DBM_Filter

NAME

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

What is a DBM Filter?

    $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?

*

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

$db->Filter_Push()

$db->Filter_Key_Push()

$db->Filter_Value_Push()

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()

$db->Filtered()

Writing a Filter

Immediate Filters

    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

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

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

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.

EXAMPLE

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

    $db->Filter_Key_Push('int32') ;

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

SEE ALSO

AUTHOR