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

       DBM_Filter -- Filter DBM keys/values

	   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_filter2', params...);

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


	   package DBM_Filter::my_filter1;

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


	   package DBM_Filter::my_filter2;

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


       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

       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.

       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.

	    The filter is applied to both keys and values.

	    The filter is applied to the key only.

	    The filter is applied to the value only.

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

       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)


	    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:


	    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.   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 { $_ = $encoding->decode($_) }
		   } ;


       In this instance the "Store" and "Fetch" methods are encapsulated inside a "Filter"

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 application.

   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

       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.

       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_File>,  GDBM_File, NDBM_File, ODBM_File, SDBM_File, perldbmfilter

       Paul Marquess <pmqs@cpan.org>

perl v5.16.3				    2013-03-04				  DBM_Filter(3pm)
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