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



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

SYNOPSIS
           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 { ... }

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

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

       DBD::File::dr
           Driver package, contains the methods DBI calls indirectly via DBI interface:

             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 package however if you need additional initialization in the
           connect method you may need to.

       DBD::File::db
           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 methods "set_versions" and "init_valid_attributes".

       DBD::File::st
           Contains the methods to deal with prepared statement handles. e.g.,

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

   DBD::File
       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.

   DBD::File::dr
       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";

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

       Methods provided by DBD::File:

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

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

       FETCH
           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
           prefix, the driver prefix (written as $drv_prefix) is added.

           The driver prefix is extracted from the attribute name and verified against "$dbh->{ $drv_prefix
           . "valid_attrs" }" (when it exists). If the requested attribute value is not listed as a valid
           attribute, this method croaks. If the attribute is valid and readonly (listed in "$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.

       STORE
           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 database 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 "DBD::File::db::init_valid_attributes".

       set_versions
           This method sets the attribute "f_version" with the version of DBD::File.

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

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

       init_valid_attributes
           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";

       init_default_attributes
           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 validate 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 prefix).

           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_table_meta_attr" to FETCH attribute
           values and "DBD::DRV::Table::set_table_meta_attr" to STORE attribute values. This allows it to
           map meta attributes for compatibility reasons.

       get_single_table_meta
       get_file_meta
           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 $table being a regular
           expression matching against the table names (evaluated without the default table). The table name
           '*' is all currently known tables, including the default one. The table name '+' is all table
           names which conform to ANSI file name restrictions (/^[_A-Za-z0-9]+$/).

           The table meta information is retrieved using the get_table_meta and get_table_meta_attr methods
           of the table class of the implementation.

       set_single_table_meta
       set_file_meta
           Sets an attribute in a table's meta information. The method signature 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 get_file_meta.

           The table meta information is updated using the get_table_meta and set_table_meta_attr methods of
           the table class of the implementation.

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

   DBD::File::st
       Contains the methods to deal with prepared statement handles:

       FETCH
           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 prepared.  When
           SQL::Statement has additional information about a table, 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.

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

       open_table
           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

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

       file2table
           This method tries to map a filename to the associated table name.  It is called with a partially
           filled meta structure for the resulting 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
           undef.

       bootstrap_table_meta
           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 table.

           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.

       init_table_meta
           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.

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

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

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

       table_meta_attr_changed
           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 structure) and the initialized flag is removed, too. The decision
           is made based on %register_reset_on_modify.

       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 "register_reset_on_modify":

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

       register_compat_map
           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 );

       open_file
           Called to open the table's data file.

           Depending on the attributes set in the table's meta data, the following 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.

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

           Overriding this method might be reasonable in very rare cases.

       seek
           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.

       truncate
           Implements the abstract table method used when dumb table algorithms 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.

AUTHOR
       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 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::Developers(3)