DBI::DBD(3) User Contributed Perl Documentation DBI::DBD(3)NAMEDBI::DBD - Perl DBI Database Driver Writer's Guide
SYNOPSIS
perldoc DBI::DBD
Version and volatility
$Revision: 11.21 $
$Date: 2004/02/01 11:16:16 $
This document is still a minimal draft which is in need of further
work.
The changes will occur both because the DBI specification is changing
and hence the requirements on DBD drivers change, and because feedback
from people reading this document will suggest improvements to it.
Please read the DBI documentation first and fully, including the DBI
FAQ. Then reread the DBI specification again as you're reading this.
It'll help.
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. And 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 data‐
base 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 auxilliary 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 auxilliary 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 contrac‐
tion of that. Hence, DBD::Oracle has the name Oracle and the prefix
'ora_'. This information will be recorded in the DBI documentation.
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 example 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
README
MANIFEST
Driver.pm
lib/Bundle/DBD/Driver.pm
lib/DBD/Driver/Summary.pm
t/*.t
Needless to say, all these files are either text files or pure Perl.
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 prerequi‐
site 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.
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 exam‐
ple, 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 tempo‐
rary tables that are automatically removed at the end of a session,
then exploit them as often as possible. Try to make your tests inde‐
pendent 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 glob‐
bing.
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 the METADATA METHODS entry elsewhere
in this document 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 trans‐
lating 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.
In what follows I assume the name "Driver" for your new package and the
prefix 'drv_'. The minimal set of files we have to implement are Make‐
file.PL, README, MANIFEST and Driver.pm. Files in the 'nice to have'
category include '
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 "ExtU‐
tils::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 Make‐
file.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. And, 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 DBI 1.03;
use DBI::DBD;
use ExtUtils::MakeMaker;
WriteMakefile(
dbd_edit_mm_attribs( {
'NAME' => 'DBD::Driver',
'VERSION_FROM' => 'Driver.pm',
'INC' => $DBI_INC_DIR,
'dist' => { 'SUFFIX' => '.gz',
'COMPRESS' => 'gzip -9f' },
'realclean' => { FILES => '*.xsi' },
},
{ create_pp_tests => 1})
);
package MY;
sub postamble { return main::dbd_postamble(@_); }
sub libscan {
my ($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 con‐
taining 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. 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 $VER‐
SION, and the subsequent text will be used to determine the version
number. Note the commentary in the ExtUtils::MakeMaker manpage 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"(). 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 - auxilliary modules too).
See also ExtUtils::MakeMaker(3) and ExtUtils::MM_Unix(3). Consider
using CPAN::MakeMaker(3) in place of ExtUtils::MakeMaker.
README
The README file should describe what the driver is for, the pre-requi‐
sites 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. Also, it
is in your interests to ensure that your tests work as widely as possi‐
ble. 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.
A suitable skeleton for this file is shown below. 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 licencing
information at the top.
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 data‐
base.
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 implemen‐
tation. 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;
use vars qw($err $errstr $state $drh);
$err = 0; # holds error code for DBI::err
$errstr = ""; # holds error string for DBI::errstr
$sqlstate = "S1000"; # holds SQL state for DBI::state
These variables are used for storing error states and messages. Note
that most pure Perl drivers do not support the SQL standard error
indicator SQLSTATE, and for such drivers, the value "S1000" is appro‐
priate. If your database does support SQLSTATE, then initialize
$sqlstate to an empty string. However, it is crucial to understand
that you must not modify them directly; see below.
$VERSION = "1.23.00" # Version number of DBD::File
This is where the version number of your driver is specified. The
code in Makefile.PL is told to look in this file for the information.
It is recommended that you use a two-part (1.23) or three-part
(1.23.45) version number. Please ensure that any other modules added
with your driver are also version stamped so that CPAN does not get
confused. 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 RCS or CVS you can use this code:
$VERSION = sprintf "%d.%02d", '$Revision: 11.21 $ ' =~ /(\d+)\.(\d+)/;
which pads out the fractional part with leading zeros so all is well
(so long as you don't go past x.99)
$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
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";
# 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 $drh;
}
The driver method is the driver handle constructor. It's a reasonable
example of how DBI implements its handles. There are three kinds:
driver handles (typically stored in "$drh"; from now on called "drh"
of "$drh"), database handles (from now on called "dbh" or "$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).
Also needed here, in the DBD::Driver package, is a CLONE() method
that will be called by perl when an intrepreter 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 inter‐
preter:
sub CLONE {
undef $rdh;
}
The DBD::Driver::dr package
The database handle constructor
The next lines of code look as follows:
package DBD::Driver::dr; # ====== DRIVER ======
$DBD::Driver::dr::imp_data_size = 0;
Note that no @ISA is needed here, or for the other DBD::Driver::*
classes, because the DBI takes care of that for you when the driver
is loaded.
The database handle constructor is a driver method, thus we have to
change the namespace.
sub connect
{
my ($drh, $dbname, $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.
# create a 'blank' dbh (call superclass constructor)
my $dbh = DBI::_new_dbh($drh, {
'Name' => $dbname,
})
or return undef;
# Process attributes from the DSN; we assume ODBC syntax
# here, that is, the DSN looks like var1=val1;...;varN=valN
foreach my $var (split(/;/, $dbname)) {
if ($var =~ m/(.*?)=(,*)/) {
# Not !!! $dbh->{$var} = $val;
$dbh->STORE($var, $val);
}
}
$dbh;
}
The Name attribute is a standard DBI attribute.
This is mostly the same as in the driver handle constructor above.
The arguments are described in the DBI man page. See DBI(3). The
constructor _new_dbh is called, returning a database handle. The
constructor's prototype is:
$dbh = DBI::_new_dbh($drh, $public_attr, $private_attr);
with similar arguments to those in the driver handle constructor,
except that the "$class" is replaced by "$drh".
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 $HOME/.odbcini file on Unix for
ODBC data sources (DSNs). As a trivial example, consider a fixed
list of data sources:
sub data_sources
{
my($srh, $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;
}
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 cur‐
rent directory to something not existent by using the (driver-spe‐
cific) 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 argu‐
ment.
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 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.
The DBD::Driver::db package
The statement handle constructor
There's nothing much new in the statement handle constructor.
package DBD::Driver::db; # ====== DATABASE ======
$DBD::Driver::db::imp_data_size = 0;
sub prepare
{
my ($dbh, $statement, @attribs) = @_;
# create a 'blank' sth
my $sth = DBI::_new_sth($dbh, {
'Statement' => $statement,
});
# Setup module specific data
$sth->STORE('drv_params', []);
$sth->STORE('NUM_OF_PARAMS', ($statement =~ tr/?//));
$sth;
}
This is still the same: check the arguments and call the super class
constructor DBI::_new_sth. The "Statement" attribute should be
cached as shown. Note the prefix drv_ in the attribute names: it is
required that your private attributes are lowercased and use such a
prefix.
Note that we parse the statement here in order to set the attribute
NUM_OF_PARAMS. The technique illustrated is not very reliable; it
can be confused by question marks appearing in quoted strings, delim‐
ited identifiers or in SQL comments that are part of the SQL state‐
ment. We could set NUM_OF_PARAMS in the execute method instead
because the DBI specification explicitly allows a driver to defer
this, but then the user could not call bind_param.
Transaction handling
Pure Perl drivers will rarely support transactions. Thus your commit
and rollback methods will typically be quite simple:
sub commit
{
my ($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(1) 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 catch 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 actu‐
ally 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) discon‐
nect method and possibly a quote method if DBI's default isn't cor‐
rect for you.
Where reasonable use $h->SUPER:\fIs0:foo() to call the DBI's method
in some or all cases and just wrap your custom behavior around that.
If you want to use private trace flags you'll probably want to be
able to set them by name. To do that you'll need to define a
parse_trace_flag() method (note that's parse_trace_flag not
parse_trace_flags).
sub parse_trace_flag {
my ($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
The execute method
This is perhaps the most difficult method because we have to consider
parameter bindings here. We present a simplified implementation by
using the drv_params attribute from above:
package DBD::Driver::st;
$DBD::Driver::st::imp_data_size = 0;
sub bind_param
{
my ($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->FETCH('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->{Active};
my $params = (@bind_values) ?
\@bind_values : $sth->FETCH('drv_params');
my $numParam = $sth->FETCH('NUM_OF_PARAMS');
return $sth->set_err(1, "Wrong number of parameters")
if @$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') = $numFields;
@$data ⎪⎪ '0E0';
}
There are a number of things you should note here. We setup the
NUM_OF_FIELDS attribute here, because this is essential for bind_col‐
umns 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 $sth->errstr;
Fetching data
We should not implement the methods fetchrow_array,
fetchall_arrayref, ... because these are already part of DBI. All we
need is the method fetchrow_arrayref:
sub fetchrow_arrayref
{
my ($sth) = @_;
my $data = $sth->FETCH('drv_data');
my $row = shift @$data;
if (!$row) {
$sth->{Active} = 0; # mark as no longer active
return undef;
}
if ($sth->FETCH('ChopBlanks')) {
map { $_ =~ s/\s+$//; } @$row;
}
return $sth->_set_fbav($row);
}
*fetch = \&fetchrow_arrayref; # required alias for fetchrow_arrayref
sub rows { shift->FETCH('drv_rows') }
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.
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, ...
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 the stored data and do
$sth->SUPER::finish;
A "table_info" method to return details of available tables.
A "type_info_all" method to return details of supported types.
If you've defined a parse_trace_flag() method in ::db you'll also
want it in ::st, so just alias it in:
*parse_trace_flag = \&DBD::foo:db::parse_trace_flag;
And perhaps some other methods that are not part of the DBI specifi‐
cation, in particular to make metadata available. Remember that they
must have names that begin with your drivers registered prefix so
they can be installed using install_method().
Tests
The test process should conform as closely as possibly to the Perl
standard test harness.
In particular, most (all) of the tests should be run in the t sub-
directory, and should simply produce an 'ok' when run under 'make
test'. For details on how this is done, see the Camel book and the
section in Chapter 7, "The Standard Perl Library" on the Test::Harness
manpage.
The tests may need to adapt to the type of database which is being used
for testing, and to the privileges of the user testing the driver.
The DBD::Informix test code has to adapt in a number of places to the
type of database to which it is connected as different Informix data‐
bases have different capabilities. For example, some of the tests are
for databases without transaction logs; others are for databases with a
transaction log. Some versions of the server have support for blobs,
or stored procedures, or user-defined data types, and others do not.
When a complete file of tests must be skipped, you can provide a reason
in a pseudo-comment:
if ($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
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 refer‐
ence 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::Ora‐
cle 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 main‐
tained 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 $DBD::File::{dr⎪db⎪st}::imp_data_size are not
defined here, but in the XS code, because they declare the size
of certain C structures.
· Some methods are typically moved to the XS code, in particular
prepare, execute, disconnect, disconnect_all and the STORE and
FETCH methods.
· Other methods are still part of "Driver.pm", but have callbacks
to the XS code.
· If the driver-specific parts of the imp_drh_t structure need to
be formally initialized (which does not seem to be a common
requirement), then you need to add a call to an appropriate XS
function in the driver method of DBD::Driver::driver, and you
define the corresponding function in Driver.xs, and you define
the C code in dbdimp.c and the prototype in dbdimp.h.
For example, DBD::Informix has such a requirement, and adds the
following call after the call to _new_drh in Informix.pm:
DBD::Informix::dr::driver_init($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. We also ignore things
that are 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 from dbdimp.c.
See below for details.
*FIX ME* Discuss removing attributes from hash reference as an opti‐
mization to skip later calls to $dbh->STORE made by DBI->connect.
*FIX ME* Discuss removing attributes in Perl code.
*FIX ME* Discuss removing attributes in C code.
The disconnect_all method
*FIX ME* T.B.S
The data_sources method
If your data_sources method can be implemented in pure Perl, then do
so because it is easier than doing it in XS code (see the section
above for pure Perl drivers). If your data_sources method must call
onto compiled functions, then you will need to define
dbd_dr_data_sources in your dbdimp.h file, which will trigger
Driver.xst (in DBI v1.33 or greater) to generate the XS code that
calls your actual C function (see the discussion below for details)
and you do not code anything in Driver.pm to handle it.
The prepare method
The prepare method is the statement handle constructor, and most of
it is not new. Like the connect method, it now has a C callback:
package DBD::Driver::db; # ====== DATABASE ======
use strict;
sub prepare
{
my ($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 func‐
tions 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 con‐
stants 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 1.32 */
#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 and dbd_db_login6 unless you are intent on supporting
really old versions of DBI (prior to DBI 1.06) as well as modern ver‐
sions. 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 enhance your driver to use new features
for which the DBI is offering backwards compatibility via dbivport.h.
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 in an emulation of it was added to dbiv‐
port.h.
Copying dbivport.h into your driver distribution and #including it in
Driver.h, as shown above, lets you enhance your driver to use the new
DBIh_SET_ERR_CHAR macro even with versions of the DBI earlier than
1.41. 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.
Implementation header dbdimp.h
This header file has two jobs:
First it defines data structures for your private part of the handles.
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 implementa‐
tions 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.
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 spe‐
cific 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 inher‐
ited 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_xst.h */
#define dbd_init drv_dr_init
#define dbd_db_login6 drv_db_login
#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 func‐
tions, which should usually be file static if the 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 5.6 requires support for function prototypes (ANSI or ISO or
Standard C), you should write your code using function prototypes too.
Although technically DBI still supports Perl 5.005_03, which did not
mandate prototype support from the C compiler, the only platform where
prototypes are a problem is on HP-UX with the bundled C compiler (which
is strictly K&R). The solution for that is to get a copy of the GNU
Compiler Collection (GCC, aka the GNU C Compiler) for HP-UX.
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 fo the func‐
tions 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 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 data‐
base 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*. For DBIh_SET_ERR_CHAR the err_c, errstr, state, method are
char*. The err_i parameter is an IV that's used instead of err_c is
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 dbiv‐
port.h as described in the Driver.h entry elsewhere in this document
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_STATE(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. 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 constatnts for the array sizes.
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 dbd_db_login6 method
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 pri‐
vate 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";
}
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 sup‐
port 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 sepa‐
rately. 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 to 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,
but 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
notes about dbd_drv_error above 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 onto a common error format‐
ter. See the code in DBD::Informix 1.05.00 for more information.
The dbd_db_login6 function should return TRUE for success, FALSE oth‐
erwise.
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.
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 data‐
base. 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 inac‐
tive; 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 data‐
bases 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} = $value;
Its prototype is:
int dbd_db_STORE_attrib(SV* dbh, imp_dbh_t* imp_dbh, SV* keysv,
SV* valuesv);
You do not handle all attributes; on the contrary, you should not
handle DBI attributes here: leave this to DBI. (There are two excep‐
tions, AutoCommit and ChopBlanks, which you should care about.)
The return value is TRUE if you have handled the attribute or FALSE
otherwise. If you are handling an attribute and something fails, you
should call dbd_drv_error, so DBI can raise exceptions, if desired.
If dbd_drv_error returns, however, you have a problem: the user will
never know about the error, because he typically will not check
"$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(3).)
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 noncon‐
stant 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, for example, 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
perpare() 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, ... here, but DBI doesn't require that. However, if you do,
document it.
In any case you should set the IMPSET flag, as you did in dbd_db_con‐
nect 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);
Note, that you must be aware, that a statement may be executed
repeatedly. Also, you should not expect, that finish will be called
between two executions, so you'll 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 fol‐
lows:
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 you 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" state‐
ments (or any other statements that can return multiple sets of val‐
ues 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 indi‐
cate 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's no point in doing anything more than turing of
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 impor‐
tantly, 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 infor‐
mation 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 infor‐
mation 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->con‐
nect() 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 some‐
times 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 honured 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_DBIS‐
TATE(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 func‐
tions 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 con‐
secutive 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).
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 imple‐
ments 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 -e write_getinfo_pm \
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 the write_type‐
info_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 descibed in the Gen‐
erating the get_info method entry elsewhere in this document With the
pre-requisites in place, you might type:
perl -MDBI::DBD::Metadata -e write_typeinfo \
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 (eg
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 exam‐
ple, 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 (eg
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], $arr[2]);
}
return @res;
}
See also the default implementation in DBI.pm.
Writing DBD::Driver::db::quote
This method takes a value and converts it into a string suitable for
embedding in an SQL statement as a string literal.
If your DBMS accepts the SQL standard notation for strings (single
quotes around the string as a whole with any embedded single quotes
doubled up), then you do not need to write this method as DBI provides
a default method that does it for you. If your DBMS uses an alterna‐
tive notation or escape mechanism, then you need to provide an equiva‐
lent function. For example, suppose your DBMS used C notation with
double quotes around the string and backslashes escaping both double
quotes and backslashes themselves. Then you might write the function
as:
sub quote
{
my($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-argu‐
ment variant of the method.
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 con‐
nection 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. The 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 ini‐
tialize it with a pointer to the handles private data. Note: This
must be a part of the function header, because it declares a vari‐
able.
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, RaiseEr‐
ror 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_DBIS‐
TATE(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.
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 pre‐
pare 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 sub‐
class. 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 with‐
out additional work. Setting "@ISA" is not sufficient.
Overwriting methods
The first problem is, that the connect method has no idea of sub‐
classes. 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 over‐
write 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
$srh->SUPER::connect($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 <jlef‐
fler@informix.com>), Jochen Wiedmann <joe@ispsoft.de>, Steffen Goeldner
<sgoeldner@cpan.org>, and Tim Bunce <dbi-users@perl.org>.
3rd Berkeley Distribution perl v5.6.1 DBI::DBD(3)
