++ed by:
BENNING RWSTAUNER PERLANCAR GETTY MISHIN

95 PAUSE user(s)
79 non-PAUSE user(s).

Tim Bunce
and 1 contributors

NAME

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

SYNOPSIS

  perldoc DBD::File::Developers

  perldoc DBD::File::Roadmap

This document describes how DBD developers can write DBD::File based DBI drivers. It supplements DBI::DBD, 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 attributes f_version, sql_nano_version, sql_statement_version and (if not prohibited by a restrictive ${prefix}_valid_attrs) ${prefix}_version.

This method is called at the end 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, sql_valid_attrs, f_readonly_attrs and sql_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, sql_identifier_case, sql_quoted_identifier_case and sql_handler.

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_versions

This method is called by the code injected into the instantiated driver to provide the user callable driver method ${prefix}versions (e.g. dbm_versions, csv_versions, ...).

The DBD::File implementation returns all version information known by DBD::File (e.g. DBI version, Perl version, DBD::File version and the SQL handler version).

get_versions takes the $dbh as the first argument and optionally a second argument containing a table name. The second argument is not evaluated in DBD::File::db::get_versions itself - but might be in the future.

If the derived implementor class provides a method named get_${drv_prefix}versions, this is invoked and the return value of it is associated to the derived driver name:

    if (my $dgv = $dbh->{ImplementorClass}->can ("get_" . $drv_prefix . "versions") {
        (my $derived_driver = $dbh->{ImplementorClass}) =~ s/::db$//;
        $versions{$derived_driver} = &$dgv ($dbh, $table);
    }

Override it to add more version information about your module, (e.g. some kind of parser version in case of DBD::CSV, ...), if one line is not enough room to provide all relevant information.

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.

sql_parser_object

Returns a SQL::Parser instance, when sql_handler is set to "SQL::Statement". The parser instance is stored in sql_parser_object.

It is not recommended to override this method.

disconnect

Disconnects from a database. All local table information is discarded and the Active attribute is set to 0.

type_info_all

Returns information about all the types supported by DBD::File.

table_info

Returns a statement handle which is prepared to deliver information about all known tables.

list_tables

Returns a list of all known table names.

quote

Quotes a string for use in SQL statements.

commit

Warns about a useless call (if warnings enabled) and returns. DBD::File is typically a driver which commits every action instantly when executed.

rollback

Warns about a useless call (if warnings enabled) and returns. DBD::File is typically a driver which commits every action instantly when executed.

DBD::File::st

Contains the methods to deal with prepared statement handles:

bind_param

Common routine to bind placeholders to a statement for execution. It is dangerous to override this method without detailed knowledge about the DBD::File internal storage structure.

execute

Executes a previously prepared statement (with placeholders, if any).

finish

Finishes a statement handle, discards all buffered results. The prepared statement is not discarded so the statement can be executed again.

fetch

Fetches the next row from the result-set. This method may be rewritten in a later version and if it's overridden in a derived class, the derived implementation should not rely on the storage details.

fetchrow_arrayref

Alias for fetch.

FETCH

Fetches statement handle attributes. Supported attributes (for full overview see "Statement Handle Attributes" in DBI) are NAME and NULLABLE. Each column is returned as NULLABLE which might be wrong depending on the derived backend storage. If the statement handle has private attributes, they can be fetched using this method, too. Note that statement attributes are not associated with any table used in this statement.

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

STORE

Allows storing of statement private attributes.

rows

Returns the number of rows affected by the last execute. This method might return undef.

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. This method should be overridden when mapped attribute names should be returned for compatibility reasons.

set_table_meta_attr

Sets a single attribute in the table meta data. This method should be overridden when mapped attribute names should be modified for compatibility reasons.

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.

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 );
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.