NAME
DBIx::Sequence - A simple SQL92 ID generator
SYNOPSIS
use DBIx::Sequence;
my $sequence = new DBIx::Sequence({ dbh => $dbh });
my $next_id = $sequence->Next('dataset');
DESCRIPTION
This module is intended to give easier portability to Perl database
application by providing a database independant unique ID generator.
This way, an application developer is not bound to use his database's
SEQUENCE or auto_increment thus making his application portable on
multiple database environnements.
This module implements a simple Spin Locker mechanism and is garanteed
to return a unique value every time it is called, even with concurrent
processes. It uses your database for its state storage with ANSI SQL92
compliant SQL. All SQL queries inside DBIx::Sequence are pre cached and
very efficient especially under mod_perl.
INSTALLATION
perl Makefile.PL
make
make test
make install
Note:
If you decide to run extended tests for the module, you will have to
provide the make test with a DSN (connect string) to your database
(dbi:Driver:db;host=hostname) and a valid username/password combination
for a privileged user.
DBIx::Sequence uses 2 tables for its operation, namely the
dbix_sequence_state and the dbix_sequence_release tables. Those tables
will be created if you run extended tests, if not you will need to
create them yourself.
dbix_sequence_state:
| dataset | varchar(50) |
| state_id | int(11) |
dbix_sequence_release:
| dataset | varchar(50) |
| released_id | int(11) |
Those table names are overloadable at your convenience, see the
OVERLOADING section for details.
BASIC USAGE
The basic usage of this module is to generate a unique ID to replace the
use of your database's SEQUENCE of auto_increment field.
INIT
First, you need to create the sequence object:
use DBIx::Sequence;
my $sequence = new DBIx::Sequence({
db_user => 'scott',
db_pw => 'tiger',
db_dsn => 'dbi:mysql:scottdb',
allow_id_reuse => 1,
});
DBIx::Sequence can be used to manage multiple sets of ID's (perhaps you
could have one dataset per table, or one and only one dataset). This
permits you to handle multiple applications with the same sequence
class. The dataset is normally simply a token string that represents
your ID set. If the dataset does not exists, DBIx::Sequence will create
automagically for you. No special steps are involved in the creation of
a dataset.
The arguments contains the database informations, db_user, db_pw and
db_dsn and are stored in a hash reference.
At this point, the object has pre cached all of the SQL that will be
used to generate the spin locker race. It is normally a good idea to
have a shared sequence object (especially) under mod_perl to save the
prepare overhead. The 'allow_id_reuse' argument can be passed to the
constructor to either allow the use of the Release() or deny it. (True
value makes it allowed)
GETTING THE NEXT ID
To get the next id, you simpy have to use the Next() method of your
sequence while specifying the dataset you are getting the next id for.
my $next_id = $sequence->Next($dataset);
RELEASING ID'S.
Generated ID's can be _explicitly_ released in your application. When an
ID is released, the sequence will be able to give this id back to you
throught the Next() method.
This is how it is done:
$sequence->Release($dataset, $id);
Note:
You must use release only when you are _CERTAIN_ that your ID is not
used anymore and that you want it to be recycled. The Spin Locking
mechanism will also take place on released id's to ensure that no two
processes can get the same ID.
PERMANENTLY REMOVING A DATASET
To make DBIx::Sequence forget about an existing dataset, you need to use
the Delete_Dataset() method.
$sequence->Delete_Dataset($dataset);
This will clear all state and existence for this dataset and will also
clear it's released id's. Note that if your application still uses this
dataset, it will be automatically recreated blank.
BOOTSTRAPPING A DATASET FROM EXISTING DATA
It is possible to sync the state of a DBIx::Sequence dataset by using
the Bootstrap() method.
$sequence->Bootstrap('my_dataset','my_bootstrap_table','my_primary_field');
Bootstrap() takes 3 arguments.
* The dataset to bootstrap
* The table from wich you will bootstrap
* The field in the bootstrap table that will be used to bootstrap the
dataset.
Bootstrap will then sync up the DBIx::Sequence's state with the maximum
id of the 'my_primary_field' in 'my_bootstrap_table'. The bootstrap
field must be a numeric field as you can suspect. The SQL function MAX()
will be called on it during the bootstrap process.
Note: The bootstrap method _can_ be used at runtime since it will
initiate a race for updating the value thus following the same
algorithm. It is recommended though that you use Bootstrap() when no
other concurrent processes are requesting id's.
OVERLOADING
It is possible to create an overloaded class of DBIx::Sequence. This
permits you to create a DBIx::Sequence that has different properties
than the orignal one. The only thing you really have to overload to
modify the behaviour of DBIx::Sequence are some constants:
* STATE_TABLE : Defines the table used by DBIx::Sequence to store
dataset's states.
* RELEASE_TABLE : Defines the table used by DBIx::Sequence to store
released id's.
* COLUMN_PREFIX : A string to be prepended to every column in the
internal SQL statements.
* DEFAULT_INIT_VALUE : Value used to initialize a dataset when it is
first created.
* DEFAULT_ALLOW_ID_REUSE : When set to true, will allow the use of
Release() if not specified in the constructor. (allow_id_reuse)
* DEBUG_LEVEL : When set to true, will enable debugging to STDERR.
So it is very easy to specify the behaviour of DBIx::Sequence that you
wish to use by creating an overloaded class.
Also, a very important method to overload is the getDbh() method. This
is the function that returns the database handle to the DBIx::Sequence.
Your overloaded class should redefine the getDbh method.
Overloading getDbh will make your sequence class integrate more cleanly
with your application.
i.e.
package MySequence;
use DBI;
use DBIx::Sequence;
use vars qw(@ISA);
@ISA = qw(DBIx::Sequence);
use constant STATE_TABLE => 'my_state_table';
use constant RELEASE_TABLE => 'my_release_table';
use constant COLUMN_PREFIX => '';
use constant DEFAULT_INIT_VALUE => '100';
use constant DEFAULT_ALLOW_ID_REUSE => 1;
use constant DEBUG_LEVEL => 0;
sub getDbh
{
my $self = shift;
return MyApplication::MyDBModule::getDbh();
}
1;
Then, your code can use this class for its sequencing. Notice that since
we overloaded getDbh(), we don't need to pass a second parameter to
new().
use MySequence;
my $sequence = new MySequence();
my $next_id = $sequence->Next($dataset);
SPECIAL NOTE ON DATABASE HANDLE OPTIONS
DBIx::Sequence requires that the dbh object you passe to it has the
AutoCommit flag set to 1. The main reason for this is that if AutoCommit
is off, DBIx::Sequence will have to do an implicit commit() call, wich
in most cases is a bad idea, especially when the dbh passed to the
sequence object already has transactions prelogged in it.
CVS AND BLEEDING VERSIONS
For the latest development information, CVS access and Changelog, please
visit:
http://labs.turbulent.ca
If you use this module in a project, please let me know!
Your comments and rants are more than welcomed!
Commercial support for this module is available, please contact me for
info!
TODO
* Implement multiple locking mechanism (semaphore, spin, db locker)
* Implement pluggable locking module support
AUTHOR
Benoit Beausejour, <bbeausej@pobox.com>
NOTES
This code was made possible by the help of individuals:
Philippe "Gozer" M. Chiasson <gozer@cpan.org>
Thanks to Uri Guttman for documentation checks ;)
CONTRIBUTORS
Here are the people who submitted patches and changes to the module,
they have my thanks for their contributions:
Trevor Shellhorn <trevor.schellhorn-perl@marketingtips.com>
Dan Kubb <dkubb@cpan.org>
SEE ALSO
perl(1).
COPYRIGHT
Copyright (c) 2000 Benoit Beausejour <bbeausej@pobox.com> All rights
reserved. This program is free software, you can redistribute it and/or
modify it under the same terms as Perl itself.