DBM_Filter - phpMan

NAME
    DBM_Filter -- Filter DBM keys/values

SYNOPSIS
        use DBM_Filter ;
        use SDBM_File; # or DB_File, GDBM_File, 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 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 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*.

  $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 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 {
                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:

    *    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.

    *    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 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

AUTHOR
    Paul Marquess <pmqs AT cpan.org>