DBI::DBD(3pm) User Contributed Perl Documentation DBI::DBD(3pm)
NAME
DBI::DBD - Perl DBI Database Driver Writer's Guide
SYNOPSIS
perldoc DBI::DBD
Version and volatility
This document is still a minimal draft which is in need of further work.
Please read the DBI documentation first and fully. Then look at the
implementation of some high-profile and regularly maintained drivers
like DBD::Oracle, DBD::ODBC, DBD::Pg etc. (Those are no no particular
order.)
Then reread the DBI specification and the code of those drivers again as
you're reading this. It'll help. Where this document and the driver
code differ it's likely that the driver code is more correct, especially
if multiple drivers do the same thing.
This document is a patchwork of contributions from various authors.
More contributions (preferably as patches) are very welcome.
DESCRIPTION
This document is primarily intended to help people writing new database
drivers for the Perl Database Interface (Perl DBI). It may also help
others interested in discovering why the internals of a DBD driver are
written the way they are.
This is a guide. Few (if any) of the statements in it are completely
authoritative under all possible circumstances. This means you will
need to use judgement in applying the guidelines in this document. If
in any doubt at all, please do contact the dbi-dev mailing list (details
given below) where Tim Bunce and other driver authors can help.
CREATING A NEW DRIVER
The first rule for creating a new database driver for the Perl DBI is
very simple: DON'T!
There is usually a driver already available for the database you want to
use, almost regardless of which database you choose. Very often, the
database will provide an ODBC driver interface, so you can often use
DBD::ODBC to access the database. This is typically less convenient on a
Unix box than on a Microsoft Windows box, but there are numerous options
for ODBC driver managers on Unix too, and very often the ODBC driver is
provided by the database supplier.
Before deciding that you need to write a driver, do your homework to
ensure that you are not wasting your energies.
[As of December 2002, the consensus is that if you need an ODBC driver
manager on Unix, then the unixODBC driver (available from
<http://www.unixodbc.org/>) is the way to go.]
The second rule for creating a new database driver for the Perl DBI is
also very simple: Don't -- get someone else to do it for you!
Nevertheless, there are occasions when it is necessary to write a new
driver, often to use a proprietary language or API to access the
database more swiftly, or more comprehensively, than an ODBC driver can.
Then you should read this document very carefully, but with a suitably
sceptical eye.
If there is something in here that does not make any sense, question it.
You might be right that the information is bogus, but don't come to that
conclusion too quickly.
URLs and mailing lists
The primary web-site for locating DBI software and information is
http://dbi.perl.org/
There are two main and one auxiliary mailing lists for people working
with DBI. The primary lists are dbi-users@perl.org for general users of
DBI and DBD drivers, and dbi-dev@perl.org mainly for DBD driver writers
(don't join the dbi-dev list unless you have a good reason). The
auxiliary list is dbi-announce@perl.org for announcing new releases of
DBI or DBD drivers.
You can join these lists by accessing the web-site
<http://dbi.perl.org/>. The lists are closed so you cannot send email
to any of the lists unless you join the list first.
You should also consider monitoring the comp.lang.perl.* newsgroups,
especially comp.lang.perl.modules.
The Cheetah book
The definitive book on Perl DBI is the Cheetah book, so called because
of the picture on the cover. Its proper title is 'Programming the Perl
DBI: Database programming with Perl' by Alligator Descartes and Tim
Bunce, published by O'Reilly Associates, February 2000, ISBN
1-56592-699-4. Buy it now if you have not already done so, and read it.
Locating drivers
Before writing a new driver, it is in your interests to find out whether
there already is a driver for your database. If there is such a driver,
it would be much easier to make use of it than to write your own!
The primary web-site for locating Perl software is
<http://search.cpan.org/>. You should look under the various modules
listings for the software you are after. For example:
http://search.cpan.org/modlist/Database_Interfaces
Follow the DBD:: and DBIx:: links at the top to see those subsets.
See the DBI docs for information on DBI web sites and mailing lists.
Registering a new driver
Before going through any official registration process, you will need to
establish that there is no driver already in the works. You'll do that
by asking the DBI mailing lists whether there is such a driver
available, or whether anybody is working on one.
When you get the go ahead, you will need to establish the name of the
driver and a prefix for the driver. Typically, the name is based on the
name of the database software it uses, and the prefix is a contraction
of that. Hence, DBD::Oracle has the name Oracle and the prefix 'ora_'.
The prefix must be lowercase and contain no underscores other than the
one at the end.
This information will be recorded in the DBI module. Apart from
documentation purposes, registration is a prerequisite for installing
private methods.
If you are writing a driver which will not be distributed on CPAN, then
you should choose a prefix beginning with 'x_', to avoid potential
prefix collisions with drivers registered in the future. Thus, if you
wrote a non-CPAN distributed driver called DBD::CustomDB, the prefix
might be 'x_cdb_'.
This document assumes you are writing a driver called DBD::Driver, and
that the prefix 'drv_' is assigned to the driver.
Two styles of database driver
There are two distinct styles of database driver that can be written to
work with the Perl DBI.
Your driver can be written in pure Perl, requiring no C compiler. When
feasible, this is the best solution, but most databases are not written
in such a way that this can be done. Some examples of pure Perl drivers
are DBD::File and DBD::CSV.
Alternatively, and most commonly, your driver will need to use some C
code to gain access to the database. This will be classified as a C/XS
driver.
What code will you write?
There are a number of files that need to be written for either a pure
Perl driver or a C/XS driver. There are no extra files needed only by a
pure Perl driver, but there are several extra files needed only by a
C/XS driver.
Files common to pure Perl and C/XS drivers
Assuming that your driver is called DBD::Driver, these files are:
• Makefile.PL
• META.yml
• README
• MANIFEST
• Driver.pm
• lib/Bundle/DBD/Driver.pm
• lib/DBD/Driver/Summary.pm
• t/*.t
The first four files are mandatory. Makefile.PL is used to control how
the driver is built and installed. The README file tells people who
download the file about how to build the module and any prerequisite
software that must be installed. The MANIFEST file is used by the
standard Perl module distribution mechanism. It lists all the source
files that need to be distributed with your module. Driver.pm is what is
loaded by the DBI code; it contains the methods peculiar to your driver.
Although the META.yml file is not required you are advised to create
one. Of particular importance are the build_requires and
configure_requires attributes which newer CPAN modules understand. You
use these to tell the CPAN module (and CPANPLUS) that your build and
configure mechanisms require DBI. The best reference for META.yml (at
the time of writing) is
<http://module-build.sourceforge.net/META-spec-v1.4.html>. You can find
a reasonable example of a META.yml in DBD::ODBC.
The lib/Bundle/DBD/Driver.pm file allows you to specify other Perl
modules on which yours depends in a format that allows someone to type a
simple command and ensure that all the pre-requisites are in place as
well as building your driver.
The lib/DBD/Driver/Summary.pm file contains (an updated version of) the
information that was included - or that would have been included - in
the appendices of the Cheetah book as a summary of the abilities of your
driver and the associated database.
The files in the t subdirectory are unit tests for your driver. You
should write your tests as stringently as possible, while taking into
account the diversity of installations that you can encounter:
• Your tests should not casually modify operational databases.
• You should never damage existing tables in a database.
• You should code your tests to use a constrained name space within
the database. For example, the tables (and all other named objects)
that are created could all begin with 'dbd_drv_'.
• At the end of a test run, there should be no testing objects left
behind in the database.
• If you create any databases, you should remove them.
• If your database supports temporary tables that are automatically
removed at the end of a session, then exploit them as often as
possible.
• Try to make your tests independent of each other. If you have a test
t/t11dowhat.t that depends upon the successful running of
t/t10thingamy.t, people cannot run the single test case
t/t11dowhat.t. Further, running t/t11dowhat.t twice in a row is
likely to fail (at least, if t/t11dowhat.t modifies the database at
all) because the database at the start of the second run is not what
you saw at the start of the first run.
• Document in your README file what you do, and what privileges people
need to do it.
• You can, and probably should, sequence your tests by including a
test number before an abbreviated version of the test name; the
tests are run in the order in which the names are expanded by shell-
style globbing.
• It is in your interests to ensure that your tests work as widely as
possible.
Many drivers also install sub-modules DBD::Driver::SubModule for any of
a variety of different reasons, such as to support the metadata methods
(see the discussion of "METADATA METHODS" below). Such sub-modules are
conventionally stored in the directory lib/DBD/Driver. The module itself
would usually be in a file SubModule.pm. All such sub-modules should
themselves be version stamped (see the discussions far below).
Extra files needed by C/XS drivers
The software for a C/XS driver will typically contain at least four
extra files that are not relevant to a pure Perl driver.
• Driver.xs
• Driver.h
• dbdimp.h
• dbdimp.c
The Driver.xs file is used to generate C code that Perl can call to gain
access to the C functions you write that will, in turn, call down onto
your database software.
The Driver.h header is a stylized header that ensures you can access the
necessary Perl and DBI macros, types, and function declarations.
The dbdimp.h is used to specify which functions have been implemented by
your driver.
The dbdimp.c file is where you write the C code that does the real work
of translating between Perl-ish data types and what the database expects
to use and return.
There are some (mainly small, but very important) differences between
the contents of Makefile.PL and Driver.pm for pure Perl and C/XS
drivers, so those files are described both in the section on creating a
pure Perl driver and in the section on creating a C/XS driver.
Obviously, you can add extra source code files to the list.
Requirements on a driver and driver writer
To be remotely useful, your driver must be implemented in a format that
allows it to be distributed via CPAN, the Comprehensive Perl Archive
Network (<http://www.cpan.org/> and <http://search.cpan.org>). Of
course, it is easier if you do not have to meet this criterion, but you
will not be able to ask for much help if you do not do so, and no-one is
likely to want to install your module if they have to learn a new
installation mechanism.
CREATING A PURE PERL DRIVER
Writing a pure Perl driver is surprisingly simple. However, there are
some problems you should be aware of. The best option is of course
picking up an existing driver and carefully modifying one method after
the other.
Also look carefully at DBD::AnyData and DBD::Template.
As an example we take a look at the DBD::File driver, a driver for
accessing plain files as tables, which is part of the DBD::CSV package.
The minimal set of files we have to implement are Makefile.PL, README,
MANIFEST and Driver.pm.
Pure Perl version of Makefile.PL
You typically start with writing Makefile.PL, a Makefile generator. The
contents of this file are described in detail in the ExtUtils::MakeMaker
man pages. It is definitely a good idea if you start reading them. At
least you should know about the variables CONFIGURE, DEFINED, PM, DIR,
EXE_FILES, INC, LIBS, LINKTYPE, NAME, OPTIMIZE, PL_FILES, VERSION,
VERSION_FROM, clean, depend, realclean from the ExtUtils::MakeMaker man
page: these are used in almost any Makefile.PL.
Additionally read the section on Overriding MakeMaker Methods and the
descriptions of the distcheck, disttest and dist targets: They will
definitely be useful for you.
Of special importance for DBI drivers is the postamble method from the
ExtUtils::MM_Unix man page.
For Emacs users, I recommend the libscan method, which removes Emacs
backup files (file names which end with a tilde '~') from lists of
files.
Now an example, I use the word "Driver" wherever you should insert your
driver's name:
# -*- perl -*-
use ExtUtils::MakeMaker;
WriteMakefile(
dbd_edit_mm_attribs( {
'NAME' => 'DBD::Driver',
'VERSION_FROM' => 'Driver.pm',
'INC' => '',
'dist' => { 'SUFFIX' => '.gz',
'COMPRESS' => 'gzip -9f' },
'realclean' => { FILES => '*.xsi' },
'PREREQ_PM' => '1.03',
'CONFIGURE' => sub {
eval {require DBI::DBD;};
if ($@) {
warn $@;
exit 0;
}
my $dbi_arch_dir = dbd_dbi_arch_dir();
if (exists($opts{INC})) {
return {INC => "$opts{INC} -I$dbi_arch_dir"};
} else {
return {INC => "-I$dbi_arch_dir"};
}
}
},
{ create_pp_tests => 1})
);
package MY;
sub postamble { return main::dbd_postamble(@_); }
sub libscan {
my ($self, $path) = @_;
($path =~ m/\~$/) ? undef : $path;
}
Note the calls to dbd_edit_mm_attribs() and dbd_postamble().
The second hash reference in the call to dbd_edit_mm_attribs()
(containing create_pp_tests()) is optional; you should not use it unless
your driver is a pure Perl driver (that is, it does not use C and XS
code). Therefore, the call to dbd_edit_mm_attribs() is not relevant for
C/XS drivers and may be omitted; simply use the (single) hash reference
containing NAME etc as the only argument to WriteMakefile().
Note that the dbd_edit_mm_attribs() code will fail if you do not have a
t sub-directory containing at least one test case.
PREREQ_PM tells MakeMaker that DBI (version 1.03 in this case) is
required for this module. This will issue a warning that DBI 1.03 is
missing if someone attempts to install your DBD without DBI 1.03. See
CONFIGURE below for why this does not work reliably in stopping cpan
testers failing your module if DBI is not installed.
CONFIGURE is a subroutine called by MakeMaker during "WriteMakefile".
By putting the "require DBI::DBD" in this section we can attempt to load
DBI::DBD but if it is missing we exit with success. As we exit
successfully without creating a Makefile when DBI::DBD is missing cpan
testers will not report a failure. This may seem at odds with PREREQ_PM
but PREREQ_PM does not cause "WriteMakefile" to fail (unless you also
specify PREREQ_FATAL which is strongly discouraged by MakeMaker) so
"WriteMakefile" would continue to call "dbd_dbi_arch_dir" and fail.
All drivers must use dbd_postamble() or risk running into problems.
Note the specification of VERSION_FROM; the named file (Driver.pm) will
be scanned for the first line that looks like an assignment to $VERSION,
and the subsequent text will be used to determine the version number.
Note the commentary in ExtUtils::MakeMaker on the subject of correctly
formatted version numbers.
If your driver depends upon external software (it usually will), you
will need to add code to ensure that your environment is workable before
the call to WriteMakefile(). If you need to check for the existence of
an external library and perhaps modify INC to include the paths to where
the external library header files are located and you cannot find the
library or header files make sure you output a message saying they
cannot be found but "exit 0" (success) before calling "WriteMakefile" or
CPAN testers will fail your module if the external library is not found.
A full-fledged Makefile.PL can be quite large (for example, the files
for DBD::Oracle and DBD::Informix are both over 1000 lines long, and the
Informix one uses - and creates - auxiliary modules too).
See also ExtUtils::MakeMaker and ExtUtils::MM_Unix. Consider using
CPAN::MakeMaker in place of ExtUtils::MakeMaker.
README
The README file should describe what the driver is for, the pre-
requisites for the build process, the actual build process, how to
report errors, and who to report them to.
Users will find ways of breaking the driver build and test process which
you would never even have dreamed to be possible in your worst
nightmares. Therefore, you need to write this document defensively,
precisely and concisely.
As always, use the README from one of the established drivers as a basis
for your own; the version in DBD::Informix is worth a look as it has
been quite successful in heading off problems.
• Note that users will have versions of Perl and DBI that are both
older and newer than you expected, but this will seldom cause much
trouble. When it does, it will be because you are using features of
DBI that are not supported in the version they are using.
• Note that users will have versions of the database software that are
both older and newer than you expected. You will save yourself time
in the long run if you can identify the range of versions which have
been tested and warn about versions which are not known to be OK.
• Note that many people trying to install your driver will not be
experts in the database software.
• Note that many people trying to install your driver will not be
experts in C or Perl.
MANIFEST
The MANIFEST will be used by the Makefile's dist target to build the
distribution tar file that is uploaded to CPAN. It should list every
file that you want to include in your distribution, one per line.
lib/Bundle/DBD/Driver.pm
The CPAN module provides an extremely powerful bundle mechanism that
allows you to specify pre-requisites for your driver.
The primary pre-requisite is Bundle::DBI; you may want or need to add
some more. With the bundle set up correctly, the user can type:
perl -MCPAN -e 'install Bundle::DBD::Driver'
and Perl will download, compile, test and install all the Perl modules
needed to build your driver.
The prerequisite modules are listed in the "CONTENTS" section, with the
official name of the module followed by a dash and an informal name or
description.
• Listing Bundle::DBI as the main pre-requisite simplifies life.
• Don't forget to list your driver.
• Note that unless the DBMS is itself a Perl module, you cannot list
it as a pre-requisite in this file.
• You should keep the version of the bundle the same as the version of
your driver.
• You should add configuration management, copyright, and licensing
information at the top.
A suitable skeleton for this file is shown below.
package Bundle::DBD::Driver;
$VERSION = '0.01';
1;
__END__
=head1 NAME
Bundle::DBD::Driver - A bundle to install all DBD::Driver related modules
=head1 SYNOPSIS
C<perl -MCPAN -e 'install Bundle::DBD::Driver'>
=head1 CONTENTS
Bundle::DBI - Bundle for DBI by TIMB (Tim Bunce)
DBD::Driver - DBD::Driver by YOU (Your Name)
=head1 DESCRIPTION
This bundle includes all the modules used by the Perl Database
Interface (DBI) driver for Driver (DBD::Driver), assuming the
use of DBI version 1.13 or later, created by Tim Bunce.
If you've not previously used the CPAN module to install any
bundles, you will be interrogated during its setup phase.
But when you've done it once, it remembers what you told it.
You could start by running:
C<perl -MCPAN -e 'install Bundle::CPAN'>
=head1 SEE ALSO
Bundle::DBI
=head1 AUTHOR
Your Name E<lt>F<you@yourdomain.com>E<gt>
=head1 THANKS
This bundle was created by ripping off Bundle::libnet created by
Graham Barr E<lt>F<gbarr@ti.com>E<gt>, and radically simplified
with some information from Jochen Wiedmann E<lt>F<joe@ispsoft.de>E<gt>.
The template was then included in the DBI::DBD documentation by
Jonathan Leffler E<lt>F<jleffler@informix.com>E<gt>.
=cut
lib/DBD/Driver/Summary.pm
There is no substitute for taking the summary file from a driver that
was documented in the Perl book (such as DBD::Oracle or DBD::Informix or
DBD::ODBC, to name but three), and adapting it to describe the
facilities available via DBD::Driver when accessing the Driver database.
Pure Perl version of Driver.pm
The Driver.pm file defines the Perl module DBD::Driver for your driver.
It will define a package DBD::Driver along with some version
information, some variable definitions, and a function driver() which
will have a more or less standard structure.
It will also define three sub-packages of DBD::Driver:
DBD::Driver::dr
with methods connect(), data_sources() and disconnect_all();
DBD::Driver::db
with methods such as prepare();
DBD::Driver::st
with methods such as execute() and fetch().
The Driver.pm file will also contain the documentation specific to
DBD::Driver in the format used by perldoc.
In a pure Perl driver, the Driver.pm file is the core of the
implementation. You will need to provide all the key methods needed by
DBI.
Now let's take a closer look at an excerpt of File.pm as an example. We
ignore things that are common to any module (even non-DBI modules) or
really specific to the DBD::File package.
The DBD::Driver package
The header
package DBD::File;
use strict;
our $VERSION = "1.23.00" # Version number of DBD::File
This is where the version number of your driver is specified, and is
where Makefile.PL looks for this information. Please ensure that any
other modules added with your driver are also version stamped so that
CPAN does not get confused.
It is recommended that you use a two-part (1.23) or three-part (1.23.45)
version number. Also consider the CPAN system, which gets confused and
considers version 1.10 to precede version 1.9, so that using a raw CVS,
RCS or SCCS version number is probably not appropriate (despite being
very common).
For Subversion you could use:
our $VERSION = "12.012346";
(use lots of leading zeros on the second portion so if you move the code
to a shared repository like svn.perl.org the much larger revision
numbers won't cause a problem, at least not for a few years). For RCS
or CVS you can use:
our $VERSION = "11.22";
which pads out the fractional part with leading zeros so all is well (so
long as you don't go past x.99)
our $drh = undef; # holds driver handle once initialized
This is where the driver handle will be stored, once created. Note that
you may assume there is only one handle for your driver.
The driver constructor
The driver() method is the driver handle constructor. Note that the
driver() method is in the DBD::Driver package, not in one of the sub-
packages DBD::Driver::dr, DBD::Driver::db, or DBD::Driver::db.
sub driver
{
return $drh if $drh; # already created - return same one
my ($class, $attr) = @_;
$class .= "::dr";
DBD::Driver::db->install_method('drv_example_dbh_method');
DBD::Driver::st->install_method('drv_example_sth_method');
# not a 'my' since we use it above to prevent multiple drivers
$drh = DBI::_new_drh($class, {
'Name' => 'File',
'Version' => $VERSION,
'Attribution' => 'DBD::File by Jochen Wiedmann',
})
or return undef;
return $drh;
}
This is a reasonable example of how DBI implements its handles. There
are three kinds: driver handles (typically stored in $drh; from now on
called drh or $drh), database handles (from now on called dbh or $dbh)
and statement handles (from now on called sth or $sth).
The prototype of DBI::_new_drh() is
$drh = DBI::_new_drh($class, $public_attrs, $private_attrs);
with the following arguments:
$class
is typically the class for your driver, (for example,
"DBD::File::dr"), passed as the first argument to the driver()
method.
$public_attrs
is a hash ref to attributes like Name, Version, and Attribution.
These are processed and used by DBI. You had better not make any
assumptions about them nor should you add private attributes here.
$private_attrs
This is another (optional) hash ref with your private attributes.
DBI will store them and otherwise leave them alone.
The DBI::_new_drh() method and the driver() method both return "undef"
for failure (in which case you must look at $DBI::err and $DBI::errstr
for the failure information, because you have no driver handle to use).
Using install_method() to expose driver-private methods
DBD::Foo::db->install_method($method_name, \%attr);
Installs the driver-private method named by $method_name into the DBI
method dispatcher so it can be called directly, avoiding the need to use
the func() method.
It is called as a static method on the driver class to which the method
belongs. The method name must begin with the corresponding registered
driver-private prefix. For example, for DBD::Oracle $method_name must
being with '"ora_"', and for DBD::AnyData it must begin with '"ad_"'.
The "\%attr" attributes can be used to provide fine control over how the
DBI dispatcher handles the dispatching of the method. However it's
undocumented at the moment. See the IMA_* #define's in DBI.xs and the
O=>0x000x values in the initialization of %DBI::DBI_methods in DBI.pm.
(Volunteers to polish up and document the interface are very welcome to
get in touch via dbi-dev@perl.org).
Methods installed using install_method default to the standard error
handling behaviour for DBI methods: clearing err and errstr before
calling the method, and checking for errors to trigger RaiseError etc.
on return. This differs from the default behaviour of func().
Note for driver authors: The DBD::Foo::xx->install_method call won't
work until the class-hierarchy has been setup. Normally the DBI looks
after that just after the driver is loaded. This means install_method()
can't be called at the time the driver is loaded unless the class-
hierarchy is set up first. The way to do that is to call the
setup_driver() method:
DBI->setup_driver('DBD::Foo');
before using install_method().
The CLONE special subroutine
Also needed here, in the DBD::Driver package, is a CLONE() method that
will be called by perl when an interpreter is cloned. All your CLONE()
method needs to do, currently, is clear the cached $drh so the new
interpreter won't start using the cached $drh from the old interpreter:
sub CLONE {
undef $drh;
}
See
<http://search.cpan.org/dist/perl/pod/perlmod.pod#Making_your_module_threadsafe>
for details.
The DBD::Driver::dr package
The next lines of code look as follows:
package DBD::Driver::dr; # ====== DRIVER ======
$DBD::Driver::dr::imp_data_size = 0;
Note that no @ISA is needed here, or for the other DBD::Driver::*
classes, because the DBI takes care of that for you when the driver is
loaded.
*FIX ME* Explain what the imp_data_size is, so that implementors aren't
practicing cargo-cult programming.
The database handle constructor
The database handle constructor is the driver's (hence the changed
namespace) connect() method:
sub connect
{
my ($drh, $dr_dsn, $user, $auth, $attr) = @_;
# Some database specific verifications, default settings
# and the like can go here. This should only include
# syntax checks or similar stuff where it's legal to
# 'die' in case of errors.
# For example, many database packages requires specific
# environment variables to be set; this could be where you
# validate that they are set, or default them if they are not set.
my $driver_prefix = "drv_"; # the assigned prefix for this driver
# Process attributes from the DSN; we assume ODBC syntax
# here, that is, the DSN looks like var1=val1;...;varN=valN
foreach my $var ( split /;/, $dr_dsn ) {
my ($attr_name, $attr_value) = split '=', $var, 2;
return $drh->set_err($DBI::stderr, "Can't parse DSN part '$var'")
unless defined $attr_value;
# add driver prefix to attribute name if it doesn't have it already
$attr_name = $driver_prefix.$attr_name
unless $attr_name =~ /^$driver_prefix/o;
# Store attribute into %$attr, replacing any existing value.
# The DBI will STORE() these into $dbh after we've connected
$attr->{$attr_name} = $attr_value;
}
# Get the attributes we'll use to connect.
# We use delete here because these no need to STORE them
my $db = delete $attr->{drv_database} || delete $attr->{drv_db}
or return $drh->set_err($DBI::stderr, "No database name given in DSN '$dr_dsn'");
my $host = delete $attr->{drv_host} || 'localhost';
my $port = delete $attr->{drv_port} || 123456;
# Assume you can attach to your database via drv_connect:
my $connection = drv_connect($db, $host, $port, $user, $auth)
or return $drh->set_err($DBI::stderr, "Can't connect to $dr_dsn: ...");
# create a 'blank' dbh (call superclass constructor)
my ($outer, $dbh) = DBI::_new_dbh($drh, { Name => $dr_dsn });
$dbh->STORE('Active', 1 );
$dbh->{drv_connection} = $connection;
return $outer;
}
This is mostly the same as in the driver handle constructor above. The
arguments are described in DBI.
The constructor DBI::_new_dbh() is called, returning a database handle.
The constructor's prototype is:
($outer, $inner) = DBI::_new_dbh($drh, $public_attr, $private_attr);
with similar arguments to those in the driver handle constructor, except
that the $class is replaced by $drh. The Name attribute is a standard
DBI attribute (see "Database Handle Attributes" in DBI).
In scalar context, only the outer handle is returned.
Note the use of the STORE() method for setting the dbh attributes.
That's because within the driver code, the handle object you have is the
'inner' handle of a tied hash, not the outer handle that the users of
your driver have.
Because you have the inner handle, tie magic doesn't get invoked when
you get or set values in the hash. This is often very handy for speed
when you want to get or set simple non-special driver-specific
attributes.
However, some attribute values, such as those handled by the DBI like
PrintError, don't actually exist in the hash and must be read via
"$h->FETCH($attrib)" and set via "$h->STORE($attrib, $value)". If in
any doubt, use these methods.
The data_sources() method
The data_sources() method must populate and return a list of valid data
sources, prefixed with the "dbi:Driver" incantation that allows them to
be used in the first argument of the "DBI->connect()" method. An
example of this might be scanning the $HOME/.odbcini file on Unix for
ODBC data sources (DSNs).
As a trivial example, consider a fixed list of data sources:
sub data_sources
{
my($drh, $attr) = @_;
my(@list) = ();
# You need more sophisticated code than this to set @list...
push @list, "dbi:Driver:abc";
push @list, "dbi:Driver:def";
push @list, "dbi:Driver:ghi";
# End of code to set @list
return @list;
}
The disconnect_all() method
If you need to release any resources when the driver is unloaded, you
can provide a disconnect_all method.
Other driver handle methods
If you need any other driver handle methods, they can follow here.
Error handling
It is quite likely that something fails in the connect method. With
DBD::File for example, you might catch an error when setting the current
directory to something not existent by using the (driver-specific) f_dir
attribute.
To report an error, you use the set_err() method:
$h->set_err($err, $errmsg, $state);
This will ensure that the error is recorded correctly and that
RaiseError and PrintError etc are handled correctly.
Typically you'll always use the method instance, aka your method's first
argument.
As set_err() always returns "undef" your error handling code can usually
be simplified to something like this:
return $h->set_err($err, $errmsg, $state) if ...;
The DBD::Driver::db package
package DBD::Driver::db; # ====== DATABASE ======
$DBD::Driver::db::imp_data_size = 0;
The statement handle constructor
There's nothing much new in the statement handle constructor, which is
the prepare() method:
sub prepare
{
my ($dbh, $statement, @attribs) = @_;
# create a 'blank' sth
my ($outer, $sth) = DBI::_new_sth($dbh, { Statement => $statement });
$sth->STORE('NUM_OF_PARAMS', ($statement =~ tr/?//));
$sth->{drv_params} = [];
return $outer;
}
This is still the same -- check the arguments and call the super class
constructor DBI::_new_sth(). Again, in scalar context, only the outer
handle is returned. The Statement attribute should be cached as shown.
Note the prefix drv_ in the attribute names: it is required that all
your private attributes use a lowercase prefix unique to your driver.
As mentioned earlier in this document, the DBI contains a registry of
known driver prefixes and may one day warn about unknown attributes that
don't have a registered prefix.
Note that we parse the statement here in order to set the attribute
NUM_OF_PARAMS. The technique illustrated is not very reliable; it can be
confused by question marks appearing in quoted strings, delimited
identifiers or in SQL comments that are part of the SQL statement. We
could set NUM_OF_PARAMS in the execute() method instead because the DBI
specification explicitly allows a driver to defer this, but then the
user could not call bind_param().
Transaction handling
Pure Perl drivers will rarely support transactions. Thus your commit()
and rollback() methods will typically be quite simple:
sub commit
{
my ($dbh) = @_;
if ($dbh->FETCH('Warn')) {
warn("Commit ineffective while AutoCommit is on");
}
0;
}
sub rollback {
my ($dbh) = @_;
if ($dbh->FETCH('Warn')) {
warn("Rollback ineffective while AutoCommit is on");
}
0;
}
Or even simpler, just use the default methods provided by the DBI that
do nothing except return "undef".
The DBI's default begin_work() method can be used by inheritance.
The STORE() and FETCH() methods
These methods (that we have already used, see above) are called for you,
whenever the user does a:
$dbh->{$attr} = $val;
or, respectively,
$val = $dbh->{$attr};
See perltie for details on tied hash refs to understand why these
methods are required.
The DBI will handle most attributes for you, in particular attributes
like RaiseError or PrintError. All you have to do is handle your
driver's private attributes and any attributes, like AutoCommit and
ChopBlanks, that the DBI can't handle for you.
A good example might look like this:
sub STORE
{
my ($dbh, $attr, $val) = @_;
if ($attr eq 'AutoCommit') {
# AutoCommit is currently the only standard attribute we have
# to consider.
if (!$val) { die "Can't disable AutoCommit"; }
return 1;
}
if ($attr =~ m/^drv_/) {
# Handle only our private attributes here
# Note that we could trigger arbitrary actions.
# Ideally we should warn about unknown attributes.
$dbh->{$attr} = $val; # Yes, we are allowed to do this,
return 1; # but only for our private attributes
}
# Else pass up to DBI to handle for us
$dbh->SUPER::STORE($attr, $val);
}
sub FETCH
{
my ($dbh, $attr) = @_;
if ($attr eq 'AutoCommit') { return 1; }
if ($attr =~ m/^drv_/) {
# Handle only our private attributes here
# Note that we could trigger arbitrary actions.
return $dbh->{$attr}; # Yes, we are allowed to do this,
# but only for our private attributes
}
# Else pass up to DBI to handle
$dbh->SUPER::FETCH($attr);
}
The DBI will actually store and fetch driver-specific attributes (with
all lowercase names) without warning or error, so there's actually no
need to implement driver-specific any code in your FETCH() and STORE()
methods unless you need extra logic/checks, beyond getting or setting
the value.
Unless your driver documentation indicates otherwise, the return value
of the STORE() method is unspecified and the caller shouldn't use that
value.
Other database handle methods
As with the driver package, other database handle methods may follow
here. In particular you should consider a (possibly empty) disconnect()
method and possibly a quote() method if DBI's default isn't correct for
you. You may also need the type_info_all() and get_info() methods, as
described elsewhere in this document.
Where reasonable use "$h->SUPER::foo()" to call the DBI's method in some
or all cases and just wrap your custom behavior around that.
If you want to use private trace flags you'll probably want to be able
to set them by name. To do that you'll need to define a
parse_trace_flag() method (note that's "parse_trace_flag", singular, not
"parse_trace_flags", plural).
sub parse_trace_flag {
my ($h, $name) = @_;
return 0x01000000 if $name eq 'foo';
return 0x02000000 if $name eq 'bar';
return 0x04000000 if $name eq 'baz';
return 0x08000000 if $name eq 'boo';
return 0x10000000 if $name eq 'bop';
return $h->SUPER::parse_trace_flag($name);
}
All private flag names must be lowercase, and all private flags must be
in the top 8 of the 32 bits.
The DBD::Driver::st package
This package follows the same pattern the others do:
package DBD::Driver::st;
$DBD::Driver::st::imp_data_size = 0;
The execute() and bind_param() methods
This is perhaps the most difficult method because we have to consider
parameter bindings here. In addition to that, there are a number of
statement attributes which must be set for inherited DBI methods to
function correctly (see "Statement attributes" below).
We present a simplified implementation by using the drv_params attribute
from above:
sub bind_param
{
my ($sth, $pNum, $val, $attr) = @_;
my $type = (ref $attr) ? $attr->{TYPE} : $attr;
if ($type) {
my $dbh = $sth->{Database};
$val = $dbh->quote($sth, $type);
}
my $params = $sth->{drv_params};
$params->[$pNum-1] = $val;
1;
}
sub execute
{
my ($sth, @bind_values) = @_;
# start of by finishing any previous execution if still active
$sth->finish if $sth->FETCH('Active');
my $params = (@bind_values) ?
\@bind_values : $sth->{drv_params};
my $numParam = $sth->FETCH('NUM_OF_PARAMS');
return $sth->set_err($DBI::stderr, "Wrong number of parameters")
if @$params != $numParam;
my $statement = $sth->{'Statement'};
for (my $i = 0; $i < $numParam; $i++) {
$statement =~ s/?/$params->[$i]/; # XXX doesn't deal with quoting etc!
}
# Do anything ... we assume that an array ref of rows is
# created and store it:
$sth->{'drv_data'} = $data;
$sth->{'drv_rows'} = @$data; # number of rows
$sth->STORE('NUM_OF_FIELDS') = $numFields;
$sth->{Active} = 1;
@$data || '0E0';
}
There are a number of things you should note here.
We initialize the NUM_OF_FIELDS and Active attributes here, because they
are essential for bind_columns() to work.
We use attribute "$sth->{Statement}" which we created within prepare().
The attribute "$sth->{Database}", which is nothing else than the dbh,
was automatically created by DBI.
Finally, note that (as specified in the DBI specification) we return the
string '0E0' instead of the number 0, so that the result tests true but
equal to zero.
$sth->execute() or die $sth->errstr;
The execute_array(), execute_for_fetch() and bind_param_array() methods
In general, DBD's only need to implement execute_for_fetch() and
"bind_param_array". DBI's default execute_array() will invoke the DBD's
execute_for_fetch() as needed.
The following sequence describes the interaction between DBI
"execute_array" and a DBD's "execute_for_fetch":
1. App calls "$sth->execute_array(\%attrs, @array_of_arrays)"
2. If @array_of_arrays was specified, DBI processes @array_of_arrays by
calling DBD's bind_param_array(). Alternately, App may have directly
called bind_param_array()
3. DBD validates and binds each array
4. DBI retrieves the validated param arrays from DBD's ParamArray
attribute
5. DBI calls DBD's "execute_for_fetch($fetch_tuple_sub,
\@tuple_status)", where &$fetch_tuple_sub is a closure to iterate
over the returned ParamArray values, and "\@tuple_status" is an
array to receive the disposition status of each tuple.
6. DBD iteratively calls &$fetch_tuple_sub to retrieve parameter tuples
to be added to its bulk database operation/request.
7. when DBD reaches the limit of tuples it can handle in a single
database operation/request, or the &$fetch_tuple_sub indicates no
more tuples by returning undef, the DBD executes the bulk operation,
and reports the disposition of each tuple in \@tuple_status.
8. DBD repeats steps 6 and 7 until all tuples are processed.
E.g., here's the essence of DBD::Oracle's execute_for_fetch:
while (1) {
my @tuple_batch;
for (my $i = 0; $i < $batch_size; $i++) {
push @tuple_batch, [ @{$fetch_tuple_sub->() || last} ];
}
last unless @tuple_batch;
my $res = ora_execute_array($sth, \@tuple_batch,
scalar(@tuple_batch), $tuple_batch_status);
push @$tuple_status, @$tuple_batch_status;
}
Note that DBI's default execute_array()/execute_for_fetch()
implementation requires the use of positional (i.e., '?') placeholders.
Drivers which require named placeholders must either emulate positional
placeholders (e.g., see DBD::Oracle), or must implement their own
execute_array()/execute_for_fetch() methods to properly sequence bound
parameter arrays.
Fetching data
Only one method needs to be written for fetching data,
fetchrow_arrayref(). The other methods, fetchrow_array(),
fetchall_arrayref(), etc, as well as the database handle's "select*"
methods are part of DBI, and call fetchrow_arrayref() as necessary.
sub fetchrow_arrayref
{
my ($sth) = @_;
my $data = $sth->{drv_data};
my $row = shift @$data;
if (!$row) {
$sth->STORE(Active => 0); # mark as no longer active
return undef;
}
if ($sth->FETCH('ChopBlanks')) {
map { $_ =~ s/\s+$//; } @$row;
}
return $sth->_set_fbav($row);
}
*fetch = \&fetchrow_arrayref; # required alias for fetchrow_arrayref
Note the use of the method _set_fbav() -- this is required so that
bind_col() and bind_columns() work.
If an error occurs which leaves the $sth in a state where remaining rows
can't be fetched then Active should be turned off before the method
returns.
The rows() method for this driver can be implemented like this:
sub rows { shift->{drv_rows} }
because it knows in advance how many rows it has fetched. Alternatively
you could delete that method and so fallback to the DBI's own method
which does the right thing based on the number of calls to _set_fbav().
The more_results method
If your driver doesn't support multiple result sets, then don't even
implement this method.
Otherwise, this method needs to get the statement handle ready to fetch
results from the next result set, if there is one. Typically you'd start
with:
$sth->finish;
then you should delete all the attributes from the attribute cache that
may no longer be relevant for the new result set:
delete $sth->{$_}
for qw(NAME TYPE PRECISION SCALE ...);
for drivers written in C use:
hv_delete((HV*)SvRV(sth), "NAME", 4, G_DISCARD);
hv_delete((HV*)SvRV(sth), "NULLABLE", 8, G_DISCARD);
hv_delete((HV*)SvRV(sth), "NUM_OF_FIELDS", 13, G_DISCARD);
hv_delete((HV*)SvRV(sth), "PRECISION", 9, G_DISCARD);
hv_delete((HV*)SvRV(sth), "SCALE", 5, G_DISCARD);
hv_delete((HV*)SvRV(sth), "TYPE", 4, G_DISCARD);
Don't forget to also delete, or update, any driver-private attributes
that may not be correct for the next resultset.
The NUM_OF_FIELDS attribute is a special case. It should be set using
STORE:
$sth->STORE(NUM_OF_FIELDS => 0); /* for DBI <= 1.53 */
$sth->STORE(NUM_OF_FIELDS => $new_value);
for drivers written in C use this incantation:
/* Adjust NUM_OF_FIELDS - which also adjusts the row buffer size */
DBIc_NUM_FIELDS(imp_sth) = 0; /* for DBI <= 1.53 */
DBIc_STATE(imp_xxh)->set_attr_k(sth, sv_2mortal(newSVpvn("NUM_OF_FIELDS",13)), 0,
sv_2mortal(newSViv(mysql_num_fields(imp_sth->result)))
);
For DBI versions prior to 1.54 you'll also need to explicitly adjust the
number of elements in the row buffer array (DBIc_FIELDS_AV(imp_sth)) to
match the new result set. Fill any new values with newSV(0) not
&sv_undef. Alternatively you could free DBIc_FIELDS_AV(imp_sth) and set
it to null, but that would mean bind_columns() wouldn't work across
result sets.
Statement attributes
The main difference between dbh and sth attributes is, that you should
implement a lot of attributes here that are required by the DBI, such as
NAME, NULLABLE, TYPE, etc. See "Statement Handle Attributes" in DBI for
a complete list.
Pay attention to attributes which are marked as read only, such as
NUM_OF_PARAMS. These attributes can only be set the first time a
statement is executed. If a statement is prepared, then executed
multiple times, warnings may be generated.
You can protect against these warnings, and prevent the recalculation of
attributes which might be expensive to calculate (such as the NAME and
NAME_* attributes):
my $storedNumParams = $sth->FETCH('NUM_OF_PARAMS');
if (!defined $storedNumParams or $storedNumFields < 0) {
$sth->STORE('NUM_OF_PARAMS') = $numParams;
# Set other useful attributes that only need to be set once
# for a statement, like $sth->{NAME} and $sth->{TYPE}
}
One particularly important attribute to set correctly (mentioned in
"ATTRIBUTES COMMON TO ALL HANDLES" in DBI is Active. Many DBI methods,
including bind_columns(), depend on this attribute.
Besides that the STORE() and FETCH() methods are mainly the same as
above for dbh's.
Other statement methods
A trivial finish() method to discard stored data, reset any attributes
(such as Active) and do "$sth->SUPER::finish()".
If you've defined a parse_trace_flag() method in ::db you'll also want
it in ::st, so just alias it in:
*parse_trace_flag = \&DBD::foo:db::parse_trace_flag;
And perhaps some other methods that are not part of the DBI
specification, in particular to make metadata available. Remember that
they must have names that begin with your drivers registered prefix so
they can be installed using install_method().
If DESTROY() is called on a statement handle that's still active
("$sth->{Active}" is true) then it should effectively call finish().
sub DESTROY {
my $sth = shift;
$sth->finish if $sth->FETCH('Active');
}
Tests
The test process should conform as closely as possibly to the Perl
standard test harness.
In particular, most (all) of the tests should be run in the t sub-
directory, and should simply produce an "ok" when run under "make test".
For details on how this is done, see the Camel book and the section in
Chapter 7, "The Standard Perl Library" on Test::Harness.
The tests may need to adapt to the type of database which is being used
for testing, and to the privileges of the user testing the driver. For
example, the DBD::Informix test code has to adapt in a number of places
to the type of database to which it is connected as different Informix
databases have different capabilities: some of the tests are for
databases without transaction logs; others are for databases with a
transaction log; some versions of the server have support for blobs, or
stored procedures, or user-defined data types, and others do not.
When a complete file of tests must be skipped, you can provide a reason
in a pseudo-comment:
if ($no_transactions_available)
{
print "1..0 # Skip: No transactions available\n";
exit 0;
}
Consider downloading the DBD::Informix code and look at the code in
DBD/Informix/TestHarness.pm which is used throughout the DBD::Informix
tests in the t sub-directory.
CREATING A C/XS DRIVER
Please also see the section under "CREATING A PURE PERL DRIVER"
regarding the creation of the Makefile.PL.
Creating a new C/XS driver from scratch will always be a daunting task.
You can and should greatly simplify your task by taking a good reference
driver implementation and modifying that to match the database product
for which you are writing a driver.
The de facto reference driver has been the one for DBD::Oracle written
by Tim Bunce, who is also the author of the DBI package. The DBD::Oracle
module is a good example of a driver implemented around a C-level API.
Nowadays it it seems better to base on DBD::ODBC, another driver
maintained by Tim and Jeff Urlwin, because it offers a lot of metadata
and seems to become the guideline for the future development. (Also as
DBD::Oracle digs deeper into the Oracle 8 OCI interface it'll get even
more hairy than it is now.)
The DBD::Informix driver is one driver implemented using embedded SQL
instead of a function-based API. DBD::Ingres may also be worth a look.
C/XS version of Driver.pm
A lot of the code in the Driver.pm file is very similar to the code for
pure Perl modules - see above. However, there are also some subtle (and
not so subtle) differences, including:
• The variables $DBD::Driver::{dr|db|st}::imp_data_size are not
defined here, but in the XS code, because they declare the size
of certain C structures.
• Some methods are typically moved to the XS code, in particular
prepare(), execute(), disconnect(), disconnect_all() and the
STORE() and FETCH() methods.
• Other methods are still part of Driver.pm, but have callbacks to
the XS code.
• If the driver-specific parts of the imp_drh_t structure need to
be formally initialized (which does not seem to be a common
requirement), then you need to add a call to an appropriate XS
function in the driver method of DBD::Driver::driver(), and you
define the corresponding function in Driver.xs, and you define
the C code in dbdimp.c and the prototype in dbdimp.h.
For example, DBD::Informix has such a requirement, and adds the
following call after the call to _new_drh() in Informix.pm:
DBD::Informix::dr::driver_init($drh);
and the following code in Informix.xs:
# Initialize the DBD::Informix driver data structure
void
driver_init(drh)
SV *drh
CODE:
ST(0) = dbd_ix_dr_driver_init(drh) ? &sv_yes : &sv_no;
and the code in dbdimp.h declares:
extern int dbd_ix_dr_driver_init(SV *drh);
and the code in dbdimp.ec (equivalent to dbdimp.c) defines:
/* Formally initialize the DBD::Informix driver structure */
int
dbd_ix_dr_driver(SV *drh)
{
D_imp_drh(drh);
imp_drh->n_connections = 0; /* No active connections */
imp_drh->current_connection = 0; /* No current connection */
imp_drh->multipleconnections = (ESQLC_VERSION >= 600) ? True : False;
dbd_ix_link_newhead(&imp_drh->head); /* Empty linked list of connections */
return 1;
}
DBD::Oracle has a similar requirement but gets around it by
checking whether the private data part of the driver handle is
all zeroed out, rather than add extra functions.
Now let's take a closer look at an excerpt from Oracle.pm (revised
heavily to remove idiosyncrasies) as an example, ignoring things that
were already discussed for pure Perl drivers.
The connect method
The connect method is the database handle constructor. You could write
either of two versions of this method: either one which takes connection
attributes (new code) and one which ignores them (old code only).
If you ignore the connection attributes, then you omit all mention of
the $auth variable (which is a reference to a hash of attributes), and
the XS system manages the differences for you.
sub connect
{
my ($drh, $dbname, $user, $auth, $attr) = @_;
# Some database specific verifications, default settings
# and the like following here. This should only include
# syntax checks or similar stuff where it's legal to
# 'die' in case of errors.
my $dbh = DBI::_new_dbh($drh, {
'Name' => $dbname,
})
or return undef;
# Call the driver-specific function _login in Driver.xs file which
# calls the DBMS-specific function(s) to connect to the database,
# and populate internal handle data.
DBD::Driver::db::_login($dbh, $dbname, $user, $auth, $attr)
or return undef;
$dbh;
}
This is mostly the same as in the pure Perl case, the exception being
the use of the private _login() callback, which is the function that
will really connect to the database. It is implemented in Driver.xst
(you should not implement it) and calls dbd_db_login6() or
"dbd_db_login6_sv" from dbdimp.c. See below for details.
If your driver has driver-specific attributes which may be passed in the
connect method and hence end up in $attr in "dbd_db_login6" then it is
best to delete any you process so DBI does not send them again via STORE
after connect. You can do this in C like this:
DBD_ATTRIB_DELETE(attr, "my_attribute_name",
strlen("my_attribute_name"));
However, prior to DBI subversion version 11605 (and fixed post 1.607)
DBD_ATTRIB_DELETE segfaulted so if you cannot guarantee the DBI version
will be post 1.607 you need to use:
hv_delete((HV*)SvRV(attr), "my_attribute_name",
strlen("my_attribute_name"), G_DISCARD);
*FIX ME* Discuss removing attributes in Perl code.
The disconnect_all method
*FIX ME* T.B.S
The data_sources method
If your data_sources() method can be implemented in pure Perl, then do
so because it is easier than doing it in XS code (see the section above
for pure Perl drivers).
If your data_sources() method must call onto compiled functions, then
you will need to define dbd_dr_data_sources in your dbdimp.h file, which
will trigger Driver.xst (in DBI v1.33 or greater) to generate the XS
code that calls your actual C function (see the discussion below for
details) and you do not code anything in Driver.pm to handle it.
The prepare method
The prepare method is the statement handle constructor, and most of it
is not new. Like the connect() method, it now has a C callback:
package DBD::Driver::db; # ====== DATABASE ======
use strict;
sub prepare
{
my ($dbh, $statement, $attribs) = @_;
# create a 'blank' sth
my $sth = DBI::_new_sth($dbh, {
'Statement' => $statement,
})
or return undef;
# Call the driver-specific function _prepare in Driver.xs file
# which calls the DBMS-specific function(s) to prepare a statement
# and populate internal handle data.
DBD::Driver::st::_prepare($sth, $statement, $attribs)
or return undef;
$sth;
}
The execute method
*FIX ME* T.B.S
The fetchrow_arrayref method
*FIX ME* T.B.S
Other methods?
*FIX ME* T.B.S
Driver.xs
Driver.xs should look something like this:
#include "Driver.h"
DBISTATE_DECLARE;
INCLUDE: Driver.xsi
MODULE = DBD::Driver PACKAGE = DBD::Driver::dr
/* Non-standard drh XS methods following here, if any. */
/* If none (the usual case), omit the MODULE line above too. */
MODULE = DBD::Driver PACKAGE = DBD::Driver::db
/* Non-standard dbh XS methods following here, if any. */
/* Currently this includes things like _list_tables from */
/* DBD::mSQL and DBD::mysql. */
MODULE = DBD::Driver PACKAGE = DBD::Driver::st
/* Non-standard sth XS methods following here, if any. */
/* In particular this includes things like _list_fields from */
/* DBD::mSQL and DBD::mysql for accessing metadata. */
Note especially the include of Driver.xsi here: DBI inserts stub
functions for almost all private methods here which will typically do
much work for you.
Wherever you really have to implement something, it will call a private
function in dbdimp.c, and this is what you have to implement.
You need to set up an extra routine if your driver needs to export
constants of its own, analogous to the SQL types available when you say:
use DBI qw(:sql_types);
*FIX ME* T.B.S
Driver.h
Driver.h is very simple and the operational contents should look like
this:
#ifndef DRIVER_H_INCLUDED
#define DRIVER_H_INCLUDED
#define NEED_DBIXS_VERSION 93 /* 93 for DBI versions 1.00 to 1.51+ */
#define PERL_NO_GET_CONTEXT /* if used require DBI 1.51+ */
#include <DBIXS.h> /* installed by the DBI module */
#include "dbdimp.h"
#include "dbivport.h" /* see below */
#include <dbd_xsh.h> /* installed by the DBI module */
#endif /* DRIVER_H_INCLUDED */
The DBIXS.h header defines most of the interesting information that the
writer of a driver needs.
The file dbd_xsh.h header provides prototype declarations for the C
functions that you might decide to implement. Note that you should
normally only define one of dbd_db_login(), dbd_db_login6() or
"dbd_db_login6_sv" unless you are intent on supporting really old
versions of DBI (prior to DBI 1.06) as well as modern versions. The only
standard, DBI-mandated functions that you need write are those specified
in the dbd_xsh.h header. You might also add extra driver-specific
functions in Driver.xs.
The dbivport.h file should be copied from the latest DBI release into
your distribution each time you modify your driver. Its job is to allow
you to enhance your code to work with the latest DBI API while still
allowing your driver to be compiled and used with older versions of the
DBI (for example, when the DBIh_SET_ERR_CHAR() macro was added to DBI
1.41, an emulation of it was added to dbivport.h). This makes users
happy and your life easier. Always read the notes in dbivport.h to check
for any limitations in the emulation that you should be aware of.
With DBI v1.51 or better I recommend that the driver defines
PERL_NO_GET_CONTEXT before DBIXS.h is included. This can significantly
improve efficiency when running under a thread enabled perl. (Remember
that the standard perl in most Linux distributions is built with threads
enabled. So is ActiveState perl for Windows, and perl built for Apache
mod_perl2.) If you do this there are some things to keep in mind:
• If PERL_NO_GET_CONTEXT is defined, then every function that calls
the Perl API will need to start out with a "dTHX;" declaration.
• You'll know which functions need this, because the C compiler will
complain that the undeclared identifier "my_perl" is used if and
only if the perl you are using to develop and test your driver has
threads enabled.
• If you don't remember to test with a thread-enabled perl before
making a release it's likely that you'll get failure reports from
users who are.
• For driver private functions it is possible to gain even more
efficiency by replacing "dTHX;" with "pTHX_" prepended to the
parameter list and then "aTHX_" prepended to the argument list where
the function is called.
See "How multiple interpreters and concurrency are supported" in
perlguts for additional information about PERL_NO_GET_CONTEXT.
Implementation header dbdimp.h
This header file has two jobs:
First it defines data structures for your private part of the handles.
Note that the DBI provides many common fields for you. For example the
statement handle (imp_sth) already has a row_count field with an IV type
that accessed via the DBIc_ROW_COUNT(imp_sth) macro. Using this is
strongly recommended as it's built in to some DBI internals so the DBI
can 'just work' in more cases and you'll have less driver-specific code
to write. Study DBIXS.h to see what's included with each type of
handle.
Second it defines macros that rename the generic names like
dbd_db_login() to database specific names like ora_db_login(). This
avoids name clashes and enables use of different drivers when you work
with a statically linked perl.
It also will have the important task of disabling XS methods that you
don't want to implement.
Finally, the macros will also be used to select alternate
implementations of some functions. For example, the dbd_db_login()
function is not passed the attribute hash.
Since DBI v1.06, if a dbd_db_login6() macro is defined (for a function
with 6 arguments), it will be used instead with the attribute hash
passed as the sixth argument.
Since DBI post v1.607, if a dbd_db_login6_sv() macro is defined (for a
function like dbd_db_login6 but with scalar pointers for the dbname,
username and password), it will be used instead. This will allow your
login6 function to see if there are any Unicode characters in the
dbname.
Similarly defining dbd_db_do4_iv is preferred over dbd_db_do4,
dbd_st_rows_iv over dbd_st_rows, and dbd_st_execute_iv over
dbd_st_execute. The *_iv forms are declared to return the IV type
instead of an int.
People used to just pick Oracle's dbdimp.c and use the same names,
structures and types. I strongly recommend against that. At first glance
this saves time, but your implementation will be less readable. It was
just hell when I had to separate DBI specific parts, Oracle specific
parts, mSQL specific parts and mysql specific parts in DBD::mysql's
dbdimp.h and dbdimp.c. (DBD::mysql was a port of DBD::mSQL which was
based on DBD::Oracle.) [Seconded, based on the experience taking
DBD::Informix apart, even though the version inherited in 1996 was only
based on DBD::Oracle.]
This part of the driver is your exclusive part. Rewrite it from scratch,
so it will be clean and short: in other words, a better piece of code.
(Of course keep an eye on other people's work.)
struct imp_drh_st {
dbih_drc_t com; /* MUST be first element in structure */
/* Insert your driver handle attributes here */
};
struct imp_dbh_st {
dbih_dbc_t com; /* MUST be first element in structure */
/* Insert your database handle attributes here */
};
struct imp_sth_st {
dbih_stc_t com; /* MUST be first element in structure */
/* Insert your statement handle attributes here */
};
/* Rename functions for avoiding name clashes; prototypes are */
/* in dbd_xsh.h */
#define dbd_init drv_dr_init
#define dbd_db_login6_sv drv_db_login_sv
#define dbd_db_do drv_db_do
... many more here ...
These structures implement your private part of the handles.
You have to use the name "imp_dbh_{dr|db|st}" and the first field must
be of type dbih_drc_t|_dbc_t|_stc_t and must be called "com".
You should never access these fields directly, except by using the
DBIc_xxx() macros below.
Implementation source dbdimp.c
Conventionally, dbdimp.c is the main implementation file (but
DBD::Informix calls the file dbdimp.ec). This section includes a short
note on each function that is used in the Driver.xsi template and thus
has to be implemented.
Of course, you will probably also need to implement other support
functions, which should usually be file static if they are placed in
dbdimp.c. If they are placed in other files, you need to list those
files in Makefile.PL (and MANIFEST) to handle them correctly.
It is wise to adhere to a namespace convention for your functions to
avoid conflicts. For example, for a driver with prefix drv_, you might
call externally visible functions dbd_drv_xxxx. You should also avoid
non-constant global variables as much as possible to improve the support
for threading.
Since Perl requires support for function prototypes (ANSI or ISO or
Standard C), you should write your code using function prototypes too.
It is possible to use either the unmapped names such as dbd_init() or
the mapped names such as dbd_ix_dr_init() in the dbdimp.c file.
DBD::Informix uses the mapped names which makes it easier to identify
where to look for linkage problems at runtime (which will report errors
using the mapped names).
Most other drivers, and in particular DBD::Oracle, use the unmapped
names in the source code which makes it a little easier to compare code
between drivers and eases discussions on the dbi-dev mailing list. The
majority of the code fragments here will use the unmapped names.
Ultimately, you should provide implementations for most of the functions
listed in the dbd_xsh.h header. The exceptions are optional functions
(such as dbd_st_rows()) and those functions with alternative signatures,
such as "dbd_db_login6_sv", dbd_db_login6() and dbd_db_login(). Then you
should only implement one of the alternatives, and generally the newer
one of the alternatives.
The dbd_init method
#include "Driver.h"
DBISTATE_DECLARE;
void dbd_init(dbistate_t* dbistate)
{
DBISTATE_INIT; /* Initialize the DBI macros */
}
The dbd_init() function will be called when your driver is first loaded;
the bootstrap command in DBD::Driver::dr::driver() triggers this, and
the call is generated in the BOOT section of Driver.xst. These
statements are needed to allow your driver to use the DBI macros. They
will include your private header file dbdimp.h in turn. Note that
DBISTATE_INIT requires the name of the argument to dbd_init() to be
called dbistate().
The dbd_drv_error method
You need a function to record errors so DBI can access them properly.
You can call it whatever you like, but we'll call it dbd_drv_error()
here.
The argument list depends on your database software; different systems
provide different ways to get at error information.
static void dbd_drv_error(SV *h, int rc, const char *what)
{
Note that h is a generic handle, may it be a driver handle, a database
or a statement handle.
D_imp_xxh(h);
This macro will declare and initialize a variable imp_xxh with a pointer
to your private handle pointer. You may cast this to to imp_drh_t,
imp_dbh_t or imp_sth_t.
To record the error correctly, equivalent to the set_err() method, use
one of the DBIh_SET_ERR_CHAR(...) or DBIh_SET_ERR_SV(...) macros, which
were added in DBI 1.41:
DBIh_SET_ERR_SV(h, imp_xxh, err, errstr, state, method);
DBIh_SET_ERR_CHAR(h, imp_xxh, err_c, err_i, errstr, state, method);
For "DBIh_SET_ERR_SV" the err, errstr, state, and method parameters are
"SV*" (use &sv_undef instead of NULL).
For "DBIh_SET_ERR_CHAR" the err_c, errstr, state, method parameters are
"char*".
The err_i parameter is an "IV" that's used instead of err_c if err_c is
"Null".
The method parameter can be ignored.
The "DBIh_SET_ERR_CHAR" macro is usually the simplest to use when you
just have an integer error code and an error message string:
DBIh_SET_ERR_CHAR(h, imp_xxh, Nullch, rc, what, Nullch, Nullch);
As you can see, any parameters that aren't relevant to you can be
"Null".
To make drivers compatible with DBI < 1.41 you should be using
dbivport.h as described in "Driver.h" above.
The (obsolete) macros such as "DBIh_EVENT2" should be removed from
drivers.
The names "dbis" and "DBIS", which were used in previous versions of
this document, should be replaced with the DBIc_DBISTATE(imp_xxh) macro.
The name "DBILOGFP", which was also used in previous versions of this
document, should be replaced by DBIc_LOGPIO(imp_xxh).
Your code should not call the C "<stdio.h>" I/O functions; you should
use PerlIO_printf() as shown:
if (DBIc_TRACE_LEVEL(imp_xxh) >= 2)
PerlIO_printf(DBIc_LOGPIO(imp_xxh), "foobar %s: %s\n",
foo, neatsvpv(errstr,0));
That's the first time we see how tracing works within a DBI driver. Make
use of this as often as you can, but don't output anything at a trace
level less than 3. Levels 1 and 2 are reserved for the DBI.
You can define up to 8 private trace flags using the top 8 bits of
DBIc_TRACE_FLAGS(imp), that is: 0xFF000000. See the parse_trace_flag()
method elsewhere in this document.
The dbd_dr_data_sources method
This method is optional; the support for it was added in DBI v1.33.
As noted in the discussion of Driver.pm, if the data sources can be
determined by pure Perl code, do it that way. If, as in DBD::Informix,
the information is obtained by a C function call, then you need to
define a function that matches the prototype:
extern AV *dbd_dr_data_sources(SV *drh, imp_drh_t *imp_drh, SV *attrs);
An outline implementation for DBD::Informix follows, assuming that the
sqgetdbs() function call shown will return up to 100 databases names,
with the pointers to each name in the array dbsname and the name strings
themselves being stores in dbsarea.
AV *dbd_dr_data_sources(SV *drh, imp_drh_t *imp_drh, SV *attr)
{
int ndbs;
int i;
char *dbsname[100];
char dbsarea[10000];
AV *av = Nullav;
if (sqgetdbs(&ndbs, dbsname, 100, dbsarea, sizeof(dbsarea)) == 0)
{
av = NewAV();
av_extend(av, (I32)ndbs);
sv_2mortal((SV *)av);
for (i = 0; i < ndbs; i++)
av_store(av, i, newSVpvf("dbi:Informix:%s", dbsname[i]));
}
return(av);
}
The actual DBD::Informix implementation has a number of extra lines of
code, logs function entry and exit, reports the error from sqgetdbs(),
and uses "#define"'d constants for the array sizes.
The dbd_db_login6 method
int dbd_db_login6_sv(SV* dbh, imp_dbh_t* imp_dbh, SV* dbname,
SV* user, SV* auth, SV *attr);
or
int dbd_db_login6(SV* dbh, imp_dbh_t* imp_dbh, char* dbname,
char* user, char* auth, SV *attr);
This function will really connect to the database. The argument dbh is
the database handle. imp_dbh is the pointer to the handles private data,
as is imp_xxx in dbd_drv_error() above. The arguments dbname, user, auth
and attr correspond to the arguments of the driver handle's connect()
method.
You will quite often use database specific attributes here, that are
specified in the DSN. I recommend you parse the DSN (using Perl) within
the connect() method and pass the segments of the DSN via the attributes
parameter through _login() to dbd_db_login6().
Here's how you fetch them; as an example we use hostname attribute,
which can be up to 12 characters long excluding null terminator:
SV** svp;
STRLEN len;
char* hostname;
if ( (svp = DBD_ATTRIB_GET_SVP(attr, "drv_hostname", 12)) && SvTRUE(*svp)) {
hostname = SvPV(*svp, len);
DBD_ATTRIB_DELETE(attr, "drv_hostname", 12); /* avoid later STORE */
} else {
hostname = "localhost";
}
If you handle any driver specific attributes in the dbd_db_login6 method
you probably want to delete them from "attr" (as above with
DBD_ATTRIB_DELETE). If you don't delete your handled attributes DBI will
call "STORE" for each attribute after the connect/login and this is at
best redundant for attributes you have already processed.
Note: Until revision 11605 (post DBI 1.607), there was a problem with
DBD_ATTRIBUTE_DELETE so unless you require a DBI version after 1.607 you
need to replace each DBD_ATTRIBUTE_DELETE call with:
hv_delete((HV*)SvRV(attr), key, key_len, G_DISCARD)
Note that you can also obtain standard attributes such as AutoCommit and
ChopBlanks from the attributes parameter, using "DBD_ATTRIB_GET_IV" for
integer attributes.
If, for example, your database does not support transactions but
AutoCommit is set off (requesting transaction support), then you can
emulate a 'failure to connect'.
Now you should really connect to the database. In general, if the
connection fails, it is best to ensure that all allocated resources are
released so that the handle does not need to be destroyed separately. If
you are successful (and possibly even if you fail but you have allocated
some resources), you should use the following macros:
DBIc_IMPSET_on(imp_dbh);
This indicates that the driver (implementor) has allocated resources in
the imp_dbh structure and that the implementors private dbd_db_destroy()
function should be called when the handle is destroyed.
DBIc_ACTIVE_on(imp_dbh);
This indicates that the handle has an active connection to the server
and that the dbd_db_disconnect() function should be called before the
handle is destroyed.
Note that if you do need to fail, you should report errors via the drh
or imp_drh rather than via dbh or imp_dbh because imp_dbh will be
destroyed by the failure, so errors recorded in that handle will not be
visible to DBI, and hence not the user either.
Note too, that the function is passed dbh and imp_dbh, and there is a
macro "D_imp_drh_from_dbh" which can recover the imp_drh from the
imp_dbh. However, there is no DBI macro to provide you with the drh
given either the imp_dbh or the dbh or the imp_drh (and there's no way
to recover the dbh given just the imp_dbh).
This suggests that, despite the above notes about dbd_drv_error() taking
an "SV *", it may be better to have two error routines, one taking
imp_dbh and one taking imp_drh instead. With care, you can factor most
of the formatting code out so that these are small routines calling a
common error formatter. See the code in DBD::Informix 1.05.00 for more
information.
The dbd_db_login6() function should return TRUE for success, FALSE
otherwise.
Drivers implemented long ago may define the five-argument function
dbd_db_login() instead of dbd_db_login6(). The missing argument is the
attributes. There are ways to work around the missing attributes, but
they are ungainly; it is much better to use the 6-argument form. Even
later drivers will use dbd_db_login6_sv() which provides the dbname,
username and password as SVs.
The dbd_db_commit and dbd_db_rollback methods
int dbd_db_commit(SV *dbh, imp_dbh_t *imp_dbh);
int dbd_db_rollback(SV* dbh, imp_dbh_t* imp_dbh);
These are used for commit and rollback. They should return TRUE for
success, FALSE for error.
The arguments dbh and imp_dbh are the same as for dbd_db_login6() above;
I will omit describing them in what follows, as they appear always.
These functions should return TRUE for success, FALSE otherwise.
The dbd_db_disconnect method
This is your private part of the disconnect() method. Any dbh with the
ACTIVE flag on must be disconnected. (Note that you have to set it in
dbd_db_connect() above.)
int dbd_db_disconnect(SV* dbh, imp_dbh_t* imp_dbh);
The database handle will return TRUE for success, FALSE otherwise. In
any case it should do a:
DBIc_ACTIVE_off(imp_dbh);
before returning so DBI knows that dbd_db_disconnect() was executed.
Note that there's nothing to stop a dbh being disconnected while it
still have active children. If your database API reacts badly to trying
to use an sth in this situation then you'll need to add code like this
to all sth methods:
if (!DBIc_ACTIVE(DBIc_PARENT_COM(imp_sth)))
return 0;
Alternatively, you can add code to your driver to keep explicit track of
the statement handles that exist for each database handle and arrange to
destroy those handles before disconnecting from the database. There is
code to do this in DBD::Informix. Similar comments apply to the driver
handle keeping track of all the database handles.
Note that the code which destroys the subordinate handles should only
release the associated database resources and mark the handles inactive;
it does not attempt to free the actual handle structures.
This function should return TRUE for success, FALSE otherwise, but it is
not clear what anything can do about a failure.
The dbd_db_discon_all method
int dbd_discon_all (SV *drh, imp_drh_t *imp_drh);
This function may be called at shutdown time. It should make best-
efforts to disconnect all database handles - if possible. Some databases
don't support that, in which case you can do nothing but return
'success'.
This function should return TRUE for success, FALSE otherwise, but it is
not clear what anything can do about a failure.
The dbd_db_destroy method
This is your private part of the database handle destructor. Any dbh
with the IMPSET flag on must be destroyed, so that you can safely free
resources. (Note that you have to set it in dbd_db_connect() above.)
void dbd_db_destroy(SV* dbh, imp_dbh_t* imp_dbh)
{
DBIc_IMPSET_off(imp_dbh);
}
The DBI Driver.xst code will have called dbd_db_disconnect() for you, if
the handle is still 'active', before calling dbd_db_destroy().
Before returning the function must switch IMPSET to off, so DBI knows
that the destructor was called.
A DBI handle doesn't keep references to its children. But children do
keep references to their parents. So a database handle won't be
"DESTROY"'d until all its children have been "DESTROY"'d.
The dbd_db_STORE_attrib method
This function handles
$dbh->{$key} = $value;
Its prototype is:
int dbd_db_STORE_attrib(SV* dbh, imp_dbh_t* imp_dbh, SV* keysv,
SV* valuesv);
You do not handle all attributes; on the contrary, you should not handle
DBI attributes here: leave this to DBI. (There are two exceptions,
AutoCommit and ChopBlanks, which you should care about.)
The return value is TRUE if you have handled the attribute or FALSE
otherwise. If you are handling an attribute and something fails, you
should call dbd_drv_error(), so DBI can raise exceptions, if desired.
If dbd_drv_error() returns, however, you have a problem: the user will
never know about the error, because he typically will not check
"$dbh->errstr()".
I cannot recommend a general way of going on, if dbd_drv_error()
returns, but there are examples where even the DBI specification expects
that you croak(). (See the AutoCommit method in DBI.)
If you have to store attributes, you should either use your private data
structure imp_xxx, the handle hash (via "(HV*)SvRV(dbh)"), or use the
private imp_data.
The first is best for internal C values like integers or pointers and
where speed is important within the driver. The handle hash is best for
values the user may want to get/set via driver-specific attributes. The
private imp_data is an additional "SV" attached to the handle. You could
think of it as an unnamed handle attribute. It's not normally used.
The dbd_db_FETCH_attrib method
This is the counterpart of dbd_db_STORE_attrib(), needed for:
$value = $dbh->{$key};
Its prototype is:
SV* dbd_db_FETCH_attrib(SV* dbh, imp_dbh_t* imp_dbh, SV* keysv);
Unlike all previous methods this returns an "SV" with the value. Note
that you should normally execute sv_2mortal(), if you return a
nonconstant value. (Constant values are &sv_undef, &sv_no and &sv_yes.)
Note, that DBI implements a caching algorithm for attribute values. If
you think, that an attribute may be fetched, you store it in the dbh
itself:
if (cacheit) /* cache value for later DBI 'quick' fetch? */
hv_store((HV*)SvRV(dbh), key, kl, cachesv, 0);
The dbd_st_prepare method
This is the private part of the prepare() method. Note that you must not
really execute the statement here. You may, however, preparse and
validate the statement, or do similar things.
int dbd_st_prepare(SV* sth, imp_sth_t* imp_sth, char* statement,
SV* attribs);
A typical, simple, possibility is to do nothing and rely on the perl
prepare() code that set the Statement attribute on the handle. This
attribute can then be used by dbd_st_execute().
If the driver supports placeholders then the NUM_OF_PARAMS attribute
must be set correctly by dbd_st_prepare():
DBIc_NUM_PARAMS(imp_sth) = ...
If you can, you should also setup attributes like NUM_OF_FIELDS, NAME,
etc. here, but DBI doesn't require that - they can be deferred until
execute() is called. However, if you do, document it.
In any case you should set the IMPSET flag, as you did in
dbd_db_connect() above:
DBIc_IMPSET_on(imp_sth);
The dbd_st_execute method
This is where a statement will really be executed.
int dbd_st_execute(SV* sth, imp_sth_t* imp_sth);
"dbd_st_execute" should return -2 for any error, -1 if the number of
rows affected is unknown else it should be the number of affected
(updated, inserted) rows.
Note that you must be aware a statement may be executed repeatedly.
Also, you should not expect that finish() will be called between two
executions, so you might need code, like the following, near the start
of the function:
if (DBIc_ACTIVE(imp_sth))
dbd_st_finish(h, imp_sth);
If your driver supports the binding of parameters (it should!), but the
database doesn't, you must do it here. This can be done as follows:
SV *svp;
char* statement = DBD_ATTRIB_GET_PV(h, "Statement", 9, svp, "");
int numParam = DBIc_NUM_PARAMS(imp_sth);
int i;
for (i = 0; i < numParam; i++)
{
char* value = dbd_db_get_param(sth, imp_sth, i);
/* It is your drivers task to implement dbd_db_get_param, */
/* it must be setup as a counterpart of dbd_bind_ph. */
/* Look for '?' and replace it with 'value'. Difficult */
/* task, note that you may have question marks inside */
/* quotes and comments the like ... :-( */
/* See DBD::mysql for an example. (Don't look too deep into */
/* the example, you will notice where I was lazy ...) */
}
The next thing is to really execute the statement.
Note that you must set the attributes NUM_OF_FIELDS, NAME, etc when the
statement is successfully executed if the driver has not already done
so: they may be used even before a potential fetchrow(). In particular
you have to tell DBI the number of fields that the statement has,
because it will be used by DBI internally. Thus the function will
typically ends with:
if (isSelectStatement) {
DBIc_NUM_FIELDS(imp_sth) = numFields;
DBIc_ACTIVE_on(imp_sth);
}
It is important that the ACTIVE flag only be set for "SELECT" statements
(or any other statements that can return many values from the database
using a cursor-like mechanism). See dbd_db_connect() above for more
explanations.
There plans for a preparse function to be provided by DBI, but this has
not reached fruition yet. Meantime, if you want to know how ugly it can
get, try looking at the dbd_ix_preparse() in DBD::Informix dbdimp.ec and
the related functions in iustoken.c and sqltoken.c.
The dbd_st_fetch method
This function fetches a row of data. The row is stored in in an array,
of "SV"'s that DBI prepares for you. This has two advantages: it is fast
(you even reuse the "SV"'s, so they don't have to be created after the
first fetchrow()), and it guarantees that DBI handles bind_cols() for
you.
What you do is the following:
AV* av;
int numFields = DBIc_NUM_FIELDS(imp_sth); /* Correct, if NUM_FIELDS
is constant for this statement. There are drivers where this is
not the case! */
int chopBlanks = DBIc_is(imp_sth, DBIcf_ChopBlanks);
int i;
if (!fetch_new_row_of_data(...)) {
... /* check for error or end-of-data */
DBIc_ACTIVE_off(imp_sth); /* turn off Active flag automatically */
return Nullav;
}
/* get the fbav (field buffer array value) for this row */
/* it is very important to only call this after you know */
/* that you have a row of data to return. */
av = DBIc_DBISTATE(imp_sth)->get_fbav(imp_sth);
for (i = 0; i < numFields; i++) {
SV* sv = fetch_a_field(..., i);
if (chopBlanks && SvOK(sv) && type_is_blank_padded(field_type[i])) {
/* Remove white space from end (only) of sv */
}
sv_setsv(AvARRAY(av)[i], sv); /* Note: (re)use! */
}
return av;
There's no need to use a fetch_a_field() function returning an "SV*".
It's more common to use your database API functions to fetch the data as
character strings and use code like this:
sv_setpvn(AvARRAY(av)[i], char_ptr, char_count);
"NULL" values must be returned as "undef". You can use code like this:
SvOK_off(AvARRAY(av)[i]);
The function returns the "AV" prepared by DBI for success or "Nullav"
otherwise.
*FIX ME* Discuss what happens when there's no more data to fetch.
Are errors permitted if another fetch occurs after the first fetch
that reports no more data. (Permitted, not required.)
If an error occurs which leaves the $sth in a state where remaining rows
can't be fetched then Active should be turned off before the method
returns.
The dbd_st_finish3 method
The "$sth->finish()" method can be called if the user wishes to indicate
that no more rows will be fetched even if the database has more rows to
offer, and the DBI code can call the function when handles are being
destroyed. See the DBI specification for more background details.
In both circumstances, the DBI code ends up calling the dbd_st_finish3()
method (if you provide a mapping for dbd_st_finish3() in dbdimp.h), or
dbd_st_finish() otherwise. The difference is that dbd_st_finish3()
takes a third argument which is an "int" with the value 1 if it is being
called from a destroy() method and 0 otherwise.
Note that DBI v1.32 and earlier test on dbd_db_finish3() to call
dbd_st_finish3(); if you provide dbd_st_finish3(), either define
dbd_db_finish3() too, or insist on DBI v1.33 or later.
All it needs to do is turn off the Active flag for the sth. It will
only be called by Driver.xst code, if the driver has set ACTIVE to on
for the sth.
Outline example:
int dbd_st_finish3(SV* sth, imp_sth_t* imp_sth, int from_destroy) {
if (DBIc_ACTIVE(imp_sth))
{
/* close cursor or equivalent action */
DBIc_ACTIVE_off(imp_sth);
}
return 1;
}
The from_destroy parameter is true if dbd_st_finish3() is being called
from DESTROY() - and so the statement is about to be destroyed. For
many drivers there is no point in doing anything more than turning off
the Active flag in this case.
The function returns TRUE for success, FALSE otherwise, but there isn't
a lot anyone can do to recover if there is an error.
The dbd_st_destroy method
This function is the private part of the statement handle destructor.
void dbd_st_destroy(SV* sth, imp_sth_t* imp_sth) {
... /* any clean-up that's needed */
DBIc_IMPSET_off(imp_sth); /* let DBI know we've done it */
}
The DBI Driver.xst code will call dbd_st_finish() for you, if the sth
has the ACTIVE flag set, before calling dbd_st_destroy().
The dbd_st_STORE_attrib and dbd_st_FETCH_attrib methods
These functions correspond to dbd_db_STORE() and dbd_db_FETCH() attrib
above, except that they are for statement handles. See above.
int dbd_st_STORE_attrib(SV* sth, imp_sth_t* imp_sth, SV* keysv,
SV* valuesv);
SV* dbd_st_FETCH_attrib(SV* sth, imp_sth_t* imp_sth, SV* keysv);
The dbd_bind_ph method
This function is internally used by the bind_param() method, the
bind_param_inout() method and by the DBI Driver.xst code if execute() is
called with any bind parameters.
int dbd_bind_ph (SV *sth, imp_sth_t *imp_sth, SV *param,
SV *value, IV sql_type, SV *attribs,
int is_inout, IV maxlen);
The param argument holds an "IV" with the parameter number (1, 2, ...).
The value argument is the parameter value and sql_type is its type.
If your driver does not support bind_param_inout() then you should
ignore maxlen and croak if is_inout is TRUE.
If your driver does support bind_param_inout() then you should note that
value is the "SV" after dereferencing the reference passed to
bind_param_inout().
In drivers of simple databases the function will, for example, store the
value in a parameter array and use it later in dbd_st_execute(). See
the DBD::mysql driver for an example.
Implementing bind_param_inout support
To provide support for parameters bound by reference rather than by
value, the driver must do a number of things. First, and most
importantly, it must note the references and stash them in its own
driver structure. Secondly, when a value is bound to a column, the
driver must discard any previous reference bound to the column. On each
execute, the driver must evaluate the references and internally bind the
values resulting from the references. This is only applicable if the
user writes:
$sth->execute;
If the user writes:
$sth->execute(@values);
then DBI automatically calls the binding code for each element of
@values. These calls are indistinguishable from explicit user calls to
bind_param().
C/XS version of Makefile.PL
The Makefile.PL file for a C/XS driver is similar to the code needed for
a pure Perl driver, but there are a number of extra bits of information
needed by the build system.
For example, the attributes list passed to WriteMakefile() needs to
specify the object files that need to be compiled and built into the
shared object (DLL). This is often, but not necessarily, just dbdimp.o
(unless that should be dbdimp.obj because you're building on MS
Windows).
Note that you can reliably determine the extension of the object files
from the $Config{obj_ext} values, and there are many other useful pieces
of configuration information lurking in that hash. You get access to it
with:
use Config;
Methods which do not need to be written
The DBI code implements the majority of the methods which are accessed
using the notation "DBI->function()", the only exceptions being
"DBI->connect()" and "DBI->data_sources()" which require support from
the driver.
The DBI code implements the following documented driver, database and
statement functions which do not need to be written by the DBD driver
writer.
$dbh->do()
The default implementation of this function prepares, executes and
destroys the statement. This can be replaced if there is a better
way to implement this, such as "EXECUTE IMMEDIATE" which can
sometimes be used if there are no parameters.
$h->errstr()
$h->err()
$h->state()
$h->trace()
The DBD driver does not need to worry about these routines at all.
$h->{ChopBlanks}
This attribute needs to be honored during fetch() operations, but
does not need to be handled by the attribute handling code.
$h->{RaiseError}
The DBD driver does not need to worry about this attribute at all.
$h->{PrintError}
The DBD driver does not need to worry about this attribute at all.
$sth->bind_col()
Assuming the driver uses the "DBIc_DBISTATE(imp_xxh)->get_fbav()"
function (C drivers, see below), or the "$sth->_set_fbav($data)"
method (Perl drivers) the driver does not need to do anything about
this routine.
$sth->bind_columns()
Regardless of whether the driver uses
"DBIc_DBISTATE(imp_xxh)->get_fbav()", the driver does not need to do
anything about this routine as it simply iteratively calls
"$sth->bind_col()".
The DBI code implements a default implementation of the following
functions which do not need to be written by the DBD driver writer
unless the default implementation is incorrect for the Driver.
$dbh->quote()
This should only be written if the database does not accept the ANSI
SQL standard for quoting strings, with the string enclosed in single
quotes and any embedded single quotes replaced by two consecutive
single quotes.
For the two argument form of quote, you need to implement the
type_info() method to provide the information that quote needs.
$dbh->ping()
This should be implemented as a simple efficient way to determine
whether the connection to the database is still alive. Typically
code like this:
sub ping {
my $dbh = shift;
$sth = $dbh->prepare_cached(q{
select * from A_TABLE_NAME where 1=0
}) or return 0;
$sth->execute or return 0;
$sth->finish;
return 1;
}
where A_TABLE_NAME is the name of a table that always exists (such
as a database system catalogue).
$drh->default_user
The default implementation of default_user will get the database
username and password fields from $ENV{DBI_USER} and $ENV{DBI_PASS}.
You can override this method. It is called as follows:
($user, $pass) = $drh->default_user($user, $pass, $attr)
METADATA METHODS
The exposition above ignores the DBI MetaData methods. The metadata
methods are all associated with a database handle.
Using DBI::DBD::Metadata
The DBI::DBD::Metadata module is a good semi-automatic way for the
developer of a DBD module to write the get_info() and type_info()
functions quickly and accurately.
Generating the get_info method
Prior to DBI v1.33, this existed as the method write_getinfo_pm() in the
DBI::DBD module. From DBI v1.33, it exists as the method
write_getinfo_pm() in the DBI::DBD::Metadata module. This discussion
assumes you have DBI v1.33 or later.
You examine the documentation for write_getinfo_pm() using:
perldoc DBI::DBD::Metadata
To use it, you need a Perl DBI driver for your database which implements
the get_info() method. In practice, this means you need to install
DBD::ODBC, an ODBC driver manager, and an ODBC driver for your database.
With the pre-requisites in place, you might type:
perl -MDBI::DBD::Metadata -we \
"write_getinfo_pm (qw{ dbi:ODBC:foo_db username password Driver })"
The procedure writes to standard output the code that should be added to
your Driver.pm file and the code that should be written to
lib/DBD/Driver/GetInfo.pm.
You should review the output to ensure that it is sensible.
Generating the type_info method
Given the idea of the write_getinfo_pm() method, it was not hard to
devise a parallel method, write_typeinfo_pm(), which does the analogous
job for the DBI type_info_all() metadata method. The write_typeinfo_pm()
method was added to DBI v1.33.
You examine the documentation for write_typeinfo_pm() using:
perldoc DBI::DBD::Metadata
The setup is exactly analogous to the mechanism described in "Generating
the get_info method".
With the pre-requisites in place, you might type:
perl -MDBI::DBD::Metadata -we \
"write_typeinfo_pm (qw{ dbi:ODBC:foo_db username password Driver })"
The procedure writes to standard output the code that should be added to
your Driver.pm file and the code that should be written to
lib/DBD/Driver/TypeInfo.pm.
You should review the output to ensure that it is sensible.
Writing DBD::Driver::db::get_info
If you use the DBI::DBD::Metadata module, then the code you need is
generated for you.
If you decide not to use the DBI::DBD::Metadata module, you should
probably borrow the code from a driver that has done so (e.g.
DBD::Informix from version 1.05 onwards) and crib the code from there,
or look at the code that generates that module and follow that. The
method in Driver.pm will be very simple; the method in
lib/DBD/Driver/GetInfo.pm is not very much more complex unless your DBMS
itself is much more complex.
Note that some of the DBI utility methods rely on information from the
get_info() method to perform their operations correctly. See, for
example, the quote_identifier() and quote methods, discussed below.
Writing DBD::Driver::db::type_info_all
If you use the "DBI::DBD::Metadata" module, then the code you need is
generated for you.
If you decide not to use the "DBI::DBD::Metadata" module, you should
probably borrow the code from a driver that has done so (e.g.
"DBD::Informix" from version 1.05 onwards) and crib the code from there,
or look at the code that generates that module and follow that. The
method in Driver.pm will be very simple; the method in
lib/DBD/Driver/TypeInfo.pm is not very much more complex unless your
DBMS itself is much more complex.
Writing DBD::Driver::db::type_info
The guidelines on writing this method are still not really clear. No
sample implementation is available.
Writing DBD::Driver::db::table_info
*FIX ME* The guidelines on writing this method have not been written yet.
No sample implementation is available.
Writing DBD::Driver::db::column_info
*FIX ME* The guidelines on writing this method have not been written yet.
No sample implementation is available.
Writing DBD::Driver::db::primary_key_info
*FIX ME* The guidelines on writing this method have not been written yet.
No sample implementation is available.
Writing DBD::Driver::db::primary_key
*FIX ME* The guidelines on writing this method have not been written yet.
No sample implementation is available.
Writing DBD::Driver::db::foreign_key_info
*FIX ME* The guidelines on writing this method have not been written yet.
No sample implementation is available.
Writing DBD::Driver::db::tables
This method generates an array of names in a format suitable for being
embedded in SQL statements in places where a table name is expected.
If your database hews close enough to the SQL standard or if you have
implemented an appropriate table_info() function and and the appropriate
quote_identifier() function, then the DBI default version of this method
will work for your driver too.
Otherwise, you have to write a function yourself, such as:
sub tables
{
my($dbh, $cat, $sch, $tab, $typ) = @_;
my(@res);
my($sth) = $dbh->table_info($cat, $sch, $tab, $typ);
my(@arr);
while (@arr = $sth->fetchrow_array)
{
push @res, $dbh->quote_identifier($arr[0], $arr[1], $arr[2]);
}
return @res;
}
See also the default implementation in DBI.pm.
Writing DBD::Driver::db::quote
This method takes a value and converts it into a string suitable for
embedding in an SQL statement as a string literal.
If your DBMS accepts the SQL standard notation for strings (single
quotes around the string as a whole with any embedded single quotes
doubled up), then you do not need to write this method as DBI provides a
default method that does it for you.
If your DBMS uses an alternative notation or escape mechanism, then you
need to provide an equivalent function. For example, suppose your DBMS
used C notation with double quotes around the string and backslashes
escaping both double quotes and backslashes themselves. Then you might
write the function as:
sub quote
{
my($dbh, $str) = @_;
$str =~ s/["\\]/\\$&/gmo;
return qq{"$str"};
}
Handling newlines and other control characters is left as an exercise
for the reader.
This sample method ignores the $data_type indicator which is the
optional second argument to the method.
Writing DBD::Driver::db::quote_identifier
This method is called to ensure that the name of the given table (or
other database object) can be embedded into an SQL statement without
danger of misinterpretation. The result string should be usable in the
text of an SQL statement as the identifier for a table.
If your DBMS accepts the SQL standard notation for quoted identifiers
(which uses double quotes around the identifier as a whole, with any
embedded double quotes doubled up) and accepts "schema"."identifier"
(and "catalog"."schema"."identifier" when a catalog is specified), then
you do not need to write this method as DBI provides a default method
that does it for you.
In fact, even if your DBMS does not handle exactly that notation but you
have implemented the get_info() method and it gives the correct
responses, then it will work for you. If your database is fussier, then
you need to implement your own version of the function.
For example, DBD::Informix has to deal with an environment variable
DELIMIDENT. If it is not set, then the DBMS treats names enclosed in
double quotes as strings rather than names, which is usually a syntax
error. Additionally, the catalog portion of the name is separated from
the schema and table by a different delimiter (colon instead of dot),
and the catalog portion is never enclosed in quotes. (Fortunately, valid
strings for the catalog will never contain weird characters that might
need to be escaped, unless you count dots, dashes, slashes and at-signs
as weird.) Finally, an Informix database can contain objects that cannot
be accessed because they were created by a user with the DELIMIDENT
environment variable set, but the current user does not have it set. By
design choice, the quote_identifier() method encloses those identifiers
in double quotes anyway, which generally triggers a syntax error, and
the metadata methods which generate lists of tables etc omit those
identifiers from the result sets.
sub quote_identifier
{
my($dbh, $cat, $sch, $obj) = @_;
my($rv) = "";
my($qq) = (defined $ENV{DELIMIDENT}) ? '"' : '';
$rv .= qq{$cat:} if (defined $cat);
if (defined $sch)
{
if ($sch !~ m/^\w+$/o)
{
$qq = '"';
$sch =~ s/$qq/$qq$qq/gm;
}
$rv .= qq{$qq$sch$qq.};
}
if (defined $obj)
{
if ($obj !~ m/^\w+$/o)
{
$qq = '"';
$obj =~ s/$qq/$qq$qq/gm;
}
$rv .= qq{$qq$obj$qq};
}
return $rv;
}
Handling newlines and other control characters is left as an exercise
for the reader.
Note that there is an optional fourth parameter to this function which
is a reference to a hash of attributes; this sample implementation
ignores that.
This sample implementation also ignores the single-argument variant of
the method.
TRACING
Tracing in DBI is controlled with a combination of a trace level and a
set of flags which together are known as the trace settings. The trace
settings are stored in a single integer and divided into levels and
flags by a set of masks ("DBIc_TRACE_LEVEL_MASK" and
"DBIc_TRACE_FLAGS_MASK").
Each handle has it's own trace settings and so does the DBI. When you
call a method the DBI merges the handles settings into its own for the
duration of the call: the trace flags of the handle are OR'd into the
trace flags of the DBI, and if the handle has a higher trace level then
the DBI trace level is raised to match it. The previous DBI trace
settings are restored when the called method returns.
Trace Level
The trace level is the first 4 bits of the trace settings (masked by
"DBIc_TRACE_FLAGS_MASK") and represents trace levels of 1 to 15. Do not
output anything at trace levels less than 3 as they are reserved for
DBI.
For advice on what to output at each level see "Trace Levels" in DBI.
To test for a trace level you can use the "DBIc_TRACE_LEVEL" macro like
this:
if (DBIc_TRACE_LEVEL(imp_xxh) >= 2) {
PerlIO_printf(DBIc_LOGPIO(imp_xxh), "foobar");
}
Also note the use of PerlIO_printf which you should always use for
tracing and never the C "stdio.h" I/O functions.
Trace Flags
Trace flags are used to enable tracing of specific activities within the
DBI and drivers. The DBI defines some trace flags and drivers can define
others. DBI trace flag names begin with a capital letter and driver
specific names begin with a lowercase letter. For a list of DBI defined
trace flags see "Trace Flags" in DBI.
If you want to use private trace flags you'll probably want to be able
to set them by name. Drivers are expected to override the
parse_trace_flag (note the singular) and check if $trace_flag_name is a
driver specific trace flags and, if not, then call the DBIs default
parse_trace_flag(). To do that you'll need to define a
parse_trace_flag() method like this:
sub parse_trace_flag {
my ($h, $name) = @_;
return 0x01000000 if $name eq 'foo';
return 0x02000000 if $name eq 'bar';
return 0x04000000 if $name eq 'baz';
return 0x08000000 if $name eq 'boo';
return 0x10000000 if $name eq 'bop';
return $h->SUPER::parse_trace_flag($name);
}
All private flag names must be lowercase, and all private flags must be
in the top 8 of the 32 bits of DBIc_TRACE_FLAGS(imp) i.e., 0xFF000000.
If you've defined a parse_trace_flag() method in ::db you'll also want
it in ::st, so just alias it in:
*parse_trace_flag = \&DBD::foo:db::parse_trace_flag;
You may want to act on the current 'SQL' trace flag that DBI defines to
output SQL prepared/executed as DBI currently does not do SQL tracing.
Trace Macros
Access to the trace level and trace flags is via a set of macros.
DBIc_TRACE_SETTINGS(imp) returns the trace settings
DBIc_TRACE_LEVEL(imp) returns the trace level
DBIc_TRACE_FLAGS(imp) returns the trace flags
DBIc_TRACE(imp, flags, flaglevel, level)
e.g.,
DBIc_TRACE(imp, 0, 0, 4)
if level >= 4
DBIc_TRACE(imp, DBDtf_FOO, 2, 4)
if tracing DBDtf_FOO & level>=2 or level>=4
DBIc_TRACE(imp, DBDtf_FOO, 2, 0)
as above but never trace just due to level
WRITING AN EMULATION LAYER FOR AN OLD PERL INTERFACE
Study Oraperl.pm (supplied with DBD::Oracle) and Ingperl.pm (supplied
with DBD::Ingres) and the corresponding dbdimp.c files for ideas.
Note that the emulation code sets "$dbh->{CompatMode} = 1;" for each
connection so that the internals of the driver can implement behaviour
compatible with the old interface when dealing with those handles.
Setting emulation perl variables
For example, ingperl has a $sql_rowcount variable. Rather than try to
manually update this in Ingperl.pm it can be done faster in C code. In
dbd_init():
sql_rowcount = perl_get_sv("Ingperl::sql_rowcount", GV_ADDMULTI);
In the relevant places do:
if (DBIc_COMPAT(imp_sth)) /* only do this for compatibility mode handles */
sv_setiv(sql_rowcount, the_row_count);
OTHER MISCELLANEOUS INFORMATION
The imp_xyz_t types
Any handle has a corresponding C structure filled with private data.
Some of this data is reserved for use by DBI (except for using the DBIc
macros below), some is for you. See the description of the dbdimp.h file
above for examples. Most functions in dbdimp.c are passed both the
handle "xyz" and a pointer to "imp_xyz". In rare cases, however, you may
use the following macros:
D_imp_dbh(dbh)
Given a function argument dbh, declare a variable imp_dbh and
initialize it with a pointer to the handles private data. Note: This
must be a part of the function header, because it declares a
variable.
D_imp_sth(sth)
Likewise for statement handles.
D_imp_xxx(h)
Given any handle, declare a variable imp_xxx and initialize it with
a pointer to the handles private data. It is safe, for example, to
cast imp_xxx to "imp_dbh_t*", if "DBIc_TYPE(imp_xxx) == DBIt_DB".
(You can also call "sv_derived_from(h, "DBI::db")", but that's much
slower.)
D_imp_dbh_from_sth
Given a imp_sth, declare a variable imp_dbh and initialize it with a
pointer to the parent database handle's implementors structure.
Using DBIc_IMPSET_on
The driver code which initializes a handle should use DBIc_IMPSET_on()
as soon as its state is such that the cleanup code must be called. When
this happens is determined by your driver code.
Failure to call this can lead to corruption of data structures.
For example, DBD::Informix maintains a linked list of database handles
in the driver, and within each handle, a linked list of statements. Once
a statement is added to the linked list, it is crucial that it is
cleaned up (removed from the list). When DBIc_IMPSET_on() was being
called too late, it was able to cause all sorts of problems.
Using DBIc_is(), DBIc_has(), DBIc_on() and DBIc_off()
Once upon a long time ago, the only way of handling the internal DBI
boolean flags/attributes was through macros such as:
DBIc_WARN DBIc_WARN_on DBIc_WARN_off
DBIc_COMPAT DBIc_COMPAT_on DBIc_COMPAT_off
Each of these took an imp_xxh pointer as an argument.
Since then, new attributes have been added such as ChopBlanks,
RaiseError and PrintError, and these do not have the full set of macros.
The approved method for handling these is now the four macros:
DBIc_is(imp, flag)
DBIc_has(imp, flag) an alias for DBIc_is
DBIc_on(imp, flag)
DBIc_off(imp, flag)
DBIc_set(imp, flag, on) set if on is true, else clear
Consequently, the "DBIc_XXXXX" family of macros is now mostly deprecated
and new drivers should avoid using them, even though the older drivers
will probably continue to do so for quite a while yet. However...
There is an important exception to that. The ACTIVE and IMPSET flags
should be set via the DBIc_ACTIVE_on() and DBIc_IMPSET_on() macros, and
unset via the DBIc_ACTIVE_off() and DBIc_IMPSET_off() macros.
Using the get_fbav() method
THIS IS CRITICAL for C/XS drivers.
The "$sth->bind_col()" and "$sth->bind_columns()" documented in the DBI
specification do not have to be implemented by the driver writer because
DBI takes care of the details for you.
However, the key to ensuring that bound columns work is to call the
function "DBIc_DBISTATE(imp_xxh)->get_fbav()" in the code which fetches
a row of data.
This returns an "AV", and each element of the "AV" contains the "SV"
which should be set to contain the returned data.
The pure Perl equivalent is the "$sth->_set_fbav($data)" method, as
described in the part on pure Perl drivers.
Casting strings to Perl types based on a SQL type
DBI from 1.611 (and DBIXS_REVISION 13606) defines the sql_type_cast_svpv
method which may be used to cast a string representation of a value to a
more specific Perl type based on a SQL type. You should consider using
this method when processing bound column data as it provides some
support for the TYPE bind_col attribute which is rarely used in drivers.
int sql_type_cast_svpv(pTHX_ SV *sv, int sql_type, U32 flags, void *v)
"sv" is what you would like cast, "sql_type" is one of the DBI defined
SQL types (e.g., "SQL_INTEGER") and "flags" is a bitmask as follows:
DBIstcf_STRICT
If set this indicates you want an error state returned if the cast
cannot be performed.
DBIstcf_DISCARD_STRING
If set and the pv portion of the "sv" is cast then this will cause
sv's pv to be freed up.
sql_type_cast_svpv returns the following states:
-2 sql_type is not handled - sv not changed
-1 sv is undef, sv not changed
0 sv could not be cast cleanly and DBIstcf_STRICT was specified
1 sv could not be case cleanly and DBIstcf_STRICT was not specified
2 sv was cast ok
The current implementation of sql_type_cast_svpv supports "SQL_INTEGER",
"SQL_DOUBLE" and "SQL_NUMERIC". "SQL_INTEGER" uses sv_2iv and hence may
set IV, UV or NV depending on the number. "SQL_DOUBLE" uses sv_2nv so
may set NV and "SQL_NUMERIC" will set IV or UV or NV.
DBIstcf_STRICT should be implemented as the StrictlyTyped attribute and
DBIstcf_DISCARD_STRING implemented as the DiscardString attribute to the
bind_col method and both default to off.
See DBD::Oracle for an example of how this is used.
SUBCLASSING DBI DRIVERS
This is definitely an open subject. It can be done, as demonstrated by
the DBD::File driver, but it is not as simple as one might think.
(Note that this topic is different from subclassing the DBI. For an
example of that, see the t/subclass.t file supplied with the DBI.)
The main problem is that the dbh's and sth's that your connect() and
prepare() methods return are not instances of your DBD::Driver::db or
DBD::Driver::st packages, they are not even derived from it. Instead
they are instances of the DBI::db or DBI::st classes or a derived
subclass. Thus, if you write a method mymethod() and do a
$dbh->mymethod()
then the autoloader will search for that method in the package DBI::db.
Of course you can instead to a
$dbh->func('mymethod')
and that will indeed work, even if mymethod() is inherited, but not
without additional work. Setting @ISA is not sufficient.
Overwriting methods
The first problem is, that the connect() method has no idea of
subclasses. For example, you cannot implement base class and subclass in
the same file: The install_driver() method wants to do a
require DBD::Driver;
In particular, your subclass has to be a separate driver, from the view
of DBI, and you cannot share driver handles.
Of course that's not much of a problem. You should even be able to
inherit the base classes connect() method. But you cannot simply
overwrite the method, unless you do something like this, quoted from
DBD::CSV:
sub connect ($$;$$$) {
my ($drh, $dbname, $user, $auth, $attr) = @_;
my $this = $drh->DBD::File::dr::connect($dbname, $user, $auth, $attr);
if (!exists($this->{csv_tables})) {
$this->{csv_tables} = {};
}
$this;
}
Note that we cannot do a
$drh->SUPER::connect($dbname, $user, $auth, $attr);
as we would usually do in a an OO environment, because $drh is an
instance of DBI::dr. And note, that the connect() method of DBD::File is
able to handle subclass attributes. See the description of Pure Perl
drivers above.
It is essential that you always call superclass method in the above
manner. However, that should do.
Attribute handling
Fortunately the DBI specifications allow a simple, but still performant
way of handling attributes. The idea is based on the convention that any
driver uses a prefix driver_ for its private methods. Thus it's always
clear whether to pass attributes to the super class or not. For example,
consider this STORE() method from the DBD::CSV class:
sub STORE {
my ($dbh, $attr, $val) = @_;
if ($attr !~ /^driver_/) {
return $dbh->DBD::File::db::STORE($attr, $val);
}
if ($attr eq 'driver_foo') {
...
}
AUTHORS
Jonathan Leffler <jleffler@us.ibm.com> (previously
<jleffler@informix.com>), Jochen Wiedmann <joe@ispsoft.de>, Steffen
Goeldner <sgoeldner@cpan.org>, and Tim Bunce <dbi-users@perl.org>.
perl v5.40.0 2025-02-01 DBI::DBD(3pm)
Generated by dwww version 1.16 on Tue Dec 16 05:52:57 CET 2025.