callback" and returned
94 to you.
95 The cache can be used to store any perl data structures that can be
97 serialized by the Storable module. See Storable for details.
98
100 In this example a shared cache of files is maintained. The "load_call‐
101 back" reads the file from disk into the cache and the "validate_call‐
102 back" checks its modification time using stat(). Note that the
103 "load_callback" stores information into the cached object that "vali‐
104 date_callback" uses to check the freshness of the cache.
105 # the "load_callback", loads the file from disk, storing its stat()
107 # information along with the file into the cache. The key in this
108 # case is the filename to load.
109 sub load_file {
110 my $key = shift;
111 open(FILE, $key) or die "Unable to open file named $key : $!");
113 # note the modification time of this file - the 9th element of a
115 # stat() is the modification time of the file.
116 my $mtime = (stat($key))[9];
117 # read the file into the variable $contents in 1k chunks
119 my ($buffer, $contents);
120 while(read(FILE, $buffer, 1024)) { $contents .= $buffer }
121 close(FILE);
122 # prepare the record to store in the cache
124 my %record = ( mtime => $mtime, contents => $contents );
125 # this record goes into the cache associated with $key, which is
127 # the filename. Notice that we're returning a reference to the
128 # data structure here.
129 return \%record;
130 }
131 # the "validate" callback, checks the mtime of the file on disk and
133 # compares it to the cache value. The $record is a reference to the
134 # cached values array returned from load_file above.
135 sub validate_file {
136 my ($key, $record) = @_;
137 # get the modification time out of the record
139 my $stored_mtime = $record->{mtime};
140 # get the current modification time from the filesystem - the 9th
142 # element of a stat() is the modification time of the file.
143 my $current_mtime = (stat($key))[9];
144 # compare and return the appropriate result.
146 if ($stored_mtime == $current_mtime) {
147 # the cached object is valid, return true
148 return 1;
149 } else {
150 # the cached object is stale, return false - load_callback will
151 # be called to load it afresh from disk.
152 return 0;
153 }
154 }
155 # now we can construct the IPC::SharedCache object, using as a root
157 # key 'SAMS'.
158 tie %cache 'IPC::SharedCache' ipc_key => 'SAMS',
160 load_callback => \&load_file,
161 validate_callback => \&validate_file;
162 # fetch an object from the cache - if it's already in the cache and
164 # validate_file() returns 1, then we'll get the cached file. If it's
165 # not in the cache, or validate_file returns 0, then load_file is
166 # called to load the file into the cache.
167 $config_file = $cache{'/some/path/to/some.config'};
169
171 The module implements a full tied hash interface, meaning that you can
172 use exists(), delete(), keys() and each(). However, in normal usage
173 all you'll need to do is to fetch values from the cache and possible
174 delete keys. Just in case you were wondering, exists() doesn't trigger
175 a cache load - it returns 1 if the given key is already in the cache
176 and 0 if it isn't. Similarily, keys() and each() operate on key/value
177 pairs already loaded into the cache.
178 The most important thing to realize is that there is no need to
180 explicitely store into the cache since the load_callback is called
181 automatically when it is necessary to load new data. If you find your‐
182 self using more than just ""$data = $cache{'key'};"" you need to make
183 sure you really know what you're doing!
184 OPTIONS
186 There are a number parameters to tie that can be used to control the
188 behavior of IPC::SharedCache. Some of them are required, and some art
189 optional. Here's a preview:
190 tie %cache, 'IPC::SharedCache',
192 # required parameters
194 ipc_key => 'MYKI',
195 load_callback => \&load,
196 validate_callback => \&validate,
197 # optional parameters
199 ipc_mode => 0666,
200 ipc_segment_size => 1_000_000,
201 max_size => 50_000_000,
202 debug => 1;
203 ipc_key (required)
205 This is the unique identifier for the particular cache. It can be
207 specified as either a four-character string or an integer value. Any
208 script that wishes to access the cache must use the same ipc_key value.
209 You can use the ftok() function from IPC::SysV to generate this value,
210 see IPC::SysV for details. Using an ipc_key value that's already in
211 use by a non-IPC::SharedCache application will cause an error. Many
212 systems provide a utility called 'ipcs' to examine shared memory; you
213 can use it to check for existing shared memory usage before choosing
214 your ipc_key.
215 load_callback and validate_callback (required)
217 These parameters both specify callbacks for IPC::SharedCache to use
219 when the cache gets a request for a key. When you access the cache
220 ("$data = $cache{$key}"), the cache first looks to see if it already
221 has an object for the given key. If it doesn't, it calls the
222 load_callback and returns the result which is also stored in the cache.
223 Alternately, if it does have the object in the cache it calls the vali‐
224 date_callback to check if the object is still good. If the vali‐
225 date_callback returns true then object is good and is returned. If the
226 validate_callback returns false then the object is discarded and the
227 load_callback is called.
228 The load_callback recieves a single parameter - the requested key. It
230 must return a reference to the data object be stored in the cache.
231 Returning something that is not a reference results in an error.
232 The validate_callback recieves two parameters - the key and the refer‐
234 ence to the stored object. It must return true or false.
235 There are two ways to specify the callbacks. The first is simply to
237 specify a subroutine reference. This can be an anonymous subroutine or
238 a named one. Example:
239 tie %cache, 'IPC::SharedCache',
241 ipc_key => 'TEST',
242 load_callback => sub { ... },
243 validate_callback => \&validate;
244 The second method allows parameters to be passed to the subroutine when
246 it is called. This is done by specifying a reference to an array of
247 values, the first being the subroutine reference and the rest are
248 parameters for the subroutine. The extra parameters are passed in
249 before the IPC::SharedCache provided parameters. Example:
250 tie %cache, 'IPC::SharedCache',
252 ipc_key => 'TEST',
253 load_callback => [\&load, $arg1, $arg2, $arg3]
254 validate_callback => [\&validate, $self];
255 ipc_mode (optional)
257 This option specifies the access mode of the IPC cache. It defaults to
259 0666. See IPC::ShareLite for more information on IPC access modes.
260 The default should be fine for most applications.
261 ipc_segment_size (optional)
263 This option allows you to specify the "chunk size" of the IPC shared
265 memory segments. The default is 65,536, which is 64K. This is a good
266 default and is very portable. If you know that your system supports
267 larger IPC segment sizes and you know that your cache will be storing
268 large data items you might get better performance by increasing this
269 value.
270 This value places no limit on the size of an object stored in the cache
272 - IPC::SharedCache automatically spreads large objects across multiple
273 IPC segments.
274 WARNING: setting this value too low (below 1024 in my experience) can
276 cause errors.
277 max_size (optional)
279 By setting this parameter you are setting a logical maximum to the
281 ammount of data stored in the cache. When an item is stored in the
282 cache and this limit is exceded the oldest item (or items, as neces‐
283 sary) in the cache will be deleted to make room. This value is speci‐
284 fied in bytes. It defaults to 0, which specifies no limit on the size
285 of the cache.
286 Turning this feature on costs a fair ammount of performance - how much
288 depends largely on home much data is being stored into the cache versus
289 the size of max_cache. In the worst case (where the max_size is set
290 much too low) this option can cause severe "thrashing" and negate the
291 benefit of maintaining a cache entirely.
292 NOTE: The size of the cache may in fact exceed this value - the book-
294 keeping data stored in the root segment is not counted towards the
295 total. Also, extra padding imposed by the ipc_segment_size is not
296 counted. This may change in the future if I learn that it would be
297 appropriate to count this padding as used memory. It is not clear to
298 me that all IPC implementations will really waste this memory.
299 debug (optional)
301 Set this option to 1 to see a whole bunch of text on STDERR about what
303 IPC::SharedCache is doing.
304
306 Two static functions are included in this package that are meant to be
307 used from the command-line.
308 walk
310 Walk prints out a detailed listing of the contents of a shared cache at
312 a given ipc_key. It provides information the current keys stored and a
313 dump of the objects stored in each key. Be warned, this can be quite a
314 lot of data! Also, you'll need the Data::Dumper module installed to
315 use 'walk'. You can get it on CPAN.
316 You can call walk like:
318 perl -MIPC::SharedCache -e 'IPC::SharedCache::walk AKEY'"
320 Example:
322 $ perl -MIPC::SharedCache -e 'IPC::SharedCache::walk MYKI'"
324 *===================*
325 IPC::SharedCache Root
326 *===================*
327 IPC_KEY: MYKI
328 ELEMENTS: 3
329 TOTAL SIZE: 99 bytes
330 KEYS: a, b, c
331 *=======*
333 Data List
334 *=======*
335 KEY: a
337 $CONTENTS = [
338 950760892,
339 950760892,
340 950760892
341 ];
342 KEY: b
344 $CONTENTS = [
345 950760892,
346 950760892,
347 950760892
348 ];
349 KEY: c
351 $CONTENTS = [
352 950760892,
353 950760892,
354 950760892
355 ];
356 remove
358 This function totally removes an entire cache given an ipc_key value.
360 This should not be done to a running system! Still, it's an invaluable
361 tool during development when flawed data may become 'stuck' in the
362 cache.
363 $ perl -MIPC::SharedCache -e 'IPC::SharedCache::remove MYKI'
365 This function is silent and thus may be usefully called from within a
367 script if desired.
368
370 I am aware of no bugs - if you find one please email me at sam@tre‐
371 gar.com. When submitting bug reports, be sure to include full details,
372 including the VERSION of the module and a test script demonstrating the
373 problem.
374
376 I would like to thank Maurice Aubrey for making this module possible by
377 producing the excelent IPC::ShareLite.
378 The following people have contributed patches, ideas or new features:
380 Tim Bunce
382 Roland Mas
383 Drew Taylor
384 Ed Loehr
385 Maverick
386 Thanks everyone!
388
390 Sam Tregar, sam@tregar.com (you can also find me on the mailing list
391 for HTML::Template at htmltmpl@lists.vm.com - join it by sending a
392 blank message to htmltmpl-subscribe@lists.vm.com).
393
395 IPC::SharedCache - a Perl module to manage a SysV IPC shared cache.
396 Copyright (C) 2000 Sam Tregar (sam@tregar.com)
397 This program is free software; you can redistribute it and/or modify it
399 under the terms of the GNU General Public License as published by the
400 Free Software Foundation; either version 2 of the License, or (at your
401 option) any later version.
402 This program is distributed in the hope that it will be useful, but
404 WITHOUT ANY WARRANTY; without even the implied warranty of MER‐
405 CHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
406 Public License for more details.
407 You should have received a copy of the GNU General Public License along
409 with this program; if not, write to the Free Software Foundation, Inc.,
410 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
411
413 Sam Tregar, sam@tregar.com
414
416 perl(1).
417perl v5.8.8 2000-03-23 SharedCache(3)