mysql_online_migrations

Patch Rails migrations to enforce MySQL 5.6 online migrations Prior to MySQL 5.6, when adding / removing / renaming indexes and columns, MySQL would lock the writes of the whole table. MySQL 5.6 by default will try to apply the least locking possible. You however don't know what kind of locking it applies and there's situations where it can't allow writes during a migration (See Caveats). This gem enforces LOCK=NONE in all migration statements of Rails. Therefore, you're getting an error when MySQL cannot write during the migration so there's no surprise when rolling out in production.

Requirements

Built for Rails 3.2.15+, including Rails 4.

List of requirements :

• Use mysql2 adapter
• Use Rails ">= 3.2.15"
• Use MySQL or Percona Server 5.6.X with InnoDB

Scope of this gem

Patch Rails migrations to automatically add LOCK=NONE when issuing ALTER, CREATE INDEX, CREATE UNIQUE INDEX. DROP INDEX statements from any methods of ActiveRecord:

• Index management : add_index, remove_index, rename_index
• Add column : add_column, add_timestamps
• Remove column : remove_column, remove_timestamps
• Change column : change_column, change_column_null, change_column_default
• Any other method that was added in Rails 4 etc ...

Please note that it only modifies sql queries sent in Rails Migrations. This way we avoid patching all of ActiveRecord statements all the time.

Usage

In a typical Rails app, just add it to your Gemfile :

gem 'mysql_online_migrations'

Then run bundle install

You're ready for online migrations! Please read the caveats section though.

Turn it off for a whole environment

Example for environment test (your CI might not use MySQL 5.6 yet), add the following to config/environments/test.rb: config.active_record.mysql_online_migrations = false

Turn it off for a specific statement

Call your migration statement within with_lock method. Example :

with_lock do
  add_index :my_table, :my_field
end

The with_lock method will be useful when hitting the caveats of LOCK=NONE. Please read the 'Caveats' section.

Enable verbose output

To enable an 'ONLINE MIGRATION' debug statement whenever an online migration is run, simply set the MysqlOnlineMigrations.verbose module variable to true. Example (in a Rails app's config/initializers/mysql_online_migrations.rb):

MysqlOnlineMigrations.verbose = true

Caveats

The MySQL manual contains a list of which DDL statements can be run with LOCK=NONE under Table 14.5 Summary of Online Status for DDL Operations. The short version is that you can not yet:

• Index a column of type text
• Change the type of a column
• Change the length of a column
• Set a column to NOT NULL (at least not with the default SQL_MODE)
• Adding an AUTO_INCREMENT column,

If you don't use the with_lock method when online migration is not supported, you'll get a MySQL exception. No risk to lock the table by accident. It's therefore highly recommended to use it in development/test/staging environment before running migrations in production. If you have to perform such a migration without locking the table, tools such as pt-online-schema-change and LHM are viable options