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usr/lib64/perl5/vendor_perl/DBI/DBD/SqlEngine/_[ Not.Writable ]

File://usr/lib64/perl5/vendor_perl/DBI/DBD/SqlEngine/HowTo.pod

=head1 NAME

DBI::DBD::SqlEngine::HowTo - Guide to create DBI::DBD::SqlEngine based driver

=head1 SYNOPSIS

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

=head1 DESCRIPTION

This document provides a step-by-step guide, how to create a new
C<DBI::DBD::SqlEngine> based DBD. It expects that you carefully read the
L<DBI> documentation and that you're familiar with L<DBI::DBD> and had
read and understood L<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
L<DBI::DBD/CREATING A NEW DRIVER>.

=head1 CREATING DRIVER CLASSES

Do you have an entry in DBI's DBD registry? For this guide, a prefix of
C<foo_> is assumed.

=head2 Sample Skeleton

    package DBD::Foo;

    use strict;
    use warnings;
    use vars qw($VERSION);
    use base qw(DBI::DBD::SqlEngine);

    use DBI ();

    $VERSION = "0.001";

    package DBD::Foo::dr;

    use vars qw(@ISA $imp_data_size);

    @ISA = qw(DBI::DBD::SqlEngine::dr);
    $imp_data_size = 0;

    package DBD::Foo::db;

    use vars qw(@ISA $imp_data_size);

    @ISA = qw(DBI::DBD::SqlEngine::db);
    $imp_data_size = 0;

    package DBD::Foo::st;

    use vars qw(@ISA $imp_data_size);

    @ISA = qw(DBI::DBD::SqlEngine::st);
    $imp_data_size = 0;

    package DBD::Foo::Statement;

    use vars qw(@ISA);

    @ISA = qw(DBI::DBD::SqlEngine::Statement);

    package DBD::Foo::Table;

    use vars qw(@ISA);

    @ISA = qw(DBI::DBD::SqlEngine::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<SQL::Statement>. In
L<DBI::SQL::Nano> environments, this DBD can do nothing.

=head2 Deal with own attributes

Before we start doing usable stuff with our DBI driver, we need to think
about what we want to do and how we want to do it.

Do we need tunable knobs accessible by users? Do we need status
information? All this is handled in attributes of the database handles (be
careful when your DBD is running "behind" a L<DBD::Gofer> proxy).

How come the attributes into the DBD and how are they fetchable by the
user? Good question, but you should know because you've read the L<DBI>
documentation.

C<DBI::DBD::SqlEngine::db::FETCH> and C<DBI::DBD::SqlEngine::db::STORE>
taking care for you - all they need to know is which attribute names
are valid and mutable or immutable. Tell them by adding
C<init_valid_attributes> to your db class:

    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
	};
	$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
	};

	return $dbh;
    }

Woooho - but now the user cannot assign new managers? This is intended,
overwrite C<STORE> to handle it!

    sub STORE ($$$)
    {
	my ( $dbh, $attrib, $value ) = @_;

	$dbh->SUPER::STORE( $attrib, $value );

	# we're still alive, so no exception is thrown ...
	# by DBI::DBD::SqlEngine::db::STORE
	if ( $attrib eq "foo_manager_type" )
	{
	    $dbh->{foo_manager} = $dbh->{foo_manager_type}->new();
	    # ... probably correct some states based on the new
	    # foo_manager_type - see DBD::Sys for an example
	}
    }

But ... my driver runs without a manager until someone first assignes
a C<foo_manager_type>. Well, no - there're two places where you can
initialize defaults:

    sub init_default_attributes
    {
	my ($dbh, $phase) = @_;

	$dbh->SUPER::init_default_attributes($phase);

	if( 0 == $phase )
	{
	    # init all attributes which have no knowledge about
	    # user settings from DSN or the attribute hash
	    $dbh->{foo_manager_type} = "DBD::Foo::Manager";
	}
	elsif( 1 == $phase )
	{
	    # init phase with more knowledge from DSN or attribute
	    # hash
	    $dbh->{foo_manager} = $dbh->{foo_manager_type}->new();
	}

	return $dbh;
    }

So far we can prevent the users to use our database driver as data
storage for anything and everything. We care only about the real important
stuff for peace on earth and alike attributes. But in fact, the driver
still can't do anything. It can do less than nothing - meanwhile it's
not a stupid storage area anymore.

=head2 User comfort

C<DBI::DBD::SqlEngine> since C<0.05> consolidates all persistent meta data
of a table into a single structure stored in C<< $dbh->{sql_meta} >>. While
DBI::DBD::SqlEngine provides only readonly access to this structure,
modifications are still allowed.

Primarily DBI::DBD::SqlEngine provides access via the setters
C<get_sql_engine_meta>, C<get_single_table_meta>, C<set_single_table_meta>,
C<set_sql_engine_meta> and C<clear_sql_engine_meta>. Those methods are
easily accessible by the users via the C<< $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_sql_engine_meta");

DBI::DBD::SqlEngine 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
C<init_valid_attributes> (re-capture L</Deal with own attributes>):

    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 C<< $dbh->{foo_tables} >> and a tied hash for
each table's meta data in C<< $dbh->{foo_tables}->{$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 C<DBI::DBD::SqlEngine> these are the attributes
C<f_file>, C<f_dir>, C<f_ext> and C<f_lockfile>), inform DBI::DBD::SqlEngine 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 DBI::DBD::SqlEngine to re-do the
entire meta initialization process.

Any further action which needs to be taken can handled in
C<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 C<$meta>, so the attribute
changed handler can act depending on the old value.

=head2 Dealing with Tables

Let's put some life into it - it's going to be time for it.

This is a good point where a quick side step to L<SQL::Statement::Embed>
will help to shorten the next paragraph. The documentation in
SQL::Statement::Embed regarding embedding in own DBD's works pretty
fine with SQL::Statement and DBI::SQL::Nano.

Second look should go to L<DBI::DBD::SqlEngine::Developers> to get a
picture over the driver part of the table API. Usually there isn't much
to do for an easy driver.

=head2 Testing

Now you should have your first own DBD. Was easy, wasn't it?  But does
it work well? Prove it by writing tests and remember to use
dbd_edit_mm_attribs from L<DBI::DBD> to ensure testing even rare cases.

=head1 AUTHOR

This guide is written by Jens Rehsack. DBI::DBD::SqlEngine is written by
Jens Rehsack using code from DBD::File originally written by Jochen
Wiedmann and Jeff Zucker.

The module DBI::DBD::SqlEngine is currently maintained by

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

=head1 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.

=cut

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