Departure

Departure is an ActiveRecord connection adapter that allows running MySQL online and non-blocking DDL through ActiveRecord::Migration without needing to use a different DSL other than Rails' migrations DSL.

It uses pt-online-schema-change command-line tool of Percona Toolkit which runs MySQL alter table statements without downtime.

Rename from "Percona Migrator"

This project was formerly known as "Percona Migrator", but this incurs in an infringement of Percona's trade mark policy and thus has to be renamed. Said name is likely to cause confusion as to the source of the wrapper.

The next major versions will use "Departure" as gem name.

Installation

Departure relies on pt-online-schema-change from Percona Toolkit

Mac

brew install percona-toolkit

If when running a migration you see an error like:

PerconaMigrator::Error: Cannot connect to MySQL: Cannot connect to MySQL because
the Perl DBI module is not installed or not found.

You also need to install the DBI and DBD::MySQL modules from cpan.

$ sudo cpan
cpan> install DBI
cpan> install DBD::mysql

Linux

Ubuntu/Debian based

apt-get install percona-toolkit

Arch Linux

pacman -S percona-toolkit perl-dbd-mysql

Other distros

For other Linux distributions check out the Percona Toolkit download page to find the package that fits your distribution.

You can also get it from Percona's apt repository

Once installed, add this line to your application's Gemfile:

gem 'departure'

And then execute:

$ bundle

Or install it yourself as:

$ gem install departure

Usage

Once you added it to your app's Gemfile, you can create and run Rails migrations as usual.

All the ALTER TABLE statements will be executed with pt-online-schema-change, which will provide additional output to the migration.

pt-online-schema-change arguments

with environment variable

You can specify any pt-online-schema-change arguments when running the migration. All what you pass in the PERCONA_ARGS env var, will be bypassed to the binary, overwriting any default values. Note the format is the same as in pt-online-schema-change. Check the full list in Percona Toolkit documentation

$ PERCONA_ARGS='--chunk-time=1' bundle exec rake db:migrate:up VERSION=xxx

or even mulitple arguments

$ PERCONA_ARGS='--chunk-time=1 --critical-load Threads_running=55' bundle exec rake db:migrate:up VERSION=xxx

Use caution when using PERCONA_ARGS with db:migrate, as your args will be applied to every call that Departure makes to pt-osc.

with global configuration

You can specify any pt-online-schema-change arguments in global gem configuration using global_percona_args option.

Departure.configure do |config|
  config.global_percona_args = '--chunk-time=1 --critical-load Threads_running=55'
end

Unlike using PERCONA_ARGS, options provided with global configuration will be applied every time sql command is executed via pt-online-schema-change.

Arguments provided in global configuration can be overwritten with PERCONA_ARGS env variable.

We recommend using this option with caution and only when you understand the consequences.

LHM support

If you moved to Soundcloud's Lhm already, we got you covered. Departure overrides Lhm's DSL so that all the alter statements also go through pt-online-schema-change as well.

You can keep your Lhm migrations and start using Rails migration's DSL back again in your next migration.

Configuration

You can override any of the default values from an initializer:

Departure.configure do |config|
  config.tmp_path = '/tmp/'
end

It's strongly recommended to name it after this gems name, such as config/initializers/departure.rb

Only enabled on a per-migration basis

If you wish to only have Departure enabled per-migration, set config.enabled_by_default = false in the configure block of your departure initializer.

Then, add a uses_departure! statement in migrations where Departure should be used:

class UseDepartureMigration < ActiveRecord::Migration[5.2]
  uses_departure!

  def up
    # ...
  end
  # ...
end

How it works

When booting your Rails app, Departure extends the ActiveRecord::Migration#migrate method to reset the connection and reestablish it using the DepartureAdapter instead of the one you defined in your config/database.yml.

Then, when any migration DSL methods such as add_column or create_table are executed, they all go to the DepartureAdapter. There, the methods that require ALTER TABLE SQL statements, like add_column, are overriden to get executed with Departure::Runner, which deals with the pt-online-schema-change binary. All the others, like create_table, are delegated to the ActiveRecord's built in Mysql2Adapter and so they follow the regular path.

Departure::Runner spawns a new process that runs the pt-online-schema-change binary present in the system, with the appropriate arguments for the generated SQL.

When any errors occur, an ActiveRecord::StatementInvalid exception is raised and the migration is aborted, as all other ActiveRecord connection adapters.

Trouleshooting

Error creating new table: DBD::mysql::db do failed: Can't write; duplicate key in table (TABLE_NAME)

There is a known bug in percona-toolkit version 2.2.15 that prevents schema changes when a table has constraints. You should upgrade to a version later than 2.2.17 to fix the issue.

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/departurerb/departure. They need to be opened against master or v3.2 only if the changes fix a bug in Rails 3.2 apps.

Please note that this project is released with a Contributor Code of Conduct. By participating in this project you agree to abide by its terms.

Check the code of conduct here

Changelog

You can consult the changelog here

License

The gem is available as open source under the terms of the MIT License.