MirOS Manual: DBM_Filter(3p)


DBM_Filter(3p)  Perl Programmers Reference Guide   DBM_Filter(3p)

NAME

     DBM_Filter -- Filter DBM keys/values

SYNOPSIS

         use DBM_Filter ;
         use SDBM_File; # or DB_File, or GDBM_File, or NDBM_File, or ODBM_File

         $db = tie %hash, ...

         $db->Filter_Push(Fetch => sub {...},
                          Store => sub {...});

         $db->Filter_Push('my_filter1');
         $db->Filter_Push('my_filter2', params...);

         $db->Filter_Key_Push(...) ;
         $db->Filter_Value_Push(...) ;

         $db->Filter_Pop();
         $db->Filtered();

         package DBM_Filter::my_filter1;

         sub Store { ... }
         sub Fetch { ... }

         1;

         package DBM_Filter::my_filter2;

         sub Filter
         {
             my @opts = @_;
             ...
             return (
                 sub Store { ... },
                 sub Fetch { ... } );
         }

         1;

DESCRIPTION

     This module provides an interface that allows filters to be
     applied to tied Hashes associated with DBM files. It builds
     on the DBM Filter hooks that are present in all the
     *DB*_File modules included with the standard Perl source
     distribution from version 5.6.1 onwards. In addition to the
     *DB*_File modules distributed with Perl, the BerkeleyDB
     module, available on CPAN, supports the DBM Filter hooks.
     See perldbmfilter for more details on the DBM Filter hooks.

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What is a DBM Filter?
     A DBM Filter allows the keys and/or values in a tied hash to
     be modified by some user-defined code just before it is
     written to the DBM file and just after it is read back from
     the DBM file. For example, this snippet of code

         $some_hash{"abc"} = 42;

     could potentially trigger two filters, one for the writing
     of the key "abc" and another for writing the value 42.
     Similarly, this snippet

         my ($key, $value) = each %some_hash

     will trigger two filters, one for the reading of the key and
     one for the reading of the value.

     Like the existing DBM Filter functionality, this module
     arranges for the $_ variable to be populated with the key or
     value that a filter will check. This usually means that most
     DBM filters tend to be very short.

     So what's new?

     The main enhancements over the standard DBM Filter hooks
     are:

     +   A cleaner interface.

     +   The ability to easily apply multiple filters to a single
         DBM file.

     +   The ability to create "canned" filters. These allow com-
         monly used filters to be packaged into a stand-alone
         module.

METHODS

     This module will arrange for the following methods to be
     available via the object returned from the "tie" call.

     $db->Filter_Push()

     $db->Filter_Key_Push()

     $db->Filter_Value_Push()

     Add a filter to filter stack for the database, $db. The
     three formats vary only in whether they apply to the DBM
     key, the DBM value or both.

     Filter_Push
          The filter is applied to both keys and values.

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     Filter_Key_Push
          The filter is applied to the key only.

     Filter_Value_Push
          The filter is applied to the value only.

     $db->Filter_Pop()

     Removes the last filter that was applied to the DBM file
     associated with $db, if present.

     $db->Filtered()

     Returns TRUE if there are any filters applied to the DBM
     associated with $db.  Otherwise returns FALSE.

Writing a Filter

     Filters can be created in two main ways

     Immediate Filters

     An immediate filter allows you to specify the filter code to
     be used at the point where the filter is applied to a dbm.
     In this mode the Filter_*_Push methods expects to receive
     exactly two parameters.

         my $db = tie %hash, 'SDBM_File', ...
         $db->Filter_Push( Store => sub { },
                           Fetch => sub { });

     The code reference associated with "Store" will be called
     before any key/value is written to the database and the code
     reference associated with "Fetch" will be called after any
     key/value is read from the database.

     For example, here is a sample filter that adds a trailing
     NULL character to all strings before they are written to the
     DBM file, and removes the trailing NULL when they are read
     from the DBM file

         my $db = tie %hash, 'SDBM_File', ...
         $db->Filter_Push( Store => sub { $_ .= "\x00" ; },
                           Fetch => sub { s/\x00$// ;    });

     Points to note:

     1.   Both the Store and Fetch filters manipulate $_.

     Canned Filters

     Immediate filters are useful for one-off situations. For
     more generic problems it can be useful to package the filter

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     up in its own module.

     The usage is for a canned filter is:

         $db->Filter_Push("name", params)

     where

     "name"
          is the name of the module to load. If the string speci-
          fied does not contain the package separator characters
          "::", it is assumed to refer to the full module name
          "DBM_Filter::name". This means that the full names for
          canned filters, "null" and "utf8", included with this
          module are:

              DBM_Filter::null
              DBM_Filter::utf8

     params
          any optional parameters that need to be sent to the
          filter. See the encode filter for an example of a
          module that uses parameters.

     The module that implements the canned filter can take one of
     two forms. Here is a template for the first

         package DBM_Filter::null ;

         use strict;
         use warnings;

         sub Store
         {
             # store code here
         }

         sub Fetch
         {
             # fetch code here
         }

         1;

     Notes:

     1.   The package name uses the "DBM_Filter::" prefix.

     2.   The module must have both a Store and a Fetch method.
          If only one is present, or neither are present, a fatal
          error will be thrown.

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     The second form allows the filter to hold state information
     using a closure, thus:

         package DBM_Filter::encoding ;

         use strict;
         use warnings;

         sub Filter
         {
             my @params = @_ ;

             ...
             return {
                 Store   => sub { $_ = $encoding->encode($_) },
                 Fetch   => sub { $_ = $encoding->decode($_) }
                 } ;
         }

         1;

     In this instance the "Store" and "Fetch" methods are encap-
     sulated inside a "Filter" method.

Filters Included

     A number of canned filers are provided with this module.
     They cover a number of the main areas that filters are
     needed when interfacing with DBM files. They also act as
     templates for your own filters.

     The filter included are:

     * utf8
          This module will ensure that all data written to the
          DBM will be encoded in UTF-8.

          This module needs the Encode module.

     * encode
          Allows you to choose the character encoding will be
          store in the DBM file.

     * compress
          This filter will compress all data before it is written
          to the database and uncompressed it on reading.

          This module needs Compress::Zlib.

     * int32
          This module is used when interoperating with a C/C++
          application that uses a C int as either the key and/or
          value in the DBM file.

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     * null
          This module ensures that all data written to the DBM
          file is null terminated. This is useful when you have a
          perl script that needs to interoperate with a DBM file
          that a C program also uses. A fairly common issue is
          for the C application to include the terminating null
          in a string when it writes to the DBM file. This filter
          will ensure that all data written to the DBM file can
          be read by the C application.

NOTES

     Maintain Round Trip Integrity

     When writing a DBM filter it is very important to ensure
     that it is possible to retrieve all data that you have writ-
     ten when the DBM filter is in place. In practice, this means
     that whatever transformation is applied to the data in the
     Store method, the exact inverse operation should be applied
     in the Fetch method.

     If you don't provide an exact inverse transformation, you
     will find that code like this will not behave as you expect.

          while (my ($k, $v) = each %hash)
          {
              ...
          }

     Depending on the transformation, you will find that one or
     more of the following will happen

     1    The loop will never terminate.

     2    Too few records will be retrieved.

     3    Too many will be retrieved.

     4    The loop will do the right thing for a while, but it
          will unexpectedly fail.

     Don't mix filtered & non-filtered data in the same database
     file.

     This is just a restatement of the previous section. Unless
     you are completely certain you know what you are doing,
     avoid mixing filtered & non-filtered data.

EXAMPLE

     Say you need to interoperate with a legacy C application
     that stores keys as C ints and the values and null ter-
     minated UTF-8 strings. Here is how you would set that up

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         my $db = tie %hash, 'SDBM_File', ...

         $db->Filter_Key_Push('int32') ;

         $db->Filter_Value_Push('utf8');
         $db->Filter_Value_Push('null');

SEE ALSO

     <DB_File>,  GDBM_File, NDBM_File, ODBM_File, SDBM_File,
     perldbmfilter

AUTHOR

     Paul Marquess <pmqs@cpan.org>

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