DBM_Filter (3)

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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 files. It builds on the 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 supports the Filter hooks. See perldbmfilter for more details on the Filter hooks.

What is a DBM Filter?

A 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 file and just after it is read back from the 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

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 filters tend to be very short.

So what's new?

The main enhancements over the standard 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 key, the 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 file associated with $db, if present.

$db->Filtered()

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

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

character to all strings before they are written to the file, and removes the trailing when they are read from the 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 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 filter it is very important to ensure that it is possible to retrieve all data that you have written when the 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 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@cpan.org>