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, GDBM_File, 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
51       addition to the *DB*_File modules distributed with Perl, the BerkeleyDB
52       module, available on CPAN, supports the DBM Filter hooks. See
53       perldbmfilter 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
57       modified 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           $some_hash{"abc"} = 42;
62
63       could potentially trigger two filters, one for the writing of the key
64       "abc" and another for writing the value 42.  Similarly, this snippet
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66           my ($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       The main enhancements over the standard DBM Filter hooks are:
78
79       ·   A cleaner interface.
80
81       ·   The ability to easily apply multiple filters to a single DBM file.
82
83       ·   The ability to create "canned" filters. These allow commonly used
84           filters to be packaged into a stand-alone module.
85

METHODS

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

Writing a Filter

112       Filters can be created in two main ways
113
114   Immediate Filters
115       An immediate filter allows you to specify the filter code to be used at
116       the point where the filter is applied to a dbm. In this mode the
117       Filter_*_Push methods expects to receive exactly two parameters.
118
119           my $db = tie %hash, 'SDBM_File', ...
120           $db->Filter_Push( Store => sub { },
121                             Fetch => sub { });
122
123       The code reference associated with "Store" will be called before any
124       key/value is written to the database and the code reference associated
125       with "Fetch" will be called after any key/value is read from the
126       database.
127
128       For example, here is a sample filter that adds a trailing NULL
129       character to all strings before they are written to the DBM file, and
130       removes the trailing NULL when they are read from the DBM file
131
132           my $db = tie %hash, 'SDBM_File', ...
133           $db->Filter_Push( Store => sub { $_ .= "\x00" ; },
134                             Fetch => sub { s/\x00$// ;    });
135
136       Points to note:
137
138       1.   Both the Store and Fetch filters manipulate $_.
139
140   Canned Filters
141       Immediate filters are useful for one-off situations. For more generic
142       problems it can be useful to package the filter up in its own module.
143
144       The usage is for a canned filter is:
145
146           $db->Filter_Push("name", params)
147
148       where
149
150       "name"
151            is the name of the module to load. If the string specified does
152            not contain the package separator characters "::", it is assumed
153            to refer to the full module name "DBM_Filter::name". This means
154            that the full names for canned filters, "null" and "utf8",
155            included with this module are:
156
157                DBM_Filter::null
158                DBM_Filter::utf8
159
160       params
161            any optional parameters that need to be sent to the filter. See
162            the encode filter for an example of a module that uses parameters.
163
164       The module that implements the canned filter can take one of two forms.
165       Here is a template for the first
166
167           package DBM_Filter::null ;
168
169           use strict;
170           use warnings;
171
172           sub Store
173           {
174               # store code here
175           }
176
177           sub Fetch
178           {
179               # fetch code here
180           }
181
182           1;
183
184       Notes:
185
186       1.   The package name uses the "DBM_Filter::" prefix.
187
188       2.   The module must have both a Store and a Fetch method. If only one
189            is present, or neither are present, a fatal error will be thrown.
190
191       The second form allows the filter to hold state information using a
192       closure, thus:
193
194           package DBM_Filter::encoding ;
195
196           use strict;
197           use warnings;
198
199           sub Filter
200           {
201               my @params = @_ ;
202
203               ...
204               return {
205                   Store   => sub { $_ = $encoding->encode($_) },
206                   Fetch   => sub { $_ = $encoding->decode($_) }
207                   } ;
208           }
209
210           1;
211
212       In this instance the "Store" and "Fetch" methods are encapsulated
213       inside a "Filter" method.
214

Filters Included

216       A number of canned filers are provided with this module. They cover a
217       number of the main areas that filters are needed when interfacing with
218       DBM files. They also act as templates for your own filters.
219
220       The filter included are:
221
222       ·    utf8
223
224            This module will ensure that all data written to the DBM will be
225            encoded in UTF-8.
226
227            This module needs the Encode module.
228
229       ·    encode
230
231            Allows you to choose the character encoding will be store in the
232            DBM file.
233
234       ·    compress
235
236            This filter will compress all data before it is written to the
237            database and uncompressed it on reading.
238
239            This module needs Compress::Zlib.
240
241       ·    int32
242
243            This module is used when interoperating with a C/C++ application
244            that uses a C int as either the key and/or value in the DBM file.
245
246       ·    null
247
248            This module ensures that all data written to the DBM file is null
249            terminated. This is useful when you have a perl script that needs
250            to interoperate with a DBM file that a C program also uses. A
251            fairly common issue is for the C application to include the
252            terminating null in a string when it writes to the DBM file. This
253            filter will ensure that all data written to the DBM file can be
254            read by the C application.
255

NOTES

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

EXAMPLE

290       Say you need to interoperate with a legacy C application that stores
291       keys as C ints and the values and null terminated UTF-8 strings. Here
292       is how you would set that up
293
294           my $db = tie %hash, 'SDBM_File', ...
295
296           $db->Filter_Key_Push('int32') ;
297
298           $db->Filter_Value_Push('utf8');
299           $db->Filter_Value_Push('null');
300

AUTHOR

305       Paul Marquess <pmqs@cpan.org>
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309perl v5.30.2                      2020-03-27                   DBM_Filter(3pm)