PtOnlineMigration

Schema changes tend to lock tables, which can be unacceptable for your production database. The wonderful people at Percona have developed a tool which helps you avoid this problem on MySQL databases.

PTOnlineMigration patches ActiveRecord to have the option of altering tables through the pt-online-schema-change command.

It is highly recommended that you study up on said command: http://www.percona.com/doc/percona-toolkit/2.2/pt-online-schema-change.html

This gem depends on the Percona Toolkit, and therefore only works with MySQL.

Installation

Download/install the percona toolkit from their downloads page: http://www.percona.com/downloads/percona-toolkit/

Add this line to your application's Gemfile:

gem 'pt-online-migration'

And then execute:

$ bundle

Or install it yourself as:

$ gem install pt-online-migration

Usage

The syntax is very similar to change_table

e.g.

    class SimpleAlterFoo < ActiveRecord::Migration
        def up
            online_alter_table :foo_table, :execute do |t|
                t.integer :new_column_name
            end
        end

        def down
            online_alter_table :foo_table, :execute do |t|
                t.remove :new_column_name
            end
        end
    end

    class ComplexAlterFoo < ActiveRecord::Migration
        def up
            online_alter_table :foo_table, :execute, :database => 'foo_database', :critical_load => 'Threads_running:50' do |t|
                t.integer :new_column_name, :another_new_column_name, :limit => 7
                t.decimal :new_column_with_more_options, :precision => 5, :scale => 3
                t.change :foo_column, :boolean, :null => false
                t.rename :bar_column, :baz_column, :string, :limit => 140
                t.index  :foo_column, :unique => true, :name => 'foo_index'
            end
        end

        def down
            online_alter_table :foo_table, :execute, :database => 'foo_database', :critical_load => 'Threads_running:50' do |t|
                t.remove :new_column_name, :another_new_column_name, :new_column_with_more_options
                t.change :foo_column, :string
                t.rename :baz_column, :bar_column, :string
                t.remove_index :name => 'foo_index'
            end
        end
    end

but not

    class FailAlterFoo < ActiveRecord::Migration
        def change
            online_alter_table :foo_table, :execute do |t|
                t.integer :new_column_name
            end
        end
    end

Change is not supported.

The major difference is that the online_alter_table method takes a few new parameters. Without the symbol :execute, pt-online-schema-change will perform a dry-run and no actual schema change will be made. The new method also takes a hash of options which are given to the pt-online-schema-change command itself. Note in the complex example above a database is specified. If you don't specify a database ActiveRecord::Base.connection.current_database is assumed.

For a list of accepted options check out the percona-toolkit documention: http://www.percona.com/doc/percona-toolkit/2.2/pt-online-schema-change.html

Contributing

Fork it
Create your feature branch (git checkout -b my-new-feature)
Commit your changes (git commit -am 'Add some feature')
Push to the branch (git push origin my-new-feature)
Create new Pull Request