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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:
- The VP of Engineering
- A Backend Lead
- 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.
Schema Changes
Migrations that make changes to the database schema (e.g. adding a column) can only be added in the monthly release, patch releases may only contain data migrations unless schema changes are absolutely required to solve a problem.
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 totrueindicates the migration requires downtime.DOWNTIME_REASON: a String containing the reason for the migration requiring downtime. This constant must be set whenDOWNTIMEis set totrue.
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 indexes
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 indexes
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)
Timestamp column type
By default, Rails uses the timestamp data type that stores timestamp data without timezone information.
The timestamp data type is used by calling either the add_timestamps or the timestamps method.
Also Rails converts the :datetime data type to the timestamp one.
Example:
# timestamps
create_table :users do |t|
t.timestamps
end
# add_timestamps
def up
add_timestamps :users
end
# :datetime
def up
add_column :users, :last_sign_in, :datetime
end
Instead of using these methods one should use the following methods to store timestamps with timezones:
add_timestamps_with_timezonetimestamps_with_timezone
This ensures all timestamps have a time zone specified. This in turn means existing timestamps won't suddenly use a different timezone when the system's timezone changes. It also makes it very clear which timezone was used in the first place.
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.
Renaming reserved paths
When a new route for projects is introduced that could conflict with any existing records. The path for this records should be renamed, and the related data should be moved on disk.
Since we had to do this a few times already, there are now some helpers to help with this.
To use this you can include Gitlab::Database::RenameReservedPathsMigration::V1
in your migration. This will provide 3 methods which you can pass one or more
paths that need to be rejected.
rename_root_paths: This will rename the path of all namespaces with the
given name that don't have a parent_id.
rename_child_paths: This will rename the path of all namespaces with the
given name that have a parent_id.
rename_wildcard_paths: This will rename the path of all projects, and all
namespaces that have a project_id.
The path column for these rows will be renamed to their previous value followed
by an integer. For example: users would turn into users0