NAME
Database::Migrator - A system for implementing database migrations
VERSION
version 0.10
DESCRIPTION
This distribution consists of a single role, Database::Migrator::Core.
This role can be consumed by classes which implement the required
methods for the role. These classes will then implement a complete
database schema creation and migration system.
MIGRATION ARCHITECTURE
The migration system starts with a file containing the DDL (database
description language) for the full database schema. If the database
doesn't yet exist, the database will be created and this DDL will be run
against it.
This DDL file *should not* contain any sort of "CREATE DATABASE"
statement. This will be done separately before the DDL is run.
This DDL file *may* contain DDL to create users and grant them access to
the database.
Once the database exists, the migrations are run against the database.
Each migration goes into its own directory. The directory name is the
name of the migration. Migrations are applied in sorted order. If the
migrations start with numbers, they are sorted by these numbers,
otherwise they are sorted alphabetically.
The migration directory can either contain files with SQL or Perl. If a
file ends in ".sql", the migration runner code will feed it to the
appropriate command line utility for your database.
Otherwise the file is assumed to contain Perl code. This code is
expected to return a single anonymous subroutine when "eval"ed. This
subroutine will then be called with the "Database::Migrator" object as
its only argument.
Each file in a single migration's directory is run in sorted order. You
can use numeric prefixes on these files if necessary.
Once a migration has been applied, that fact is stored in the database,
and the migration will not be applied again. This is done by recording
the migration's name in a table. The name of the table is determined by
your code. I recommend something like "AppliedMigration" or
"applied_migrations", depending on your table naming scheme.
Migration Example
Let's assume a set of files like this:
migrations/
|
|-- 01-add-foo-data/
| \
| \-- 01-create-foo-table.sql
| |
| |-- 02-insert-foo-data.pl
|
|-- 02-add-bar-table/
\
\-- add-bar-table.sql
The 01-add-foo-data/01-create-foo-table.sql file might look like this:
CREATE TABLE Foo (
foo_id INT PRIMARY KEY AUTO_INCREMENT,
size INT NOT NULL,
description TEXT NOT NULL
);
The 01-add-foo-data/02-insert-foo-data.pl file might contain this:
sub {
use Text::CSV_XS;
my $migrator = shift;
my $csv = Text::CSV_XS->new( ... );
my $fh = IO::File->new( ... );
my $sql = q[INSERT INTO Foo (size, description) VALUES (?, ?)];
my $sth = $migrator->dbh()->prepare()
while ( my $foo = $csv->getline_hr($fh) ) {
$sth->execute( $foo->{size}, $foo->{description} );
}
$sth->finish();
}
The 02-add-bar-table/add-bar-table.sql file would contain DDL to create
the "Bar" table.
HOW TO USE THIS DISTRIBUTION
This distribution is not intended to be used all by itself. Instead, you
will need to start with a DBMS-specific implementation like
Database::Migrator::mysql.
To actually run migrations, you either need to create an command line
script or subclass an implementation (or both).
The Database::Migrator::Core role consumes the MooseX::Getopt::Dashes
role, making it easy to create a command line script for migrations:
#!/usr/bin/env perl
use strict;
use warnings;
use Database::Migrator::mysql;
Database::Migrator::mysql->new_with_options()->create_or_update_database();
However, all by itself, this will require quite a few command line
options to be passed. You can simplify this by subclassing the
implementation class and providing defaults for things like the
migration directory and migration table.
See the Database::Migrator::Core documentation for more details on what
attributes you can provide defaults for.
THE APPLIED MIGRATION TABLE
The fact that a migration has been applied is recorded in a table in the
database.
If you are creating a new schema from scratch, you can include this
table. It should contain a single text column as its primary key. This
column *must* be named "migration".
The DDL to create this table might look like this:
CREATE TABLE AppliedMigration (
migration TEXT PRIMARY KEY
);
Bootstrapping This Table
If you are migrating an existing schema to use this migration system,
you will need to add this table to the schema. This can be done using
the migration system itself. If the schema already exists but the table
does not exist, it assumes that no migrations have been applied.
In this case, you must ensure that the first migration adds this table.
migrations/
|
|-- 00-add-applied-migration-table
| \
| \-- create-applied-migration-table.sql
|
|-- 01-add-foo-data/
| \
| \-- 01-create-foo-table.sql
| |
| |-- 02-insert-foo-data.pl
|
|-- 02-add-bar-table/
\
\-- add-bar-table.sql
The 00-add-applied-migration-table/create-applied-migration-table.sql
file would contain the DDL to create the table.
IDEMPOTENT MIGRATIONS
Under normal operation, no migration should ever be applied twice.
However, I still strongly recommend that you make all your migrations
idempotent. This is much safer. For example, if the process applying
migrations is killed, it's possible that it will be killed after a
migration is applied but before that fact has been recorded.
SUPPORT
Please submit bugs to the CPAN RT system at
http://rt.cpan.org/NoAuth/Bugs.html?Dist=Database-Migrator or via email
at bug-database-migrator@rt.cpan.org.
AUTHOR
Dave Rolsky <autarch@urth.org>
COPYRIGHT AND LICENSE
This software is Copyright (c) 2014 by MaxMind, Inc..
This is free software, licensed under:
The Artistic License 2.0 (GPL Compatible)