DBD::File::HowTo

Section: User Contributed Perl Documentation (3)
Updated: 2019-07-26
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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 is 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.


 

Index

NAME
SYNOPSIS
DESCRIPTION
CREATING DRIVER CLASSES
Sample Skeleton
Start over
User comfort
Testing
AUTHOR
COPYRIGHT AND LICENSE
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