Check WAL locations before sleeping

This saves us from the unnecessary sleep if the replica is already up to
date.

Based on previous tests, 50% of the time, the replica is already up to
date so we don't need the sleep.

This also reduces the sleep time because we did not see any stale
replica errors with the previous value. This suggests that the delay may
be too high.

lib/gitlab/database/load_balancing/sidekiq_server_middleware.rb

@@ -6,7 +6,7 @@ module Gitlab
       class SidekiqServerMiddleware
         JobReplicaNotUpToDate = Class.new(StandardError)
 
-        MINIMUM_DELAY_INTERVAL = 1
+        MINIMUM_DELAY_INTERVAL_SECONDS = 0.8
 
         def call(worker, job, _queue)
           worker_class = worker.class
@@ -46,11 +46,13 @@ module Gitlab
 
           return :primary_no_wal if wal_locations.blank?
 
+          # Happy case: we can read from a replica.
+          return replica_strategy(worker_class, job) if databases_in_sync?(wal_locations)
+
           sleep_if_needed(job)
 
           if databases_in_sync?(wal_locations)
-            # Happy case: we can read from a replica.
-            retried_before?(worker_class, job) ? :replica_retried : :replica
+            replica_strategy(worker_class, job)
           elsif can_retry?(worker_class, job)
             # Optimistic case: The worker allows retries and we have retries left.
             :retry
@@ -61,9 +63,9 @@ module Gitlab
         end
 
         def sleep_if_needed(job)
-          remaining_delay = MINIMUM_DELAY_INTERVAL - (Time.current.to_f - job['created_at'].to_f)
+          remaining_delay = MINIMUM_DELAY_INTERVAL_SECONDS - (Time.current.to_f - job['created_at'].to_f)
 
-          sleep remaining_delay if remaining_delay > 0 && remaining_delay < MINIMUM_DELAY_INTERVAL
+          sleep remaining_delay if remaining_delay > 0 && remaining_delay < MINIMUM_DELAY_INTERVAL_SECONDS
         end
 
         def get_wal_locations(job)
@@ -80,6 +82,10 @@ module Gitlab
           worker_class.get_data_consistency == :delayed && not_yet_retried?(job)
         end
 
+        def replica_strategy(worker_class, job)
+          retried_before?(worker_class, job) ? :replica_retried : :replica
+        end
+
         def retried_before?(worker_class, job)
           worker_class.get_data_consistency == :delayed && !not_yet_retried?(job)
         end

spec/lib/gitlab/database/load_balancing/sidekiq_server_middleware_spec.rb

@@ -122,7 +122,7 @@ RSpec.describe Gitlab::Database::LoadBalancing::SidekiqServerMiddleware, :clean_
 
     shared_examples_for 'sleeps when necessary' do
       context 'when WAL locations are blank', :freeze_time do
-        let(:job) { { "retry" => 3, "job_id" => "a180b47c-3fd6-41b8-81e9-34da61c3400e", "wal_locations" => {}, "created_at" => Time.current.to_f - (described_class::MINIMUM_DELAY_INTERVAL - 0.3) } }
+        let(:job) { { "retry" => 3, "job_id" => "a180b47c-3fd6-41b8-81e9-34da61c3400e", "wal_locations" => {}, "created_at" => Time.current.to_f - (described_class::MINIMUM_DELAY_INTERVAL_SECONDS - 0.3) } }
 
         it 'does not sleep' do
           expect(middleware).not_to receive(:sleep)
@@ -135,17 +135,39 @@ RSpec.describe Gitlab::Database::LoadBalancing::SidekiqServerMiddleware, :clean_
         let(:job) { { "retry" => 3, "job_id" => "a180b47c-3fd6-41b8-81e9-34da61c3400e", 'wal_locations' => wal_locations, "created_at" => Time.current.to_f - elapsed_time } }
 
         context 'when delay interval has not elapsed' do
-          let(:elapsed_time) { described_class::MINIMUM_DELAY_INTERVAL - 0.3 }
+          let(:elapsed_time) { described_class::MINIMUM_DELAY_INTERVAL_SECONDS - 0.3 }
+
+          context 'when replica is up to date' do
+            before do
+              Gitlab::Database::LoadBalancing.each_load_balancer do |lb|
+                allow(lb).to receive(:select_up_to_date_host).and_return(true)
+              end
+            end
+
+            it 'does not sleep' do
+              expect(middleware).not_to receive(:sleep)
+
+              run_middleware
+            end
+          end
+
+          context 'when replica is not up to date' do
+            before do
+              Gitlab::Database::LoadBalancing.each_load_balancer do |lb|
+                allow(lb).to receive(:select_up_to_date_host).and_return(false, true)
+              end
+            end
 
             it 'sleeps until the minimum delay is reached' do
-            expect(middleware).to receive(:sleep).with(be_within(0.01).of(described_class::MINIMUM_DELAY_INTERVAL - elapsed_time))
+              expect(middleware).to receive(:sleep).with(be_within(0.01).of(described_class::MINIMUM_DELAY_INTERVAL_SECONDS - elapsed_time))
 
               run_middleware
             end
           end
+        end
 
         context 'when delay interval has elapsed' do
-          let(:elapsed_time) { described_class::MINIMUM_DELAY_INTERVAL + 0.3 }
+          let(:elapsed_time) { described_class::MINIMUM_DELAY_INTERVAL_SECONDS + 0.3 }
 
           it 'does not sleep' do
             expect(middleware).not_to receive(:sleep)
@@ -179,7 +201,7 @@ RSpec.describe Gitlab::Database::LoadBalancing::SidekiqServerMiddleware, :clean_
 
       context 'when delay interval has not elapsed', :freeze_time do
         let(:job) { { "retry" => 3, "job_id" => "a180b47c-3fd6-41b8-81e9-34da61c3400e", 'wal_locations' => wal_locations, "created_at" => Time.current.to_f - elapsed_time } }
-        let(:elapsed_time) { described_class::MINIMUM_DELAY_INTERVAL - 0.3 }
+        let(:elapsed_time) { described_class::MINIMUM_DELAY_INTERVAL_SECONDS - 0.3 }
 
         it 'does not sleep' do
           expect(middleware).not_to receive(:sleep)