This is the Shopify fork of SoundCloud's LHM. The following description, originally from SoundCloud (with minor updates by Shopify), gives some of the flavor around its original creation, and its choice of name...

Rails-style database migrations are a useful way to evolve your database 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 or billions of records. At this point, the locking nature of ALTER TABLE may take your site down for hours 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. InnoDB provides facilities for online index creation, but that 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 Ruby Gem that provides facilities for online ActiveRecord migrations.

The Large Hadron Collider at CERN near Geneva, Switzerland.

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.

LHM is a test-driven Ruby solution which can easily be dropped into an ActiveRecord migration. It presumes a single auto-incremented numeric primary key called id as per the Rails convention. Unlike Matt Freels's table_migrator solution, it does not require the presence of an indexed updated_at column.

LHM currently only works with MySQL databases and requires an established ActiveRecord connection.

Due to the Chunker implementation, LHM requires that the table to migrate has a a single integer numeric key column named id.

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

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'
)

# 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

Note: LHM does not delete the old, leftover table. This is intentional, in order to prevent accidental data loss. After successful or failed LHM migrations, these leftover tables must be cleaned up.

LHM uses a throttling mechanism to read data in your original table. By default, 2,000 rows are read each 0.1 second. If you want to change that behaviour, you can pass an instance of a throttler with the throttler option. In this example, 1,000 rows will be read with a 10 second delay between each processing:

my_throttler = Lhm::Throttler::Time.new(stride: 1000, delay: 10)

Lhm.change_table :users, throttler: my_throttler  do |m|
  ...
end

Lhm uses by default the time throttler, however a better solution is to throttle the copy of the data depending on the time that the slaves are behind. To use the SlaveLag throttler:

Lhm.change_table :users, throttler: :slave_lag_throttler  do |m|
  ...
end

Or to set that as default throttler, use the following (for instance in a Rails initializer):

Lhm.setup_throttler(:slave_lag_throttler)

If you don't have access to connect directly to your replicas, you can also throttle based on the number of threads running in MySQL, as a proxy for "is this operation causing excessive load":

Lhm.change_table :users, throttler: :threads_running_throttler do |m|
  ...
end

Or to set that as default throttler, use the following (for instance in a Rails initializer):

Lhm.setup_throttler(:threads_running_throttler)

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

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 clause so be sure to use INNER JOIN or OUTER JOIN syntax and not comma-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.

If an LHM migration is interrupted, it may leave behind the temporary tables and/or triggers 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 or triggers.

To see what LHM tables/triggers are found:

Lhm.cleanup

To remove any LHM tables/triggers found:

Lhm.cleanup(true)

Optionally, only remove tables up to a specific time, if you want to retain previous migrations.

Rails:

Lhm.cleanup(true, until: 1.day.ago)

Ruby:

Lhm.cleanup(true, until: Time.now - 86400)

To run the tests:

bundle exec rake unit # unit tests
bundle exec rake integration # integration tests
bundle exec rake unit # all tests

You can run an individual test as follows:

bundle exec rake unit TEST=spec/integration/atomic_switcher_spec.rb

You can check the code coverage reporting for an individual test as follows:

rm -rf coverage
COV=1 bundle exec rake unit TEST=spec/integration/atomic_switcher_spec.rb
open coverage/index.html

To check the code coverage for all tests:

rm -rf coverage
COV=1 bundle exec rake unit && bundle exec rake integration
open coverage/index.html

The integration tests rely on a replicated configuration for MySQL. We're using dbdeployer to manage the test nodes required to set up these configurations.

The license is included as LICENSE in this directory.

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