DBM_Filter(3pm)

1DBM_Filter(3pm)        Perl Programmers Reference Guide        DBM_Filter(3pm)
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NAME

6       DBM_Filter -- Filter DBM keys/values
7

SYNOPSIS

9           use DBM_Filter ;
10           use SDBM_File; # or DB_File, or GDBM_File, or NDBM_File, or ODBM_File
11
12           $db = tie %hash, ...
13
14           $db->Filter_Push(Fetch => sub {...},
15                            Store => sub {...});
16
17           $db->Filter_Push('my_filter1');
18           $db->Filter_Push('my_filter2', params...);
19
20           $db->Filter_Key_Push(...) ;
21           $db->Filter_Value_Push(...) ;
22
23           $db->Filter_Pop();
24           $db->Filtered();
25
26           package DBM_Filter::my_filter1;
27
28           sub Store { ... }
29           sub Fetch { ... }
30
31           1;
32
33           package DBM_Filter::my_filter2;
34
35           sub Filter
36           {
37               my @opts = @_;
38               ...
39               return (
40                   sub Store { ... },
41                   sub Fetch { ... } );
42           }
43
44           1;
45

DESCRIPTION

47       This module provides an interface that allows filters to be applied to
48       tied Hashes associated with DBM files. It builds on the DBM Filter
49       hooks that are present in all the *DB*_File modules included with the
50       standard Perl source distribution from version 5.6.1 onwards. In addi‐
51       tion to the *DB*_File modules distributed with Perl, the BerkeleyDB
52       module, available on CPAN, supports the DBM Filter hooks. See perldbm‐
53       filter for more details on the DBM Filter hooks.
54

What is a DBM Filter?

56       A DBM Filter allows the keys and/or values in a tied hash to be modi‐
57       fied by some user-defined code just before it is written to the DBM
58       file and just after it is read back from the DBM file. For example,
59       this snippet of code
60
61           $some_hash{"abc"} = 42;
62
63       could potentially trigger two filters, one for the writing of the key
64       "abc" and another for writing the value 42.  Similarly, this snippet
65
66           my ($key, $value) = each %some_hash
67
68       will trigger two filters, one for the reading of the key and one for
69       the reading of the value.
70
71       Like the existing DBM Filter functionality, this module arranges for
72       the $_ variable to be populated with the key or value that a filter
73       will check. This usually means that most DBM filters tend to be very
74       short.
75
76       So what's new?
77
78       The main enhancements over the standard DBM Filter hooks are:
79
80       ·   A cleaner interface.
81
82       ·   The ability to easily apply multiple filters to a single DBM file.
83
84       ·   The ability to create "canned" filters. These allow commonly used
85           filters to be packaged into a stand-alone module.
86

METHODS

88       This module will arrange for the following methods to be available via
89       the object returned from the "tie" call.
90
91       $db->Filter_Push()
92
93       $db->Filter_Key_Push()
94
95       $db->Filter_Value_Push()
96
97       Add a filter to filter stack for the database, $db. The three formats
98       vary only in whether they apply to the DBM key, the DBM value or both.
99
100       Filter_Push
101            The filter is applied to both keys and values.
102
103       Filter_Key_Push
104            The filter is applied to the key only.
105
106       Filter_Value_Push
107            The filter is applied to the value only.
108
109       $db->Filter_Pop()
110
111       Removes the last filter that was applied to the DBM file associated
112       with $db, if present.
113
114       $db->Filtered()
115
116       Returns TRUE if there are any filters applied to the DBM associated
117       with $db.  Otherwise returns FALSE.
118

Writing a Filter

120       Filters can be created in two main ways
121
122       Immediate Filters
123
124       An immediate filter allows you to specify the filter code to be used at
125       the point where the filter is applied to a dbm. In this mode the Fil‐
126       ter_*_Push methods expects to receive exactly two parameters.
127
128           my $db = tie %hash, 'SDBM_File', ...
129           $db->Filter_Push( Store => sub { },
130                             Fetch => sub { });
131
132       The code reference associated with "Store" will be called before any
133       key/value is written to the database and the code reference associated
134       with "Fetch" will be called after any key/value is read from the data‐
135       base.
136
137       For example, here is a sample filter that adds a trailing NULL charac‐
138       ter to all strings before they are written to the DBM file, and removes
139       the trailing NULL when they are read from the DBM file
140
141           my $db = tie %hash, 'SDBM_File', ...
142           $db->Filter_Push( Store => sub { $_ .= "\x00" ; },
143                             Fetch => sub { s/\x00$// ;    });
144
145       Points to note:
146
147       1.   Both the Store and Fetch filters manipulate $_.
148
149       Canned Filters
150
151       Immediate filters are useful for one-off situations. For more generic
152       problems it can be useful to package the filter up in its own module.
153
154       The usage is for a canned filter is:
155
156           $db->Filter_Push("name", params)
157
158       where
159
160       "name"
161            is the name of the module to load. If the string specified does
162            not contain the package separator characters "::", it is assumed
163            to refer to the full module name "DBM_Filter::name". This means
164            that the full names for canned filters, "null" and "utf8",
165            included with this module are:
166
167                DBM_Filter::null
168                DBM_Filter::utf8
169
170       params
171            any optional parameters that need to be sent to the filter. See
172            the encode filter for an example of a module that uses parameters.
173
174       The module that implements the canned filter can take one of two forms.
175       Here is a template for the first
176
177           package DBM_Filter::null ;
178
179           use strict;
180           use warnings;
181
182           sub Store
183           {
184               # store code here
185           }
186
187           sub Fetch
188           {
189               # fetch code here
190           }
191
192           1;
193
194       Notes:
195
196       1.   The package name uses the "DBM_Filter::" prefix.
197
198       2.   The module must have both a Store and a Fetch method. If only one
199            is present, or neither are present, a fatal error will be thrown.
200
201       The second form allows the filter to hold state information using a
202       closure, thus:
203
204           package DBM_Filter::encoding ;
205
206           use strict;
207           use warnings;
208
209           sub Filter
210           {
211               my @params = @_ ;
212
213               ...
214               return {
215                   Store   => sub { $_ = $encoding->encode($_) },
216                   Fetch   => sub { $_ = $encoding->decode($_) }
217                   } ;
218           }
219
220           1;
221
222       In this instance the "Store" and "Fetch" methods are encapsulated
223       inside a "Filter" method.
224

Filters Included

226       A number of canned filers are provided with this module. They cover a
227       number of the main areas that filters are needed when interfacing with
228       DBM files. They also act as templates for your own filters.
229
230       The filter included are:
231
232       * utf8
233            This module will ensure that all data written to the DBM will be
234            encoded in UTF-8.
235
236            This module needs the Encode module.
237
238       * encode
239            Allows you to choose the character encoding will be store in the
240            DBM file.
241
242       * compress
243            This filter will compress all data before it is written to the
244            database and uncompressed it on reading.
245
246            This module needs Compress::Zlib.
247
248       * int32
249            This module is used when interoperating with a C/C++ application
250            that uses a C int as either the key and/or value in the DBM file.
251
252       * null
253            This module ensures that all data written to the DBM file is null
254            terminated. This is useful when you have a perl script that needs
255            to interoperate with a DBM file that a C program also uses. A
256            fairly common issue is for the C application to include the termi‐
257            nating null in a string when it writes to the DBM file. This fil‐
258            ter will ensure that all data written to the DBM file can be read
259            by the C application.
260

NOTES

262       Maintain Round Trip Integrity
263
264       When writing a DBM filter it is very important to ensure that it is
265       possible to retrieve all data that you have written when the DBM filter
266       is in place. In practice, this means that whatever transformation is
267       applied to the data in the Store method, the exact inverse operation
268       should be applied in the Fetch method.
269
270       If you don't provide an exact inverse transformation, you will find
271       that code like this will not behave as you expect.
272
273            while (my ($k, $v) = each %hash)
274            {
275                ...
276            }
277
278       Depending on the transformation, you will find that one or more of the
279       following will happen
280
281       1    The loop will never terminate.
282
283       2    Too few records will be retrieved.
284
285       3    Too many will be retrieved.
286
287       4    The loop will do the right thing for a while, but it will unex‐
288            pectedly fail.
289
290       Don't mix filtered & non-filtered data in the same database file.
291
292       This is just a restatement of the previous section. Unless you are com‐
293       pletely certain you know what you are doing, avoid mixing filtered &
294       non-filtered data.
295

EXAMPLE

297       Say you need to interoperate with a legacy C application that stores
298       keys as C ints and the values and null terminated UTF-8 strings. Here
299       is how you would set that up
300
301           my $db = tie %hash, 'SDBM_File', ...
302
303           $db->Filter_Key_Push('int32') ;
304
305           $db->Filter_Value_Push('utf8');
306           $db->Filter_Value_Push('null');
307

AUTHOR

312       Paul Marquess <pmqs@cpan.org>
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316perl v5.8.8                       2001-09-21                   DBM_Filter(3pm)