Enable by default scheduled execution of batched background migrations

app/workers/database/batched_background_migration_worker.rb

@@ -16,7 +16,7 @@ module Database
     INTERVAL_VARIANCE = 5.seconds.freeze
 
     def perform
-      return unless Feature.enabled?(:execute_batched_migrations_on_schedule, type: :ops) && active_migration
+      return unless Feature.enabled?(:execute_batched_migrations_on_schedule, type: :ops, default_enabled: :yaml) && active_migration
 
       with_exclusive_lease(active_migration.interval) do
         # Now that we have the exclusive lease, reload migration in case another process has changed it.

changelogs/unreleased/329511-enable-by-default-execute-batched-migrations-on-schedule.yml

+---
+title: Enable by default scheduled execution of batched background migrations
+merge_request: 61316
+author:
+type: added

config/feature_flags/ops/execute_batched_migrations_on_schedule.yml

@@ -5,4 +5,4 @@ rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/326241
 milestone: '13.11'
 type: ops
 group: group::database
-default_enabled: false
+default_enabled: true

doc/update/index.md

@@ -107,6 +107,10 @@ Sidekiq::Queue.new("background_migration").size
 Sidekiq::ScheduledSet.new.select { |r| r.klass == 'BackgroundMigrationWorker' }.size
 ```
 
+### Batched background migrations
+
+See the documentation on [batched background migrations](../user/admin_area/monitoring/background_migrations.md).
+
 ### What do I do if my background migrations are stuck?
 
 WARNING:

doc/user/admin_area/monitoring/background_migrations.md

+---
+stage: Enablement
+group: Database
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
+---
+
+# Batched Background Migrations **(FREE SELF)**
+
+> - [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/51332) in GitLab 13.11.
+> - [Deployed behind a feature flag](../../../user/feature_flags.md), disabled by default.
+> - [Enabled by default](https://gitlab.com/gitlab-org/gitlab/-/issues/329511) in GitLab 13.12.
+> - Enabled on GitLab.com.
+> - Recommended for production use.
+> - For GitLab self-managed instances, GitLab administrators can opt to [disable it](#enable-or-disable-batched-background-migrations). **(FREE SELF)**
+
+There can be [risks when disabling released features](../../../user/feature_flags.md#risks-when-disabling-released-features).
+Refer to this feature's version history for more details.
+
+To update database tables in batches, GitLab can use batched background migrations. These migrations
+are created by GitLab developers and run automatically on upgrade. However, such migrations are
+limited in scope to help with migrating some `integer` database columns to `bigint`. This is needed to
+prevent integer overflow for some tables.
+
+All migrations must be finished before upgrading GitLab. To check the status of the existing
+migrations, execute this command:
+
+```ruby
+Gitlab::Database::BackgroundMigration::BatchedMigration.pluck(:id, :table_name, :status)
+```
+
+## Enable or disable Batched Background Migrations **(FREE SELF)**
+
+Batched Background Migrations is under development but ready for production use.
+It is deployed behind a feature flag that is **enabled by default**.
+[GitLab administrators with access to the GitLab Rails console](../../../administration/feature_flags.md)
+can opt to disable it.
+
+To enable it:
+
+```ruby
+Feature.enable(:execute_batched_migrations_on_schedule)
+```
+
+To disable it:
+
+```ruby
+Feature.disable(:execute_batched_migrations_on_schedule)
+```