Respond to DB health in background migrations

This changes the BackgroundMigration worker so it checks for the health
of the DB before performing a background migration. This in turn allows
us to reduce the minimum interval, without having to worry about blowing
things up if we schedule too many migrations.

In this setup, the BackgroundMigration worker will reschedule jobs as
long as the database is considered to be in an unhealthy state. Once the
database has recovered, the migration can be performed.

To determine if the database is in a healthy state, we look at the
replication lag of any replication slots defined on the primary. If the
lag is deemed to great (100 MB by default) for too many slots, the
migration is rescheduled for a later point in time.

The health checking code is hidden behind a feature flag, allowing us to
disable it if necessary.

2 files changed, 7 insertions, 3 deletions

diff --git a/lib/gitlab/background_migration.rb b/lib/gitlab/background_migration.rb
index d3f66877672..36c85dec544 100644
--- a/lib/gitlab/background_migration.rb
+++ b/lib/gitlab/background_migration.rb
@@ -46,7 +46,11 @@ module Gitlab
     # arguments - The arguments to pass to the background migration's "perform"
     #             method.
     def self.perform(class_name, arguments)
-      const_get(class_name).new.perform(*arguments)
+      migration_class_for(class_name).new.perform(*arguments)
+    end
+
+    def self.migration_class_for(class_name)
+      const_get(class_name)
     end
   end
 end
diff --git a/lib/gitlab/database/migration_helpers.rb b/lib/gitlab/database/migration_helpers.rb
index 4fe5b4cc835..f39b3b6eb5b 100644
--- a/lib/gitlab/database/migration_helpers.rb
+++ b/lib/gitlab/database/migration_helpers.rb
@@ -979,8 +979,8 @@ into similar problems in the future (e.g. when new tables are created).
 
         # To not overload the worker too much we enforce a minimum interval both
         # when scheduling and performing jobs.
-        if delay_interval < BackgroundMigrationWorker::MIN_INTERVAL
-          delay_interval = BackgroundMigrationWorker::MIN_INTERVAL
+        if delay_interval < BackgroundMigrationWorker.minimum_interval
+          delay_interval = BackgroundMigrationWorker.minimum_interval
         end
 
         model_class.each_batch(of: batch_size) do |relation, index|