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DBD::File::HowTo(3)                  User Contributed Perl Documentation                 DBD::File::HowTo(3)



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}". While DBD::File provides only readonly access to this structure,
       modifications are still allowed.

       Primarily DBD::File provides access via setters "get_file_meta", "set_file_meta" and
       "clear_file_meta". Those methods are easily accessible by the users via the "$dbh->func ()" interface
       provided by DBI. Well, many users don't feel comfortize when calling

           # don't require extension for tables cars
           $dbh->func ("cars", "f_ext", ".csv", "set_file_meta");

       DBD::File will inject a method into your driver to increase the user comfort to allow:

           # don't require extension for tables cars
           $dbh->foo_set_meta ("cars", "f_ext", ".csv");

       Better, but here and there users likes to do:

           # don't require extension for tables cars
           $dbh->{foo_tables}->{cars}->{f_ext} = ".csv";

       This interface is provided when derived DBD's define following in "init_valid_attributes" (please
       compare carefully with the example in DBI::DBD::SqlEngine::HowTo):

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

               $dbh->SUPER::init_valid_attributes ();

               $dbh->{foo_valid_attrs} = {
                   foo_version         => 1,   # contains version of this driver
                   foo_valid_attrs     => 1,   # contains the valid attributes of foo drivers
                   foo_readonly_attrs  => 1,   # contains immutable attributes of foo drivers
                   foo_bar             => 1,   # contains the bar attribute
                   foo_baz             => 1,   # contains the baz attribute
                   foo_manager         => 1,   # contains the manager of the driver instance
                   foo_manager_type    => 1,   # contains the manager class of the driver instance
                   foo_meta            => 1,   # contains the public interface to modify table meta attributes
               };
               $dbh->{foo_readonly_attrs} = {
                   foo_version         => 1,   # ensure no-one modifies the driver version
                   foo_valid_attrs     => 1,   # do not permit to add more valid attributes ...
                   foo_readonly_attrs  => 1,   # ... or make the immutable mutable
                   foo_manager         => 1,   # manager is set internally only
                   foo_meta            => 1,   # ensure public interface to modify table meta attributes are immutable
               };

               $dbh->{foo_meta} = "foo_tables";

               return $dbh;
           }

       This provides a tied hash in "$dbh->{foo_tables}" and a tied hash for each table's meta data in
       "$dbh->{foo_tables}->{$table_name}".  Modifications on the table meta attributes are done using the
       table methods:

           sub get_table_meta_attr { ... }
           sub set_table_meta_attr { ... }

       Both methods can adjust the attribute name for compatibility reasons, e.g.  when former versions of
       the DBD allowed different names to be used for the same flag:

           my %compat_map = (
                              abc => 'foo_abc',
                              xyz => 'foo_xyz',
                            );
           __PACKAGE__->register_compat_map( \%compat_map );

       If any user modification on a meta attribute needs reinitialization of the meta structure (in case of
       "DBD::File" these are the attributes "f_file", "f_dir", "f_ext" and "f_lockfile"), inform DBD::File
       by doing

           my %reset_on_modify = (
                                   foo_xyz => "foo_bar",
                                   foo_abc => "foo_bar",
                                 );
           __PACKAGE__->register_reset_on_modify( \%reset_on_modify );

       The next access to the table meta data will force DBD::File to re-do the entire meta initialization
       process.

       Any further action which needs to be taken can handled in "table_meta_attr_changed":

           sub table_meta_attr_changed
           {
               my ($class, $meta, $attrib, $value) = @_;
               ...
               $class->SUPER::table_meta_attr_changed ($meta, $attrib, $value);
           }

       This is done before the new value is set in $meta, so the attribute changed handler can act depending
       on the old value.

   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.



perl v5.12.5                                     2010-09-16                              DBD::File::HowTo(3)