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Migration Style Guide

When writing migrations for GitLab, you have to take into account that these will be ran by hundreds of thousands of organizations of all sizes, some with many years of data in their database.

In addition, having to take a server offline for a a upgrade small or big is a big burden for most organizations. For this reason it is important that your migrations are written carefully, can be applied online and adhere to the style guide below.

Migrations are not allowed to require GitLab installations to be taken offline unless absolutely necessary. Downtime assumptions should be based on the behaviour of a migration when performed using PostgreSQL, as various operations in MySQL may require downtime without there being alternatives.

When downtime is necessary the migration has to be approved by:

  1. The VP of Engineering
  2. A Backend Lead
  3. A Database Specialist

An up-to-date list of people holding these titles can be found at https://about.gitlab.com/team/.

The document "What Requires Downtime?" specifies various database operations, whether they require downtime and how to work around that whenever possible.

When writing your migrations, also consider that databases might have stale data or inconsistencies and guard for that. Try to make as few assumptions as possible about the state of the database.

Please don't depend on GitLab-specific code since it can change in future versions. If needed copy-paste GitLab code into the migration to make it forward compatible.

Commit Guidelines

Each migration must be added in its own commit with a descriptive commit message. If a commit adds a migration it should only include the migration and any corresponding changes to db/schema.rb. This makes it easy to revert a database migration without accidentally reverting other changes.

Downtime Tagging

Every migration must specify if it requires downtime or not, and if it should require downtime it must also specify a reason for this. This is required even if 99% of the migrations won't require downtime as this makes it easier to find the migrations that do require downtime.

To tag a migration, add the following two constants to the migration class' body:

  • DOWNTIME: a boolean that when set to true indicates the migration requires downtime.
  • DOWNTIME_REASON: a String containing the reason for the migration requiring downtime. This constant must be set when DOWNTIME is set to true.

For example:

class MyMigration < ActiveRecord::Migration
  DOWNTIME = true
  DOWNTIME_REASON = 'This migration requires downtime because ...'

  def change
    ...
  end
end

It is an error (that is, CI will fail) if the DOWNTIME constant is missing from a migration class.

Reversibility

Your migration must be reversible. This is very important, as it should be possible to downgrade in case of a vulnerability or bugs.

In your migration, add a comment describing how the reversibility of the migration was tested.

Multi Threading

Sometimes a migration might need to use multiple Ruby threads to speed up a migration. For this to work your migration needs to include the module Gitlab::Database::MultiThreadedMigration:

class MyMigration < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers
  include Gitlab::Database::MultiThreadedMigration
end

You can then use the method with_multiple_threads to perform work in separate threads. For example:

class MyMigration < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers
  include Gitlab::Database::MultiThreadedMigration

  def up
    with_multiple_threads(4) do
      disable_statement_timeout

      # ...
    end
  end
end

Here the call to disable_statement_timeout will use the connection local to the with_multiple_threads block, instead of re-using the global connection pool. This ensures each thread has its own connection object, and won't time out when trying to obtain one.

NOTE: PostgreSQL has a maximum amount of connections that it allows. This limit can vary from installation to installation. As a result it's recommended you do not use more than 32 threads in a single migration. Usually 4-8 threads should be more than enough.

Removing indices

When removing an index make sure to use the method remove_concurrent_index instead of the regular remove_index method. The remove_concurrent_index method automatically drops concurrent indexes when using PostgreSQL, removing the need for downtime. To use this method you must disable transactions by calling the method disable_ddl_transaction! in the body of your migration class like so:

class MyMigration < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers
  disable_ddl_transaction!

  def up
    remove_concurrent_index :table_name, :column_name if index_exists?(:table_name, :column_name)
  end
end

Adding indices

If you need to add a unique index please keep in mind there is the possibility of existing duplicates being present in the database. This means that should always first add a migration that removes any duplicates, before adding the unique index.

When adding an index make sure to use the method add_concurrent_index instead of the regular add_index method. The add_concurrent_index method automatically creates concurrent indexes when using PostgreSQL, removing the need for downtime. To use this method you must disable transactions by calling the method disable_ddl_transaction! in the body of your migration class like so:

class MyMigration < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers

  disable_ddl_transaction!

  def up
    add_concurrent_index :table, :column
  end

  def down
    remove_index :table, :column if index_exists?(:table, :column)
  end
end

Adding Columns With Default Values

When adding columns with default values you must use the method add_column_with_default. This method ensures the table is updated without requiring downtime. This method is not reversible so you must manually define the up and down methods in your migration class.

For example, to add the column foo to the projects table with a default value of 10 you'd write the following:

class MyMigration < ActiveRecord::Migration
  include Gitlab::Database::MigrationHelpers
  disable_ddl_transaction!

  def up
    add_column_with_default(:projects, :foo, :integer, default: 10)
  end

  def down
    remove_column(:projects, :foo)
  end
end

Keep in mind that this operation can easily take 10-15 minutes to complete on larger installations (e.g. GitLab.com). As a result you should only add default values if absolutely necessary.

Integer column type

By default, an integer column can hold up to a 4-byte (32-bit) number. That is a max value of 2,147,483,647. Be aware of this when creating a column that will hold file sizes in byte units. If you are tracking file size in bytes this restricts the maximum file size to just over 2GB.

To allow an integer column to hold up to an 8-byte (64-bit) number, explicitly set the limit to 8-bytes. This will allow the column to hold a value up to 9,223,372,036,854,775,807.

Rails migration example:

add_column_with_default(:projects, :foo, :integer, default: 10, limit: 8)

# or

add_column(:projects, :foo, :integer, default: 10, limit: 8)

Testing

Make sure that your migration works with MySQL and PostgreSQL with data. An empty database does not guarantee that your migration is correct.

Make sure your migration can be reversed.

Data migration

Please prefer Arel and plain SQL over usual ActiveRecord syntax. In case of using plain SQL you need to quote all input manually with quote_string helper.

Example with Arel:

users = Arel::Table.new(:users)
users.group(users[:user_id]).having(users[:id].count.gt(5))

#update other tables with these results

Example with plain SQL and quote_string helper:

select_all("SELECT name, COUNT(id) as cnt FROM tags GROUP BY name HAVING COUNT(id) > 1").each do |tag|
  tag_name = quote_string(tag["name"])
  duplicate_ids = select_all("SELECT id FROM tags WHERE name = '#{tag_name}'").map{|tag| tag["id"]}
  origin_tag_id = duplicate_ids.first
  duplicate_ids.delete origin_tag_id

  execute("UPDATE taggings SET tag_id = #{origin_tag_id} WHERE tag_id IN(#{duplicate_ids.join(",")})")
  execute("DELETE FROM tags WHERE id IN(#{duplicate_ids.join(",")})")
end

If you need more complex logic you can define and use models local to a migration. For example:

class MyMigration < ActiveRecord::Migration
  class Project < ActiveRecord::Base
    self.table_name = 'projects'
  end
end

When doing so be sure to explicitly set the model's table name so it's not derived from the class name or namespace.