DBD::File::HowTo

Section: User Contributed Perl Documentation (3)
Updated: 2021-01-27
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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 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

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.