NAME
    Database::Migrator - A system for implementing database migrations

VERSION
    version 0.11

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)

