Manpages - DBM_Filter.3perl

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.

$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.

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.

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 $_.

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.

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.

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.