Large Hadron Migrator

Rails style database migrations are a useful way to evolve your data schema in an agile manner. Most Rails projects start like this, and at first, making changes is fast and easy.

That is until your tables grow to millions of records. At this point, the locking nature of ALTER TABLE may take your site down for an hour or more while critical tables are migrated. In order to avoid this, developers begin to design around the problem by introducing join tables or moving the data into another layer. Development gets less and less agile as tables grow and grow. To make the problem worse, adding or changing indices to optimize data access becomes just as difficult.

Side effects may include black holes and universe implosion.

There are few things that can be done at the server or engine level. It is possible to change default values in an ALTER TABLE without locking the table. The InnoDB Plugin provides facilities for online index creation, which is great if you are using this engine, but only solves half the problem.

At SoundCloud we started having migration pains quite a while ago, and after looking around for third party solutions, we decided to create our own. We called it Large Hadron Migrator, and it is a gem for online ActiveRecord and DataMapper migrations.

The Large Hadron collider at CERN

The idea

The basic idea is to perform the migration online while the system is live, without locking the table. In contrast to OAK and the facebook tool, we only use a copy table and triggers.

The Large Hadron is a test driven Ruby solution which can easily be dropped into an ActiveRecord or DataMapper migration. It presumes a single auto incremented numerical primary key called id as per the Rails convention. Unlike the twitter solution, it does not require the presence of an indexed updated_at column.

Requirements

Lhm currently only works with MySQL databases and requires an established ActiveRecord or DataMapper connection.

It is compatible and continuously tested with MRI 1.9.x, ActiveRecord 2.3.x and 3.x (mysql and mysql2 adapters), as well as DataMapper 1.2 (dm-mysql-adapter).

Lhm also works with dm-master-slave-adapter, it'll bind to the master before running the migrations.

The test suite is also run against MRI 2.0.0 in Continuous Integration, but there are a few bugs left to fix.

Limitations

Lhm requires a monotonically increasing numeric Primary Key on the table, due to how the Chunker works.

Installation

Install it via gem install lhm or add gem "lhm" to your Gemfile.

Usage

You can invoke Lhm directly from a plain ruby file after connecting ActiveRecord to your mysql instance:

require 'lhm'

ActiveRecord::Base.establish_connection(
  :adapter => 'mysql',
  :host => '127.0.0.1',
  :database => 'lhm'
)

# or with DataMapper
Lhm.setup(DataMapper.setup(:default, 'mysql://127.0.0.1/lhm'))

# and migrate
Lhm.change_table :users do |m|
  m.add_column :arbitrary, "INT(12)"
  m.add_index  [:arbitrary_id, :created_at]
  m.ddl("alter table %s add column flag tinyint(1)" % m.name)
end

To use Lhm from an ActiveRecord::Migration in a Rails project, add it to your Gemfile, then invoke as follows:

require 'lhm'

class MigrateUsers < ActiveRecord::Migration
  def self.up
    Lhm.change_table :users do |m|
      m.add_column :arbitrary, "INT(12)"
      m.add_index  [:arbitrary_id, :created_at]
      m.ddl("alter table %s add column flag tinyint(1)" % m.name)
    end
  end

  def self.down
    Lhm.change_table :users do |m|
      m.remove_index  [:arbitrary_id, :created_at]
      m.remove_column :arbitrary
    end
  end
end

Using dm-migrations, you'd define all your migrations as follows, and then call migrate_up! or migrate_down! as normal.

require 'dm-migrations/migration_runner'
require 'lhm'

migration 1, :migrate_users do
  up do
    Lhm.change_table :users do |m|
      m.add_column :arbitrary, "INT(12)"
      m.add_index  [:arbitrary_id, :created_at]
      m.ddl("alter table %s add column flag tinyint(1)" % m.name)
    end
  end

  down do
    Lhm.change_table :users do |m|
      m.remove_index  [:arbitrary_id, :created_at]
      m.remove_column :arbitrary
    end
  end
end

Note: Lhm won't delete the old, leftover table. This is on purpose, in order to prevent accidental data loss.

Table rename strategies

There are two different table rename strategies available: LockedSwitcher and AtomicSwitcher.

The LockedSwitcher strategy locks the table being migrated and issues two ALTER TABLE statements. The AtomicSwitcher uses a single atomic RENAME TABLE query and is the favored solution.

Lhm chooses AtomicSwitcher if no strategy is specified, unless your version of MySQL is affected by binlog bug #39675. If your version is affected, Lhm will raise an error if you don't specify a strategy. You're recommended to use the LockedSwitcher in these cases to avoid replication issues.

To specify the strategy in your migration:

Lhm.change_table :users, :atomic_switch => true do |m|
  # ...
end

Limiting the data that is migrated

For instances where you want to limit the data that is migrated to the new table by some conditions, you may tell the migration to filter by a set of conditions:

Lhm.change_table(:sounds) do |m|
  m.filter("inner join users on users.`id` = sounds.`user_id` and sounds.`public` = 1")
end

Note that this SQL will be inserted into the copy directly after the "from" statement - so be sure to use inner/outer join syntax and not cross joins. These conditions will not affect the triggers, so any modifications to the table during the run will happen on the new table as well.

Cleaning up after an interrupted Lhm run

If an Lhm migration is interrupted, it may leave behind the temporary tables used in the migration. If the migration is re-started, the unexpected presence of these tables will cause an error. In this case, Lhm.cleanup can be used to drop any orphaned Lhm temporary tables.

To see what Lhm tables are found:

Lhm.cleanup

To remove any Lhm tables found:

Lhm.cleanup(true)

Contributing

We'll check out your contribution if you:

• Provide a comprehensive suite of tests for your fork.
• Have a clear and documented rationale for your changes.
• Package these up in a pull request.

We'll do our best to help you out with any contribution issues you may have.

License

The license is included as LICENSE in this directory.

Similar solutions

• OAK: online alter table
• Facebook
• Twitter
• pt-online-schema-change