Data Migrations

The very first thing I would like to say about ActiveColumn Cassandra data migration is that I stole most of the code for this from the Rails gem (in ActiveSupport). I made the necessary changes to update a Cassandra database instead of a relational DB. These changes were sort of significant, but I just wanted to give credit where credit is due.

With that out of the way, we can discuss how you would use ActiveColumn to perform data migrations.

Creating keyspaces

First we will create our project's keyspaces.

1. Make sure your cassandra 0.7 (or above) server is running.

2. Make sure you have your config/cassandra.yml file created. The README has an example of this file.

The ActiveColumn gem gives you several rake tasks within the ks: namespace. "ks" stands for keyspace, which is the equivalent of a database in MySQL (or other relational dbs). To see the available tasks, run this rake command:

rake -T ks

1. Create your keyspaces with the ks:create:all rake task:

rake ks:create:all

Voila! You have now successfully created your keyspaces. Now let's generate some migration files.

Creating and running migrations

1. ActiveColumn includes a generator to help you create blank migration files. To create a new migration, run this command:

rails g active_column:migration NameOfYourMigration

If you are using Rails 2, run this command instead:

./script/generate ks_migration NameOfYourMigration

The name of the migration might be something like "CreateUsersColumnFamily". After you run this command, you should see a new file that is located here:

ks/migrate/20101229183849_create_users_column_family.rb

Note that the date stamp on the file will be different depending on when you create the migration. The migration file will look like this:

class CreateUsersColumnFamily < ActiveColumn::Migration

  def self.up

  end

  def self.down

  end

end

1. Edit your new migration file to do what you want it to. For this migration, it would probably wind up looking like this:

class CreateUsersColumnFamily < ActiveColumn::Migration

  def self.up
    create_column_family :users do |cf|
      cf.comment = 'Users column family'
      cf.comparator_type = :string
    end
  end

  def self.down
    drop_column_family :users
  end

end

1. Run the migrate rake task (for development):

rake ks:migrate

This will create the column family for your development environment. But you also need it in your test environment.

1. Prepare the test environment keyspace:

rake ks:test:prepare

And BAM! You have your development and test keyspaces set up correctly.

Inside your migrations

ActiveColumn::Migration, which all migrations extend by default, offers some useful functions. They are documented via rdoc in the code itself.

But I'm using Sinatra!

If you are using Rails, you don't need to do anything beyond including the active_column gem in your Gemfile. However, if you are using Sinatra (or some other framework), you can get these rake tasks to work merely by adding the following line to your Rakefile:

require 'active_column'

Please note, however, that the Rails generator is only available if you are using Rails. If you are not using Rails, you will have to create your migrations by hand.