DBM_Filter(3pm) Perl Programmers Reference Guide DBM_Filter(3pm)
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:
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.
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:
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.
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>
perl v5.26.3 2018-03-01 DBM_Filter(3pm)
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