DBD::File::HowTo

NAME

DBD::File::HowTo − Guide to create DBD::File based driver

SYNOPSIS

perldoc DBD::File::HowTo
perldoc DBI
perldoc DBI::DBD
perldoc DBD::File::Developers
perldoc DBI::DBD::SqlEngine::Developers
perldoc DBI::DBD::SqlEngine
perldoc SQL::Eval
perldoc DBI::DBD::SqlEngine::HowTo
perldoc SQL::Statement::Embed
perldoc DBD::File
perldoc DBD::File::HowTo
perldoc DBD::File::Developers

DESCRIPTION

This document provides a step-by-step guide, how to create a new "DBD::File" based DBD. It expects that you carefully read the DBI documentation and that you’re familiar with DBI::DBD and had read and understood DBD::ExampleP.

This document addresses experienced developers who are really sure that they need to invest time when writing a new DBI Driver. Writing a DBI Driver is neither a weekend project nor an easy job for hobby coders after work. Expect one or two man-month of time for the first start.

Those who are still reading, should be able to sing the rules of " CREATING A NEW DRIVER" in DBI::DBD .

Of course, DBD::File is a DBI::DBD::SqlEngine and you surely read DBI::DBD::SqlEngine::HowTo before continuing here.

CREATING DRIVER CLASSES

Do you have an entry in DBI ’s DBD registry? For this guide, a prefix of "foo_" is assumed.

Sample Skeleton
package DBD::Foo;
use strict;
use warnings;
use vars qw(@ISA $VERSION);
use base qw(DBD::File);
use DBI ();
$VERSION = "0.001";
package DBD::Foo::dr;
use vars qw(@ISA $imp_data_size);
@ISA = qw(DBD::File::dr);
$imp_data_size = 0;
package DBD::Foo::db;
use vars qw(@ISA $imp_data_size);
@ISA = qw(DBD::File::db);
$imp_data_size = 0;
package DBD::Foo::st;
use vars qw(@ISA $imp_data_size);
@ISA = qw(DBD::File::st);
$imp_data_size = 0;
package DBD::Foo::Statement;
use vars qw(@ISA);
@ISA = qw(DBD::File::Statement);
package DBD::Foo::Table;
use vars qw(@ISA);
@ISA = qw(DBD::File::Table);
1;

Tiny, eh? And all you have now is a DBD named foo which will be able to deal with temporary tables, as long as you use SQL::Statement. In DBI::SQL::Nano environments, this DBD can do nothing.

Start over
Based on DBI::DBD::SqlEngine::HowTo, we’re now having a driver which could do basic things. Of course, it should now derive from DBD::File instead of DBI::DBD::SqlEngine, shouldn’t it?

DBD::File extends DBI::DBD::SqlEngine to deal with any kind of files. In principle, the only extensions required are to the table class:

package DBD::Foo::Table;
sub bootstrap_table_meta
{
my ( $self, $dbh, $meta, $table ) = @_;
# initialize all $meta attributes which might be relevant for
# file2table
return $self−>SUPER::bootstrap_table_meta($dbh, $meta, $table);
}
sub init_table_meta
{
my ( $self, $dbh, $meta, $table ) = @_;
# called after $meta contains the results from file2table
# initialize all missing $meta attributes
$self−>SUPER::init_table_meta( $dbh, $meta, $table );
}

In case "DBD::File::Table::open_file" doesn’t open the files as the driver needs that, override it!

sub open_file
{
my ( $self, $meta, $attrs, $flags ) = @_;
# ensure that $meta−>{f_dontopen} is set
$self−>SUPER::open_file( $meta, $attrs, $flags );
# now do what ever needs to be done
}

Combined with the methods implemented using the SQL::Statement::Embed guide, the table is full working and you could try a start over.

User comfort
"DBD::File" since 0.39 consolidates all persistent meta data of a table into a single structure stored in "$dbh−>{f_meta}". With "DBD::File" version 0.41 and "DBI::DBD::SqlEngine" version 0.05, this consolidation moves to DBI::DBD::SqlEngine. It’s still the "$dbh−>{$drv_prefix . "_meta"}" attribute which cares, so what you learned at this place before, is still valid.

sub init_valid_attributes
{
my $dbh = $_[0];
$dbh−>SUPER::init_valid_attributes ();
$dbh−>{foo_valid_attrs} = { ... };
$dbh−>{foo_readonly_attrs} = { ... };
$dbh−>{foo_meta} = "foo_tables";
return $dbh;
}

See updates at "User comfort" in DBI::DBD::SqlEngine::HowTo.

Testing
Now you should have your own DBD::File based driver. Was easy, wasn’t it? But does it work well? Prove it by writing tests and remember to use dbd_edit_mm_attribs from DBI::DBD to ensure testing even rare cases.

AUTHOR

This guide is written by Jens Rehsack. DBD::File is written by Jochen Wiedmann and Jeff Zucker.

The module DBD::File is currently maintained by

H.Merijn Brand < h.m.brand at xs4all.nl > and Jens Rehsack < rehsack at googlemail.com >

COPYRIGHT AND LICENSE

Copyright (C) 2010 by H.Merijn Brand & Jens Rehsack

All rights reserved.

You may freely distribute and/or modify this module under the terms of either the GNU General Public License ( GPL ) or the Artistic License, as specified in the Perl README file.