Large Hadron Migrator: Update huge SQL tables without going offline

With all the NoSQL hotness out
there, believe it or not, some people are still using relational
databases. (I know, right?).

When it comes to dealing with schema changes, the Active Record
Migrations in Rails make schema changes so easy, developers often take
them for granted. However, for extremely large sets of data, running an
ALTER TABLE might mean taking your database offline for hours. After
considering other projects Rany
Keddo and the smart folks at
Soundcloud developed their own solution.

Large Hadron
Migrator, named for
CERN’s high energy particle
accelerator,
uses a combination of copy table, triggers, and a journal table to move
data bit by bit into a new table while capturing everything still coming
into the source table in the live application.

Usage

To install, configure the gem in your Gemfile:

gem 'large-hadron-migrator'

… and run bundle install.

Next, write your migration as you normally would, using the
LargeHadronMigration class instead:

class AddIndexToEmails < LargeHadronMigration
  def self.up
    large_hadron_migrate :emails, :wait => 0.2 do |table_name|
      execute %Q{
        alter table %s
          add index index_emails_on_hashed_address (hashed_address)
      } % table_name
    end
  end
end

Be sure to check out the project
repo or blog post for advanced
usage and caveats.

[Source on GitHub]