NAME
Ima::DBI - Database connection caching and organization
SYNOPSIS
package Foo;
use base 'Ima::DBI';
# Class-wide methods.
Foo->set_db($db_name, $data_source, $user, $password);
Foo->set_db($db_name, $data_source, $user, $password, \%attr);
my @database_names = Foo->db_names;
my @database_handles = Foo->db_handles;
Foo->set_sql($sql_name, $statement, $db_name);
Foo->set_sql($sql_name, $statement, $db_name, $cache);
my @statement_names = Foo->sql_names;
# Object methods.
$dbh = $obj->db_*; # Where * is the name of the db connection.
$sth = $obj->sql_*; # Where * is the name of the sql statement.
$sth = $obj->sql_*(@sql_pieces);
$obj->DBIwarn($what, $doing);
my $rc = $obj->commit;
my $rc = $obj->commit(@db_names);
my $rc = $obj->rollback;
my $rc = $obj->rollback(@db_names);
DESCRIPTION
Ima::DBI attempts to organize and facilitate caching and more efficient
use of database connections and statement handles by storing DBI and SQL
information with your class (instead of as seperate objects). This
allows you to pass around just one object without worrying about a trail
of DBI handles behind it.
One of the things I always found annoying about writing large programs
with DBI was making sure that I didn't have duplicate database handles
open. I was also annoyed by the somewhat wasteful nature of the
prepare/execute/finish route I'd tend to go through in my subroutines.
The new DBI->connect_cached and DBI->prepare_cached helped a lot, but I
still had to throw around global datasource, username and password
information.
So, after a while I grew a small library of DBI helper routines and
techniques. Ima::DBI is the culmination of all this, put into a nice(?),
clean(?) class to be inherited from.
Why should I use this thing?
Ima::DBI is a little odd, and it's kinda hard to explain. So lemme
explain why you'd want to use this thing...
* Consolidation of all SQL statements and database information
No matter what, embedding one language into another is messy. DBI
alleviates this somewhat, but I've found a tendency to have that
scatter the SQL around inside the Perl code. Ima::DBI allows you to
easily group the SQL statements in one place where they are easier
to maintain (especially if one developer is writing the SQL, another
writing the Perl). Alternatively, you can place your SQL statement
alongside the code which uses it. Whatever floats your boat.
Database connection information (data source, username, password,
atrributes, etc...) can also be consolidated together and tracked.
Both the SQL and the connection info are probably going to change a
lot, so having them well organized and easy to find in the code is a
Big Help.
* Holds off opening a database connection until necessary.
While Ima::DBI is informed of all your database connections and SQL
statements at compile-time, it will not connect to the database
until you actually prepare a statement on that connection.
This is obviously very good for programs that sometimes never touch
the database. It's also good for code that has lots of possible
connections and statements, but which typically only use a few.
Kinda like an autoloader.
* Easy integration of the DBI handles into your class
Ima::DBI causes each database handle to be associated with your
class, allowing you to pull handles from an instance of your object,
as well as making many oft-used DBI methods available directly from
your instance.
This gives you a cleaner OO design, since you can now just throw
around the object as usual and it will carry its associated DBI
baggage with it.
* Honors taint mode
It always struck me as a design deficiency that tainted SQL
statements could be passed to $sth->prepare(). For example:
# $user is from an untrusted source and is tainted.
$user = get_user_data_from_the_outside_world;
$sth = $dbh->prepare('DELETE FROM Users WHERE User = $user');
Looks innocent enough... but what if $user was the string "1 OR User
LIKE '%'". You just blew away all your users. Hope you have backups.
Ima::DBI turns on the DBI->connect Taint attribute so that all DBI
methods (except execute()) will no longer accept tainted data. See
"Taint" in DBI for details.
* Taints returned data
Databases should be like any other system call. It's the scary
Outside World, thus it should be tainted. Simple. Ima::DBI turns on
DBI's Taint attribute on each connection. This feature is
overridable by passing your own Taint attribute to set_db as normal
for DBI. See "Taint" in DBI for details.
* Encapsulation of some of the more repetitive bits of everyday DBI
usage
I get lazy a lot and I forget to do things I really should, like
using bind_cols(), or rigorous error checking. Ima::DBI does some of
this stuff automatically, other times it just makes it more
convenient.
* Encapsulation of DBI's cache system
DBI's automatic handle caching system is relatively new, and some
people aren't aware of its use. Ima::DBI uses it automatically, so
you don't have to worry about it. (It even makes it a bit more
efficient)
* Sharing of database and sql information amongst inherited classes
Any SQL statements and connections created by a class are available
to its children via normal method inheritance.
* Guarantees one connection per program.
One program, one database connection (per database user). One
program, one prepared statement handle (per statement, per database
user). That's what Ima::DBI enforces. Extremely handy in persistant
environments (servers, daemons, mod_perl, FastCGI, etc...)
* Encourages use of bind parameters and columns
Bind parameters are safer and more efficient than embedding the
column information straight into the SQL statement. Bind columns are
more efficient than normal fetching. Ima::DBI pretty much requires
the usage of the former, and eases the use of the latter.
Why shouldn't I use this thing.
* It's all about OO
Although it is possible to use Ima::DBI as a stand-alone module as
part of a function-oriented design, its generally not to be used
unless integrated into an object-oriented design.
* Overkill for small programs
* Overkill for programs with only one or two SQL statements
Its up to you whether the trouble of setting up a class and jumping
through the necessary Ima::DBI hoops is worth it for small programs.
To me, it takes just as much time to set up an Ima::DBI subclass as
it would to access DBI without it... but then again I wrote the
module. YMMV.
* Overkill for programs that only use their SQL statements once
Ima::DBI's caching might prove to be an unecessary performance hog
if you never use the same SQL statement twice. Not sure, I haven't
looked into it.
USAGE
The basic steps to "DBIing" a class are:
1 Inherit from Ima::DBI
2 Set up and name all your database connections via set_db()
3 Set up and name all your SQL statements via set_sql()
4 Use sql_* to retrieve your statement handles ($sth) as needed and
db_* to retreive database handles ($dbh).
Have a look at EXAMPLE below.
TAINTING
Ima::DBI, by default, uses DBI's Taint flag on all connections.
This means that Ima::DBI methods do not accept tainted data, and that
all data fetched from the database will be tainted. This may be
different from the DBI behavior you're used to. See "Taint" in DBI for
details.
Class Methods
set_db
Foo->set_db($db_name, $data_source, $user, $password);
Foo->set_db($db_name, $data_source, $user, $password, \%attr);
This method is used in place of DBI->connect to create your database
handles. It sets up a new DBI database handle associated to $db_name.
All other arguments are passed through to DBI->connect_cached.
A new method is created for each db you setup. This new method is called
"db_$db_name"... so, for example, Foo->set_db("foo", ...) will create a
method called "db_foo()". (Spaces in $db_name will be translated into
underscores: '_')
%attr is combined with a set of defaults (RaiseError => 1, AutoCommit =>
0, PrintError => 0, Taint => 1). This is a better default IMHO, however
it does give databases without transactions (such as MySQL when used
with the default MyISAM table type) a hard time. Be sure to turn
AutoCommit back on if your database does not support transactions.
The actual database handle creation (and thus the database connection)
is held off until a prepare is attempted with this handle.
set_sql
Foo->set_sql($sql_name, $statement, $db_name);
Foo->set_sql($sql_name, $statement, $db_name, $cache);
This method is used in place of DBI->prepare to create your statement
handles. It sets up a new statement handle associated to $sql_name using
the database connection associated with $db_name. $statement is passed
through to either DBI->prepare or DBI->prepare_cached (depending on
$cache) to create the statement handle.
If $cache is true or isn't given, then prepare_cached() will be used to
prepare the statement handle and it will be cached. If $cache is false
then a normal prepare() will be used and the statement handle will be
recompiled on every sql_*() call. If you have a statement which changes
a lot or is used very infrequently you might not want it cached.
A new method is created for each statement you set up. This new method
is "sql_$sql_name"... so, as with set_db(), Foo->set_sql("bar", ...,
"foo"); will create a method called "sql_bar()" which uses the database
connection from "db_foo()". Again, spaces in $sql_name will be
translated into underscores ('_').
The actual statement handle creation is held off until sql_* is first
called on this name.
transform_sql
To make up for the limitations of bind parameters, $statement can
contain sprintf() style formatting (ie. %s and such) to allow
dynamically generated SQL statements (so to get a real percent sign, use
'%%').
The translation of the SQL happens in transform_sql(), which can be
overridden to do more complex transformations. See Class::DBI for an
example.
db_names / db_handles
my @database_names = Foo->db_names;
my @database_handles = Foo->db_handles;
my @database_handles = Foo->db_handles(@db_names);
Returns a list of the database handles set up for this class using
set_db(). This includes all inherited handles.
db_names() simply returns the name of the handle, from which it is
possible to access it by converting it to a method name and calling that
db method...
my @db_names = Foo->db_names;
my $db_meth = 'db_'.$db_names[0];
my $dbh = $foo->$db_meth;
Icky, eh? Fortunately, db_handles() does this for you and returns a list
of database handles in the same order as db_names(). Use this sparingly
as it will connect you to the database if you weren't already connected.
If given @db_names, db_handles() will return only the handles for those
connections.
These both work as either class or object methods.
sql_names
my @statement_names = Foo->sql_names;
Similar to db_names() this returns the names of all SQL statements set
up for this class using set_sql(), inherited or otherwise.
There is no corresponding sql_handles() because we can't know what
arguments to pass in.
Object Methods
db_*
$dbh = $obj->db_*;
This is how you directly access a database handle you set up with
set_db.
The actual particular method name is derived from what you told set_db.
db_* will handle all the issues of making sure you're already connected
to the database.
sql_*
$sth = $obj->sql_*;
$sth = $obj->sql_*(@sql_pieces);
sql_*() is a catch-all name for the methods you set up with set_sql().
For instance, if you did:
Foo->set_sql('GetAllFoo', 'Select * From Foo', 'SomeDb');
you'd run that statement with sql_GetAllFoo().
sql_* will handle all the issues of making sure the database is already
connected, and the statement handle is prepared. It returns a prepared
statement handle for you to use. (You're expected to execute() it)
If sql_*() is given a list of @sql_pieces it will use them to fill in
your statement, assuming you have sprintf() formatting tags in your
statement. For example:
Foo->set_sql('GetTable', 'Select * From %s', 'Things');
# Assuming we have created an object... this will prepare the
# statement 'Select * From Bar'
$sth = $obj->sql_Search('Bar');
Be very careful with what you feed this function. It cannot do any
quoting or escaping for you, so it is totally up to you to take care of
that. Fortunately if you have tainting on you will be spared the worst.
It is recommended you only use this in cases where bind parameters will
not work.
DBIwarn
$obj->DBIwarn($what, $doing);
Produces a useful error for exceptions with DBI.
I'm not particularly happy with this interface
Most useful like this:
eval {
$self->sql_Something->execute($self->{ID}, @stuff);
};
if($@) {
$self->DBIwarn($self->{ID}, 'Something');
return;
}
Modified database handle methods
Ima::DBI makes some of the methods available to your object that are
normally only available via the database handle. In addition, it spices
up the API a bit.
commit
$rc = $obj->commit;
$rc = $obj->commit(@db_names);
Derived from $dbh->commit() and basically does the same thing.
If called with no arguments, it causes commit() to be called on all
database handles associated with $obj. Otherwise it commits all database
handles whose names are listed in @db_names.
Alternatively, you may like to do: $rc = $obj->db_Name->commit;
If all the commits succeeded it returns true, false otherwise.
rollback
$rc = $obj->rollback;
$rc = $obj->rollback(@db_names);
Derived from $dbh->rollback, this acts just like Ima::DBI->commit,
except that it calls rollback().
Alternatively, you may like to do: $rc = $obj->db_Name->rollback;
If all the rollbacks succeeded it returns true, false otherwise.
EXAMPLE
package Foo;
use base qw(Ima::DBI);
# Set up database connections (but don't connect yet)
Foo->set_db('Users', 'dbi:Oracle:Foo', 'admin', 'passwd');
Foo->set_db('Customers', 'dbi:Oracle:Foo', 'Staff', 'passwd');
# Set up SQL statements to be used through out the program.
Foo->set_sql('FindUser', <<"SQL", 'Users');
SELECT *
FROM Users
WHERE Name LIKE ?
SQL
Foo->set_sql('ChangeLanguage', <<"SQL", 'Customers');
UPDATE Customers
SET Language = ?
WHERE Country = ?
SQL
# rest of the class as usual.
package main;
$obj = Foo->new;
eval {
# Does connect & prepare
my $sth = $obj->sql_FindUser;
# bind_params, execute & bind_columns
$sth->execute(['Likmi%'], [\($name)]);
while( $sth->fetch ) {
print $name;
}
# Uses cached database and statement handles
$sth = $obj->sql_FindUser;
# bind_params & execute.
$sth->execute('%Hock');
@names = $sth->fetchall;
# connects, prepares
$rows_altered = $obj->sql_ChangeLanguage->execute(qw(es_MX mx));
};
unless ($@) {
# Everything went okay, commit the changes to the customers.
$obj->commit('Customers');
}
else {
$obj->rollback('Customers');
warn "DBI failure: $@";
}
USE WITH MOD_PERL, FASTCGI, ETC.
To help with use in forking environments, Ima::DBI database handles keep
track of the PID of the process they were openend under. If they notice
a change (because you forked a new process), a new handle will be opened
in the new process. This prevents a common problem seen in environments
like mod_perl where people would open a handle in the parent process and
then run into trouble when they try to use it from a child process.
Because Ima::DBI handles keeping database connections persistent and
prevents problems with handles openend before forking, it is not
necessary to use Apache::DBI when using Ima::DBI. However, there is one
feature of Apache::DBI which you will need in a mod_perl or FastCGI
environment, and that's the automatic rollback it does at the end of
each request. This rollback provides safety from transactions left
hanging when some perl code dies -- a serious problem which could grind
your database to a halt with stale locks.
To replace this feature on your own under mod_perl, you can add
something like this in a handler at any phase of the request:
$r->push_handlers(PerlCleanupHandler => sub {
MyImaDBI->rollback();
});
Here "MyImaDBI" is your subclass of Ima::DBI. You could also make this
into an actual module and set the PerlCleanupHandler from your
httpd.conf. A similar approach should work in any long-running
environment which has a hook for running some code at the end of each
request.
TODO, Caveat, BUGS, etc....
I seriously doubt that it's thread safe.
You can bet cupcackes to sno-cones that much havoc will be wrought
if Ima::DBI is used in a threaded Perl.
Should make use of private_* handle method to store information
The docs stink.
The docs were originally written when I didn't have a good handle on
the module and how it will be used in practical cases. I need to
rewrite the docs from the ground up.
Need to add debugging hooks.
The thing which immediately comes to mind is a Verbose flag to print
out SQL statements as they are made as well as mention when database
connections are made, etc...
MAINTAINERS
Tony Bowden <tony@tmtm.com> and Perrin Harkins <perrin@elem.com>
ORIGINAL AUTHOR
Michael G Schwern <schwern@pobox.com>
LICENSE
This module is free software. You may distribute under the same terms as
Perl itself. IT COMES WITHOUT WARRANTY OF ANY KIND.
THANKS MUCHLY
Tim Bunce, for enduring many DBI questions and adding Taint,
prepare_cached and connect_cached methods to DBI, simplifying this
greatly!
Arena Networks, for effectively paying for Mike to write most of this
module.
SEE ALSO
DBI.
You may also choose to check out Class::DBI which hides most of this
from view.