phpman > perldoc > DBD::File::Developers(3pm)

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_data { ... }

        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::TableSource::FileSystem
    Provides data sources and table information on database driver and database handle level.

      package DBD::File::TableSource::FileSystem;

      sub data_sources ($;$)
      {
          my ($class, $drh, $attrs) = @_;
          ...
          }

      sub avail_tables
      {
          my ($class, $drh) = @_;
          ...
          }

    The "data_sources" method is called when the user invokes any of the following:

      @ary = DBI->data_sources ($driver);
      @ary = DBI->data_sources ($driver, \%attr);

      @ary = $dbh->data_sources ();
      @ary = $dbh->data_sources (\%attr);

    The "avail_tables" method is called when the user invokes any of the following:

      @names = $dbh->tables ($catalog, $schema, $table, $type);

      $sth   = $dbh->table_info ($catalog, $schema, $table, $type);
      $sth   = $dbh->table_info ($catalog, $schema, $table, $type, \%attr);

      $dbh->func ("list_tables");

    Every time where an "\%attr" argument can be specified, this "\%attr" object's
    "sql_table_source" attribute is preferred over the $dbh attribute or the driver default.

  DBD::File::DataSource::Stream
      package DBD::File::DataSource::Stream;

      @DBD::File::DataSource::Stream::ISA = 'DBI::DBD::SqlEngine::DataSource';

      sub complete_table_name
      {
          my ($self, $meta, $file, $respect_case) = @_;
          ...
          }

    Clears all meta attributes identifying a file: "f_fqfn", "f_fqbn" and "f_fqln". The table name
    is set according to $respect_case and "$meta->{sql_identifier_case}" (SQL_IC_LOWER,
    SQL_IC_UPPER).

      package DBD::File::DataSource::Stream;

      sub apply_encoding
      {
          my ($self, $meta, $fn) = @_;
          ...
          }

    Applies the encoding from *meta information* ("$meta->{f_encoding}") to the file handled opened
    in "open_data".

      package DBD::File::DataSource::Stream;

      sub open_data
      {
          my ($self, $meta, $attrs, $flags) = @_;
          ...
          }

    Opens ("dup (2)") the file handle provided in "$meta->{f_file}".

      package DBD::File::DataSource::Stream;

      sub can_flock { ... }

    Returns whether "flock (2)" is available or not (avoids retesting in subclasses).

  DBD::File::DataSource::File
      package DBD::File::DataSource::File;

      sub complete_table_name ($$;$)
      {
          my ($self, $meta, $table, $respect_case) = @_;
          ...
          }

    The method "complete_table_name" 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.

      package DBD::File::DataSource::File;

      sub open_data ($)
      {
          my ($self, $meta, $attrs, $flags) = @_;
          ...
          }

    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.

  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 parametrized 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.

    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" and "f_lockfile" 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) 2010-2013 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.