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DBD::File::Developers(User Contributed Perl DocumentatDBD::File::Developers(3)

       DBD::File::Developers - Developers documentation for DBD::File

	   package DBD::myDriver;

	   use base qw(DBD::File);

	   sub driver
	       my $drh = $proto->SUPER::driver($attr);
	       return $drh->{class};

	   sub CLONE { ... }

	   package DBD::myDriver::dr;

	   @ISA = qw(DBD::File::dr);

	   sub data_sources { ... }

	   package DBD::myDriver::db;

	   @ISA = qw(DBD::File::db);

	   sub init_valid_attributes { ... }
	   sub init_default_attributes { ... }
	   sub set_versions { ... }
	   sub validate_STORE_attr { my ($dbh, $attrib, $value) = @_; ... }
	   sub validate_FETCH_attr { my ($dbh, $attrib) = @_; ... }
	   sub get_myd_versions { ... }

	   package DBD::myDriver::st;

	   @ISA = qw(DBD::File::st);

	   sub FETCH { ... }
	   sub STORE { ... }

	   package DBD::myDriver::Statement;

	   @ISA = qw(DBD::File::Statement);

	   package DBD::myDriver::Table;

	   @ISA = qw(DBD::File::Table);

	   my %reset_on_modify = (
				   myd_abc => "myd_foo",
				   myd_mno => "myd_bar",
	   __PACKAGE__->register_reset_on_modify( \%reset_on_modify );
	   my %compat_map = (
			      abc => 'foo_abc',
			      xyz => 'foo_xyz',
	   __PACKAGE__->register_compat_map( \%compat_map );

	   sub bootstrap_table_meta { ... }
	   sub init_table_meta { ... }
	   sub table_meta_attr_changed { ... }
	   sub open_file { ... }

	   sub fetch_row { ... }
	   sub push_row { ... }
	   sub push_names { ... }

	   # optimize the SQL engine by add one or more of
	   sub update_current_row { ... }
	   # or
	   sub update_specific_row { ... }
	   # or
	   sub update_one_row { ... }
	   # or
	   sub insert_new_row { ... }
	   # or
	   sub delete_current_row { ... }
	   # or
	   sub delete_one_row { ... }

       This document describes how DBD developers can write DBD::File based
       DBI drivers. It supplements DBI::DBD and DBI::DBD::SqlEngine::Develop‐
       ers, which you should read first.

       Each DBI driver must provide a package global "driver" method and three
       DBI related classes:

	   Driver package, contains the methods DBI calls indirectly via DBI

	     DBI->connect ('DBI:DBM:', undef, undef, {})

	     # invokes
	     package DBD::DBM::dr;
	     @DBD::DBM::dr::ISA = qw(DBD::File::dr);

	     sub connect ($$;$$$)

	   Similar for "data_sources ()" and "disconnect_all()".

	   Pure Perl DBI drivers derived from DBD::File do not usually need to
	   override any of the methods provided through the DBD::XXX::dr pack‐
	   age however if you need additional initialization in the connect
	   method you may need to.

	   Contains the methods which are called through DBI database handles
	   ($dbh). e.g.,

	     $sth = $dbh->prepare ("select * from foo");
	     # returns the f_encoding setting for table foo
	     $dbh->csv_get_meta ("foo", "f_encoding");

	   DBD::File provides the typical methods required here. Developers
	   who write DBI drivers based on DBD::File need to override the meth‐
	   ods "set_versions" and "init_valid_attributes".

	   Contains the methods to deal with prepared statement handles. e.g.,

	     $sth->execute () or die $sth->errstr;


       This is the main package containing the routines to initialize
       DBD::File based DBI drivers. Primarily the "DBD::File::driver" method
       is invoked, either directly from DBI when the driver is initialized or
       from the derived class.

	 package DBD::DBM;

	 use base qw( DBD::File );

	 sub driver
	     my ( $class, $attr ) = @_;
	     my $drh = $class->SUPER::driver( $attr );
	     return $drh;

       It is not necessary to implement your own driver method as long as
       additional initialization (e.g. installing more private driver methods)
       is not required.	 You do not need to call "setup_driver" as DBD::File
       takes care of it.


       The driver package contains the methods DBI calls indirectly via the
       DBI interface (see "DBI Class Methods" in DBI).

       DBD::File based DBI drivers usually do not need to implement anything
       here, it is enough to do the basic initialization:

	 package DBD:XXX::dr;

	 @DBD::XXX::dr::ISA = qw (DBD::File::dr);
	 $DBD::XXX::dr::imp_data_size	  = 0;
	 $DBD::XXX::dr::data_sources_attr = undef;
	 $DBD::XXX::ATTRIBUTION = "DBD::XXX $DBD::XXX::VERSION by Hans Mustermann";


       This package defines the database methods, which are called via the DBI
       database handle $dbh.

       Methods provided by DBD::File:

	   Simply returns the content of the "Active" attribute. Override when
	   your driver needs more complicated actions here.

	   Prepares a new SQL statement to execute. Returns a statement han‐
	   dle, $sth - instance of the DBD:XXX::st. It is neither required nor
	   recommended to override this method.

	   Fetches an attribute of a DBI database object. Private handle
	   attributes must have a prefix (this is mandatory). If a requested
	   attribute is detected as a private attribute without a valid pre‐
	   fix, the driver prefix (written as $drv_prefix) is added.

	   The driver prefix is extracted from the attribute name and verified
	   against "$dbh->{ $drv_prefix . "valid_attrs" }" (when it exists).
	   If the requested attribute value is not listed as a valid
	   attribute, this method croaks. If the attribute is valid and read‐
	   only (listed in "$dbh->{ $drv_prefix . "readonly_attrs" }" when it
	   exists), a real copy of the attribute value is returned. So it's
	   not possible to modify "f_valid_attrs" from outside of
	   DBD::File::db or a derived class.

	   Stores a database private attribute. Private handle attributes must
	   have a prefix (this is mandatory). If a requested attribute is
	   detected as a private attribute without a valid prefix, the driver
	   prefix (written as $drv_prefix) is added. If the database handle
	   has an attribute "${drv_prefix}_valid_attrs" - for attribute names
	   which are not listed in that hash, this method croaks. If the data‐
	   base handle has an attribute "${drv_prefix}_readonly_attrs", only
	   attributes which are not listed there can be stored (once they are
	   initialized). Trying to overwrite such an immutable attribute
	   forces this method to croak.

	   An example of a valid attributes list can be found in

	   This method sets the attribute "f_version" with the version of

	   This method is called at the begin of the "connect ()" phase.

	   When overriding this method, do not forget to invoke the superior

	   This method is called after the database handle is instantiated as
	   the first attribute initialization.

	   "DBD::File::db::init_valid_attributes" initializes the attributes
	   "f_valid_attrs" and "f_readonly_attrs".

	   When overriding this method, do not forget to invoke the superior
	   one, preferably before doing anything else. Compatibility table
	   attribute access must be initialized here to allow DBD::File to
	   instantiate the map tie:

	       # for DBD::CSV
	       $dbh->{csv_meta} = "csv_tables";
	       # for DBD::DBM
	       $dbh->{dbm_meta} = "dbm_tables";
	       # for DBD::AnyData
	       $dbh->{ad_meta}	= "ad_tables";

	   This method is called after the database handle is instantiated to
	   initialize the default attributes.

	   "DBD::File::db::init_default_attributes" initializes the attributes
	   "f_dir", "f_meta", "f_meta_map", "f_version".

	   When the derived implementor class provides the attribute to vali‐
	   date attributes (e.g. "$dbh->{dbm_valid_attrs} = {...};") or the
	   attribute containing the immutable attributes (e.g.
	   "$dbh->{dbm_readonly_attrs} = {...};"), the attributes
	   "drv_valid_attrs", "drv_readonly_attrs", "drv_version" and
	   "drv_meta" are added (when available) to the list of valid and
	   immutable attributes (where "drv_" is interpreted as the driver

	   If "drv_meta" is set, an attribute with the name in "drv_meta" is
	   initialized providing restricted read/write access to the meta data
	   of the tables using "DBD::File::TieTables" in the first (table)
	   level and "DBD::File::TieMeta" for the meta attribute level.
	   "DBD::File::TieTables" uses "DBD::DRV::Table::get_table_meta" to
	   initialize the second level tied hash on FETCH/STORE. The
	   "DBD::File::TieMeta" class uses "DBD::DRV::Table::get_ta‐
	   ble_meta_attr" to FETCH attribute values and "DBD::DRV::Ta‐
	   ble::set_table_meta_attr" to STORE attribute values. This allows it
	   to map meta attributes for compatibility reasons.

	   Retrieve an attribute from a table's meta information. The method
	   signature is "get_file_meta ($dbh, $table, $attr)". This method is
	   called by the injected db handle method "${drv_prefix}get_meta".

	   While get_file_meta allows $table or $attr to be a list of tables
	   or attributes to retrieve, get_single_table_meta allows only one
	   table name and only one attribute name. A table name of '.' (single
	   dot) is interpreted as the default table and this will retrieve the
	   appropriate attribute globally from the dbh. This has the same
	   restrictions as "$dbh->{$attrib}".

	   get_file_meta allows '+' and '*' as wildcards for table names and
	   $table being a regular expression matching against the table names
	   (evaluated without the default table). The table name '*' is all
	   currently known tables, including the default one. The table name
	   '+' is all table names which conform to ANSI file name restrictions

	   The table meta information is retrieved using the get_table_meta
	   and get_table_meta_attr methods of the table class of the implemen‐

	   Sets an attribute in a table's meta information. The method signa‐
	   ture is "set_file_meta ($dbh, $table, $attr, $value)". This method
	   is called by the injected db handle method "${drv_prefix}set_meta".

	   While set_file_meta allows $table to be a list of tables and $attr
	   to be a hash of several attributes to set, set_single_table_meta
	   allows only one table name and only one attribute name/value pair.

	   The wildcard characters for the table name are the same as for

	   The table meta information is updated using the get_table_meta and
	   set_table_meta_attr methods of the table class of the implementa‐

	   Clears all meta information cached about a table. The method signa‐
	   ture is "clear_file_meta ($dbh, $table)". This method is called by
	   the injected db handle method "${drv_prefix}clear_meta".


       Contains the methods to deal with prepared statement handles:

	   Fetches statement handle attributes. Supported attributes (for full
	   overview see "Statement Handle Attributes" in DBI) are "NAME",
	   "TYPE", "PRECISION" and "NULLABLE" in case that SQL::Statement is
	   used as SQL execution engine and a statement is successful pre‐
	   pared.  When SQL::Statement has additional information about a ta‐
	   ble, those information are returned. Otherwise, the same defaults
	   as in DBI::DBD::SqlEngine are used.

	   This method usually requires extending in a derived implementation.
	   See DBD::CSV or DBD::DBM for some example.


       Derives from DBI::SQL::Nano::Statement to provide following method:

	   Implements the open_table method required by SQL::Statement and
	   DBI::SQL::Nano. All the work for opening the file(s) belonging to
	   the table is handled and parameterized in DBD::File::Table. Unless
	   you intend to add anything to the following implementation, an
	   empty DBD::XXX::Statement package satisfies DBD::File.

	     sub open_table ($$$$$)
		 my ($self, $data, $table, $createMode, $lockMode) = @_;

		 my $class = ref $self;
		 $class =~ s/::Statement/::Table/;

		 my $flags = {
		     createMode	   => $createMode,
		     lockMode	   => $lockMode,
		 $self->{command} eq "DROP" and $flags->{dropMode} = 1;

		 return $class->new ($data, { table => $table }, $flags);
		 } # open_table


       Derives from DBI::SQL::Nano::Table and provides physical file access
       for the table data which are stored in the files.

	   This method tries to map a filename to the associated table name.
	   It is called with a partially filled meta structure for the result‐
	   ing table containing at least the following attributes: "f_ext",
	   "f_dir", "f_lockfile" and "sql_identifier_case".

	   If a file/table map can be found then this method sets the
	   "f_fqfn", "f_fqbn", "f_fqln" and "table_name" attributes in the
	   meta structure. If a map cannot be found the table name will be

	   Initializes a table meta structure. Can be safely overridden in a
	   derived class, as long as the "SUPER" method is called at the end
	   of the overridden method.

	   It copies the following attributes from the database into the table
	   meta data "f_dir", "f_ext", "f_encoding", "f_lock", "f_schema",
	   "f_lockfile" and "sql_identifier_case" and makes them sticky to the

	   This method should be called before you attempt to map between file
	   name and table name to ensure the correct directory, extension etc.
	   are used.

	   Initializes more attributes of the table meta data - usually more
	   expensive ones (e.g. those which require class instantiations) -
	   when the file name and the table name could mapped.

	   Returns the table meta data. If there are none for the required ta‐
	   ble, a new one is initialized. When it fails, nothing is returned.
	   On success, the name of the table and the meta data structure is

	   Returns a single attribute from the table meta data. If the
	   attribute name appears in %compat_map, the attribute name is
	   updated from there.

	   Sets a single attribute in the table meta data. If the attribute
	   name appears in %compat_map, the attribute name is updated from

	   Called when an attribute of the meta data is modified.

	   If the modified attribute requires to reset a calculated attribute,
	   the calculated attribute is reset (deleted from meta data struc‐
	   ture) and the initialized flag is removed, too. The decision is
	   made based on %register_reset_on_modify.

	   Allows "set_table_meta_attr" to reset meta attributes when special
	   attributes are modified. For DBD::File, modifying one of "f_file",
	   "f_dir", "f_ext" or "f_lockfile" will reset "f_fqfn". DBD::DBM
	   extends the list for "dbm_type" and "dbm_mldbm" to reset the value
	   of "dbm_tietype".

	   If your DBD has calculated values in the meta data area, then call

	       my %reset_on_modify = ( "xxx_foo" => "xxx_bar" );
	       __PACKAGE__->register_reset_on_modify( \%reset_on_modify );

	   Allows "get_table_meta_attr" and "set_table_meta_attr" to update
	   the attribute name to the current favored one:

	       # from DBD::DBM
	       my %compat_map = ( "dbm_ext" => "f_ext" );
	       __PACKAGE__->register_compat_map( \%compat_map );

	   Called to open the table's data file.

	   Depending on the attributes set in the table's meta data, the fol‐
	   lowing steps are performed. Unless "f_dontopen" is set to a true
	   value, "f_fqfn" must contain the full qualified file name for the
	   table to work on (file2table ensures this). The encoding in
	   "f_encoding" is applied if set and the file is opened. If "<f_fqln
	   "> (full qualified lock name) is set, this file is opened, too.
	   Depending on the value in "f_lock", the appropriate lock is set on
	   the opened data file or lock file.

	   After this is done, a derived class might add more steps in an
	   overridden "open_file" method.

       new Instantiates the table. This is done in 3 steps:

	    1. get the table meta data
	    2. open the data file
	    3. bless the table data structure using inherited constructor new

	   It is not recommended to override the constructor of the table
	   class.  Find a reasonable place to add you extensions in one of the
	   above four methods.

	   Implements the abstract table method for the "DROP" command. Dis‐
	   cards table meta data after all files belonging to the table are
	   closed and unlinked.

	   Overriding this method might be reasonable in very rare cases.

	   Implements the abstract table method used when accessing the table
	   from the engine. "seek" is called every time the engine uses dumb
	   algorithms for iterating over the table content.

	   Implements the abstract table method used when dumb table algo‐
	   rithms for "UPDATE" or "DELETE" need to truncate the table storage
	   after the last written row.

       You should consult the documentation of "SQL::Eval::Table" (see
       SQL::Eval) to get more information about the abstract methods of the
       table's base class you have to override and a description of the table
       meta information expected by the SQL engines.

       The module DBD::File is currently maintained by

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

       The original author is Jochen Wiedmann.

       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 v5.8.8			  2010-09-16	      DBD::File::Developers(3)
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