


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.

What is a DBM Filter?
       A DBM Filter allows the keys and/or values in a tied hash



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       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:

       o   A cleaner interface.

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

       o   The ability to create "canned" filters. These allow
           commonly 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.

       Filter_Key_Push
            The filter is applied to the key only.

       Filter_Value_Push
            The filter is applied to the value only.





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   $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 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
            specified does not contain the package separator
            characters "::", it is assumed to refer to the full



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            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.

       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 {



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

           1;

       In this instance the "Store" and "Fetch" methods are
       encapsulated 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:

       o    utf8

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

            This module needs the Encode module.

       o    encode

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

       o    compress

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

            This module needs Compress::Zlib.

       o    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.

       o    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



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            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
       written 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
       terminated UTF-8 strings. Here is how you would set that
       up

           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



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


AUTHOR
       Paul Marquess <pmqs@cpan.org>























































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