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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
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.
perl v5.12.1 2010-07-02 DBD::File::Developers(3)
