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

=head1 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

=head1 DESCRIPTION

This document provides a step-by-step guide, how to create a new
C&lt;DBD::File&gt; based DBD. It expects that you carefully read the L&lt;DBI&gt;
documentation and that you&#039;re familiar with L&lt;DBI::DBD&gt; and had read and
understood L&lt;DBD::ExampleP&gt;.

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
L&lt;DBI::DBD/CREATING A NEW DRIVER&gt;.

Of course, DBD::File is a DBI::DBD::SqlEngine and you surely read
L&lt;DBI::DBD::SqlEngine::HowTo&gt; before continuing here.

=head1 CREATING DRIVER CLASSES

Do you have an entry in DBI&#039;s DBD registry? For this guide, a prefix of
C&lt;foo_&gt; is assumed.

=head2 Sample Skeleton

    package DBD::Foo;

    use strict;
    use warnings;
    use vars qw(@ISA $VERSION);
    use base qw(DBD::File);

    use DBI ();

    $VERSION = &quot;0.001&quot;;

    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 is able to
deal with temporary tables, as long as you use L&lt;SQL::Statement&gt;. In
L&lt;DBI::SQL::Nano&gt; environments, this DBD can do nothing.

=head2 Start over

Based on L&lt;DBI::DBD::SqlEngine::HowTo&gt;, we&#039;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&#039;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 $self-&gt;SUPER::bootstrap_table_meta($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-&gt;SUPER::init_table_meta( $dbh, $meta, $table );
    }

In case C&lt;DBD::File::Table::open_file&gt; doesn&#039;t open the files as the driver
needs that, override it!

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

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

=head2 User comfort

C&lt;DBD::File&gt; since C&lt;0.39&gt; consolidates all persistent meta data of a table
into a single structure stored in C&lt;&lt; $dbh-&gt;{f_meta} &gt;&gt;. With C&lt;DBD::File&gt;
version C&lt;0.41&gt; and C&lt;DBI::DBD::SqlEngine&gt; version C&lt;0.05&gt;, this
consolidation moves to L&lt;DBI::DBD::SqlEngine&gt;. It&#039;s still the
C&lt;&lt; $dbh-&gt;{$drv_prefix . &quot;_meta&quot;} &gt;&gt; attribute which cares, so what you
learned at this place before, is still valid.

    sub init_valid_attributes
    {
	my $dbh = $_[0];

	$dbh-&gt;SUPER::init_valid_attributes ();

	$dbh-&gt;{foo_valid_attrs} = { ... };
	$dbh-&gt;{foo_readonly_attrs} = { ...  };

	$dbh-&gt;{foo_meta} = &quot;foo_tables&quot;;

	return $dbh;
    }

See updates at L&lt;DBI::DBD::SqlEngine::HowTo/User comfort&gt;.

=head2 Testing

Now you should have your own DBD::File based driver. Was easy, wasn&#039;t it?
But does it work well?  Prove it by writing tests and remember to use
dbd_edit_mm_attribs from L&lt;DBI::DBD&gt; to ensure testing even rare cases.

=head1 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 &lt; h.m.brand at xs4all.nl &gt; and
Jens Rehsack  &lt; rehsack at googlemail.com &gt;

=head1 COPYRIGHT AND LICENSE

Copyright (C) 2010 by H.Merijn Brand &amp; 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.

=cut
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