Use a background migration for issues.closed_atchange-issues-closed-at-background-migration

In a previous attempt (rolled back in
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16021) we tried
to migrate `issues.closed_at` from timestamp to timestamptz using a
regular migration. This has a bad impact on GitLab.com and as such was
rolled back.

This commit re-implements the original migrations using generic
background migrations, allowing us to still migrate the data in a single
release but without a negative impact on availability.

To ensure the database schema is up to date the background migrations
are performed inline in development and test environments. We also make
sure to not migrate that that doesn't need migrating in the first place
or has already been migrated.

1 files changed, 57 insertions, 0 deletions

diff --git a/doc/development/what_requires_downtime.md b/doc/development/what_requires_downtime.md
index 05e0a64af18..9d0c62ecc35 100644
--- a/doc/development/what_requires_downtime.md
+++ b/doc/development/what_requires_downtime.md
@@ -195,6 +195,63 @@ end
 
 And that's it, we're done!
 
+## Changing Column Types For Large Tables
+
+While `change_column_type_concurrently` can be used for changing the type of a
+column 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.
+`change_column_type_concurrently` 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
+`change_column_type_using_background_migration` when migrating a column in a
+large table (e.g. `issues`). This method works similar to
+`change_column_type_concurrently` but uses background migration to spread the
+work / load over a longer time period, without slowing down deployments.
+
+Usage of this method is fairly simple:
+
+```ruby
+class ExampleMigration < ActiveRecord::Migration
+  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 `issues.closed_at` to `timestamp with time zone`.
+
+Keep in mind that the relation passed to
+`change_column_type_using_background_migration` _must_ include `EachBatch`,
+otherwise it will raise a `TypeError`.
+
 ## Adding Indexes
 
 Adding indexes is an expensive process that blocks INSERT and UPDATE queries for