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

       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 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

   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.

       This module will arrange for the following methods to be available via
       the object returned from the "tie" call.

       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

       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" 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.

   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.

       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 v5.10.1			  2009-02-12			 DBM_Filter(3)

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