Run background migrations with a minimum interval

This adds a minimum interval to BackgroundMigrationWorker, ensuring
background migrations of the same class only run once every 5 minutes.
This prevents a thundering herd problem where scheduled migrations all
run at once due to their delays having been expired (e.g. as the result
of a queue being paused for a long time).

If a job was recently executed it's rescheduled with a delay that equals
the remaining time of the job's lease. This means that if the lease
expires in two minutes we only need to wait two minutes, instead of
five.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/41624

app/workers/background_migration_worker.rb

 class BackgroundMigrationWorker
   include ApplicationWorker
 
+  # The minimum amount of time between processing two jobs of the same migration
+  # class.
+  #
+  # This interval is set to 5 minutes so autovacuuming and other maintenance
+  # related tasks have plenty of time to clean up after a migration has been
+  # performed.
+  MIN_INTERVAL = 5.minutes.to_i
+
   # Performs the background migration.
   #
   # See Gitlab::BackgroundMigration.perform for more information.
+  #
+  # class_name - The class name of the background migration to run.
+  # arguments - The arguments to pass to the migration class.
   def perform(class_name, arguments = [])
-    Gitlab::BackgroundMigration.perform(class_name, arguments)
+    should_perform, ttl = perform_and_ttl(class_name)
+
+    if should_perform
+      Gitlab::BackgroundMigration.perform(class_name, arguments)
+    else
+      # If the lease could not be obtained this means either another process is
+      # running a migration of this class or we ran one recently. In this case
+      # we'll reschedule the job in such a way that it is picked up again around
+      # the time the lease expires.
+      self.class.perform_in(ttl || MIN_INTERVAL, class_name, arguments)
+    end
+  end
+
+  def perform_and_ttl(class_name)
+    if always_perform?
+      # In test environments `perform_in` will run right away. This can then
+      # lead to stack level errors in the above `#perform`. To work around this
+      # we'll just perform the migration right away in the test environment.
+      [true, nil]
+    else
+      lease = lease_for(class_name)
+
+      [lease.try_obtain, lease.ttl]
+    end
+  end
+
+  def lease_for(class_name)
+    Gitlab::ExclusiveLease
+      .new("#{self.class.name}:#{class_name}", timeout: MIN_INTERVAL)
+  end
+
+  def always_perform?
+    Rails.env.test?
   end
 end

changelogs/unreleased/delay-background-migrations.yml

+---
+title: Run background migrations with a minimum interval
+merge_request:
+author:
+type: changed

lib/gitlab/database/migration_helpers.rb

@@ -842,6 +842,12 @@ into similar problems in the future (e.g. when new tables are created).
       def queue_background_migration_jobs_by_range_at_intervals(model_class, job_class_name, delay_interval, batch_size: BACKGROUND_MIGRATION_BATCH_SIZE)
         raise "#{model_class} does not have an ID to use for batch ranges" unless model_class.column_names.include?('id')
 
+        # 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
+        end
+
         model_class.each_batch(of: batch_size) do |relation, index|
           start_id, end_id = relation.pluck('MIN(id), MAX(id)').first
 

lib/gitlab/exclusive_lease.rb

@@ -71,5 +71,16 @@ module Gitlab
         redis.exists(@redis_shared_state_key)
       end
     end
+
+    # Returns the TTL of the Redis key.
+    #
+    # This method will return `nil` if no TTL could be obtained.
+    def ttl
+      Gitlab::Redis::SharedState.with do |redis|
+        ttl = redis.ttl(@redis_shared_state_key)
+
+        ttl if ttl.positive?
+      end
+    end
   end
 end

spec/lib/gitlab/database/migration_helpers_spec.rb

@@ -1006,12 +1006,12 @@ describe Gitlab::Database::MigrationHelpers do
       context 'with batch_size option' do
         it 'queues jobs correctly' do
           Sidekiq::Testing.fake! do
-            model.queue_background_migration_jobs_by_range_at_intervals(User, 'FooJob', 10.seconds, batch_size: 2)
+            model.queue_background_migration_jobs_by_range_at_intervals(User, 'FooJob', 10.minutes, batch_size: 2)
 
             expect(BackgroundMigrationWorker.jobs[0]['args']).to eq(['FooJob', [id1, id2]])
-            expect(BackgroundMigrationWorker.jobs[0]['at']).to eq(10.seconds.from_now.to_f)
+            expect(BackgroundMigrationWorker.jobs[0]['at']).to eq(10.minutes.from_now.to_f)
             expect(BackgroundMigrationWorker.jobs[1]['args']).to eq(['FooJob', [id3, id3]])
-            expect(BackgroundMigrationWorker.jobs[1]['at']).to eq(20.seconds.from_now.to_f)
+            expect(BackgroundMigrationWorker.jobs[1]['at']).to eq(20.minutes.from_now.to_f)
           end
         end
       end
@@ -1019,10 +1019,10 @@ describe Gitlab::Database::MigrationHelpers do
       context 'without batch_size option' do
         it 'queues jobs correctly' do
           Sidekiq::Testing.fake! do
-            model.queue_background_migration_jobs_by_range_at_intervals(User, 'FooJob', 10.seconds)
+            model.queue_background_migration_jobs_by_range_at_intervals(User, 'FooJob', 10.minutes)
 
             expect(BackgroundMigrationWorker.jobs[0]['args']).to eq(['FooJob', [id1, id3]])
-            expect(BackgroundMigrationWorker.jobs[0]['at']).to eq(10.seconds.from_now.to_f)
+            expect(BackgroundMigrationWorker.jobs[0]['at']).to eq(10.minutes.from_now.to_f)
           end
         end
       end

spec/lib/gitlab/exclusive_lease_spec.rb

@@ -73,4 +73,19 @@ describe Gitlab::ExclusiveLease, :clean_gitlab_redis_shared_state do
       described_class.new(key, timeout: 3600).try_obtain
     end
   end
+
+  describe '#ttl' do
+    it 'returns the TTL of the Redis key' do
+      lease = described_class.new('kittens', timeout: 100)
+      lease.try_obtain
+
+      expect(lease.ttl <= 100).to eq(true)
+    end
+
+    it 'returns nil when the lease does not exist' do
+      lease = described_class.new('kittens', timeout: 10)
+
+      expect(lease.ttl).to be_nil
+    end
+  end
 end

spec/migrations/normalize_ldap_extern_uids_spec.rb

@@ -27,11 +27,11 @@ describe NormalizeLdapExternUids, :migration, :sidekiq do
         migrate!
 
         expect(BackgroundMigrationWorker.jobs[0]['args']).to eq([described_class::MIGRATION, [1, 2]])
-        expect(BackgroundMigrationWorker.jobs[0]['at']).to eq(10.seconds.from_now.to_f)
+        expect(BackgroundMigrationWorker.jobs[0]['at']).to eq(5.minutes.from_now.to_f)
         expect(BackgroundMigrationWorker.jobs[1]['args']).to eq([described_class::MIGRATION, [3, 4]])
-        expect(BackgroundMigrationWorker.jobs[1]['at']).to eq(20.seconds.from_now.to_f)
+        expect(BackgroundMigrationWorker.jobs[1]['at']).to eq(10.minutes.from_now.to_f)
         expect(BackgroundMigrationWorker.jobs[2]['args']).to eq([described_class::MIGRATION, [5, 5]])
-        expect(BackgroundMigrationWorker.jobs[2]['at']).to eq(30.seconds.from_now.to_f)
+        expect(BackgroundMigrationWorker.jobs[2]['at']).to eq(15.minutes.from_now.to_f)
         expect(BackgroundMigrationWorker.jobs.size).to eq 3
       end
     end

spec/workers/background_migration_worker_spec.rb

 require 'spec_helper'
 
-describe BackgroundMigrationWorker, :sidekiq do
+describe BackgroundMigrationWorker, :sidekiq, :clean_gitlab_redis_shared_state do
+  let(:worker) { described_class.new }
+
   describe '.perform' do
     it 'performs a background migration' do
       expect(Gitlab::BackgroundMigration)
         .to receive(:perform)
         .with('Foo', [10, 20])
 
-      described_class.new.perform('Foo', [10, 20])
+      worker.perform('Foo', [10, 20])
+    end
+
+    it 'reschedules a migration if it was performed recently' do
+      expect(worker)
+        .to receive(:always_perform?)
+        .and_return(false)
+
+      worker.lease_for('Foo').try_obtain
+
+      expect(Gitlab::BackgroundMigration)
+        .not_to receive(:perform)
+
+      expect(described_class)
+        .to receive(:perform_in)
+        .with(a_kind_of(Numeric), 'Foo', [10, 20])
+
+      worker.perform('Foo', [10, 20])
     end
   end
 end