What requires downtime?
When working with a database certain operations can be performed without taking GitLab offline, others do require a downtime period. This guide describes various operations, their impact, and how to perform them without requiring downtime.
You can safely add a new column to an existing table as long as it does not have a default value. For example, this query would not require downtime:
ALTER TABLE projects ADD COLUMN random_value int;
Add a column with a default however does require downtime. For example, consider this query:
ALTER TABLE projects ADD COLUMN random_value int DEFAULT 42;
This requires updating every single row in the
projects table so that
random_value is set to
42 by default. This requires updating all rows and
indexes in a table. This in turn acquires enough locks on the table for it to
effectively block any other queries.
Adding a column with a default value can be done without requiring downtime
when using the migration helper method
Gitlab::Database::MigrationHelpers#add_column_with_default. This method works
add_column except it updates existing rows in batches without
blocking access to the table being modified. See "Adding Columns With Default
Values" for more
information on how to use this method.
Note that usage of
allow_null: false to also add
NOT NULL constraint is discouraged.
Removing columns is tricky because running GitLab processes may still be using the columns. To work around this safely, you will need three steps in three releases:
- Ignoring the column (release M)
- Dropping the column (release M+1)
- Removing the ignore rule (release M+2)
The reason we spread this out across three releases is that dropping a column is a destructive operation that can't be rolled back easily.
Following this procedure helps us to make sure there are no deployments to GitLab.com and upgrade processes for self-managed installations that lump together any of these steps.
Step 1: Ignoring the column (release M)
The first step is to ignore the column in the application code. This is
necessary because Rails caches the columns and re-uses this cache in various
places. This can be done by defining the columns to ignore. For example, to ignore
updated_at in the User model you'd use the following:
class User < ApplicationRecord include IgnorableColumns ignore_column :updated_at, remove_with: '12.7', remove_after: '2019-12-22' end
Multiple columns can be ignored, too:
ignore_columns %i[updated_at created_at], remove_with: '12.7', remove_after: '2019-12-22'
We require indication of when it is safe to remove the column ignore with:
remove_with: set to a GitLab release typically two releases (M+2) after adding the column ignore.
remove_after: set to a date after which we consider it safe to remove the column ignore, typically within the development cycle of release M+2.
This information allows us to reason better about column ignores and makes sure we don't remove column ignores too early for both regular releases and deployments to GitLab.com. For example, this avoids a situation where we deploy a bulk of changes that include both changes to ignore the column and subsequently remove the column ignore (which would result in a downtime).
In this example, the change to ignore the column went into release 12.5.
Step 2: Dropping the column (release M+1)
Continuing our example, dropping the column goes into a post-deployment migration in release 12.6:
remove_column :user, :updated_at
Step 3: Removing the ignore rule (release M+2)
With the next release, in this example 12.7, we set up another merge request to remove the ignore rule.
This removes the
ignore_column line and - if not needed anymore - also the inclusion of
This should only get merged with the release indicated with
remove_with and once
remove_after date has passed.
Renaming columns the normal way requires downtime as an application may continue using the old column name during/after a database migration. To rename a column without requiring downtime we need two migrations: a regular migration, and a post-deployment migration. Both these migration can go in the same release.
Step 1: Add The Regular Migration
First we need to create the regular migration. This migration should use
Gitlab::Database::MigrationHelpers#rename_column_concurrently to perform the
renaming. For example
# A regular migration in db/migrate class RenameUsersUpdatedAtToUpdatedAtTimestamp < ActiveRecord::Migration[4.2] include Gitlab::Database::MigrationHelpers disable_ddl_transaction! def up rename_column_concurrently :users, :updated_at, :updated_at_timestamp end def down undo_rename_column_concurrently :users, :updated_at, :updated_at_timestamp end end
This will take care of renaming the column, ensuring data stays in sync, copying over indexes and foreign keys, etc.
NOTE: if a column contains 1 or more indexes that do not contain the name of the original column, the above procedure will fail. In this case you will first need to rename these indexes.
Step 2: Add A Post-Deployment Migration
The renaming procedure requires some cleaning up in a post-deployment migration.
We can perform this cleanup using
# A post-deployment migration in db/post_migrate class CleanupUsersUpdatedAtRename < ActiveRecord::Migration[4.2] include Gitlab::Database::MigrationHelpers disable_ddl_transaction! def up cleanup_concurrent_column_rename :users, :updated_at, :updated_at_timestamp end def down undo_cleanup_concurrent_column_rename :users, :updated_at, :updated_at_timestamp end end
NOTE: Note: If you're renaming a large table, please carefully consider the state when the first migration has run but the second cleanup migration hasn't been run yet. With Canary it is possible that the system runs in this state for a significant amount of time.
Changing Column Constraints
Adding or removing a NOT NULL clause (or another constraint) can typically be
done without requiring downtime. However, this does require that any application
changes are deployed first. Thus, changing the constraints of a column should
happen in a post-deployment migration.
NOTE: Avoid using
change_column as it produces inefficient query because it re-defines
the whole column type. For example, to add a NOT NULL constraint, prefer
Changing Column Types
Changing the type of a column can be done using
method works similarly to
rename_column_concurrently. For example, let's say
we want to change the type of
Step 1: Create A Regular Migration
A regular migration is used to create a new column with a temporary name along with setting up some triggers to keep data in sync. Such a migration would look as follows:
# A regular migration in db/migrate class ChangeUsersUsernameStringToText < ActiveRecord::Migration[4.2] include Gitlab::Database::MigrationHelpers disable_ddl_transaction! def up change_column_type_concurrently :users, :username, :text end def down cleanup_concurrent_column_type_change :users, :username end end
Step 2: Create A Post Deployment Migration
Next we need to clean up our changes using a post-deployment migration:
# A post-deployment migration in db/post_migrate class ChangeUsersUsernameStringToTextCleanup < ActiveRecord::Migration[4.2] include Gitlab::Database::MigrationHelpers disable_ddl_transaction! def up cleanup_concurrent_column_type_change :users, :username end def down change_column_type_concurrently :users, :username, :string end end
And that's it, we're done!
Changing The Schema For Large Tables
rename_column_concurrently can be
used for changing the schema of a table without downtime, it doesn't work very
well for large tables. Because all of the work happens in sequence the migration
can take a very long time to complete, preventing a deployment from proceeding.
They can also produce a lot of pressure on the database due to it rapidly
updating many rows in sequence.
To reduce database pressure you should instead use
when migrating a column in a large table (e.g.
issues). These methods work
similarly to the concurrent counterparts but uses background migration to spread
the work / load over a longer time period, without slowing down deployments.
For example, to change the column type using a background migration:
class ExampleMigration < ActiveRecord::Migration[4.2] include Gitlab::Database::MigrationHelpers disable_ddl_transaction! class Issue < ActiveRecord::Base self.table_name = 'issues' include EachBatch def self.to_migrate where('closed_at IS NOT NULL') end end def up change_column_type_using_background_migration( Issue.to_migrate, :closed_at, :datetime_with_timezone ) end def down change_column_type_using_background_migration( Issue.to_migrate, :closed_at, :datetime ) end end
This would change the type of
timestamp with time zone.
Keep in mind that the relation passed to
change_column_type_using_background_migration must include
otherwise it will raise a
This migration then needs to be followed in a separate release (not a patch release) by a cleanup migration, which should steal from the queue and handle any remaining rows. For example:
class MigrateRemainingIssuesClosedAt < ActiveRecord::Migration[4.2] include Gitlab::Database::MigrationHelpers DOWNTIME = false disable_ddl_transaction! class Issue < ActiveRecord::Base self.table_name = 'issues' include EachBatch end def up Gitlab::BackgroundMigration.steal('CopyColumn') Gitlab::BackgroundMigration.steal('CleanupConcurrentTypeChange') migrate_remaining_rows if migrate_column_type? end def down # Previous migrations already revert the changes made here. end def migrate_remaining_rows Issue.where('closed_at_for_type_change IS NULL AND closed_at IS NOT NULL').each_batch do |batch| batch.update_all('closed_at_for_type_change = closed_at') end cleanup_concurrent_column_type_change(:issues, :closed_at) end def migrate_column_type? # Some environments may have already executed the previous version of this # migration, thus we don't need to migrate those environments again. column_for('issues', 'closed_at').type == :datetime # rubocop:disable Migration/Datetime end end
The same applies to
- Create a migration using the helper, which will schedule background migrations to spread the writes over a longer period of time.
- In the next monthly release, create a clean-up migration to steal from the Sidekiq queues, migrate any missing rows, and cleanup the rename. This migration should skip the steps after stealing from the Sidekiq queues if the column has already been renamed.
For more information, see the documentation on cleaning up background migrations.
Adding indexes is an expensive process that blocks INSERT and UPDATE queries for
the duration. You can work around this by using the
CREATE INDEX CONCURRENTLY index_name ON projects (column_name);
Migrations can take advantage of this by using the method
add_concurrent_index. For example:
class MyMigration < ActiveRecord::Migration[4.2] def up add_concurrent_index :projects, :column_name end def down remove_index(:projects, :column_name) if index_exists?(:projects, :column_name) end end
add_concurrent_index can not be reversed automatically, thus you
need to manually define
Dropping an index does not require downtime.
This operation is safe as there's no code using the table just yet.
Dropping tables can be done safely using a post-deployment migration, but only if the application no longer uses the table.
Renaming tables requires downtime as an application may continue using the old table name during/after a database migration.
Adding Foreign Keys
Adding foreign keys usually works in 3 steps:
- Start a transaction
ALTER TABLEto add the constraint(s)
- Check all existing data
ALTER TABLE typically acquires an exclusive lock until the end of a
transaction this means this approach would require downtime.
GitLab allows you to work around this by using
Gitlab::Database::MigrationHelpers#add_concurrent_foreign_key. This method
ensures that no downtime is needed.
Removing Foreign Keys
This operation does not require downtime.
Data migrations can be tricky. The usual approach to migrate data is to take a 3 step approach:
- Migrate the initial batch of data
- Deploy the application code
- Migrate any remaining data
Usually this works, but not always. For example, if a field's format is to be changed from JSON to something else we have a bit of a problem. If we were to change existing data before deploying application code we'll most likely run into errors. On the other hand, if we were to migrate after deploying the application code we could run into the same problems.
If you merely need to correct some invalid data, then a post-deployment migration is usually enough. If you need to change the format of data (e.g. from JSON to something else) it's typically best to add a new column for the new data format, and have the application use that. In such a case the procedure would be:
- Add a new column in the new format
- Copy over existing data to this new column
- Deploy the application code
- In a post-deployment migration, copy over any remaining data
In general there is no one-size-fits-all solution, therefore it's best to discuss these kind of migrations in a merge request to make sure they are implemented in the best way possible.