Update export workers to read from replica if available

Changelog: changed

 - Group/Project export workers do a lot of reads from
   the database in order to perform the export with
   a handful of writes
 - Because of this reason, read from replicas to reduce
   the load of primary database

config/feature_flags/development/load_balancing_for_export_workers.yml

+---
+name: load_balancing_for_export_workers
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/68153
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/338615
+milestone: '14.2'
+type: development
+group: group::import
+default_enabled: false

lib/gitlab/import_export/json/streaming_serializer.rb

@@ -31,10 +31,12 @@ module Gitlab
         end
 
         def execute
-          serialize_root
+          read_from_replica_if_available do
+            serialize_root
 
-          includes.each do |relation_definition|
-            serialize_relation(relation_definition)
+            includes.each do |relation_definition|
+              serialize_relation(relation_definition)
+            end
           end
         end
 
@@ -166,6 +168,13 @@ module Gitlab
             )
           ])
         end
+
+        def read_from_replica_if_available(&block)
+          return yield unless ::Feature.enabled?(:load_balancing_for_export_workers, type: :development, default_enabled: :yaml)
+          return yield unless ::Gitlab::Database::LoadBalancing.enable?
+
+          ::Gitlab::Database::LoadBalancing::Session.current.use_replicas_for_read_queries(&block)
+        end
       end
     end
   end

spec/lib/gitlab/import_export/json/streaming_serializer_spec.rb

@@ -156,6 +156,41 @@ RSpec.describe Gitlab::ImportExport::Json::StreamingSerializer do
         subject.execute
       end
     end
+
+    describe 'load balancing' do
+      context 'when feature flag load_balancing_for_export_workers is enabled' do
+        before do
+          stub_feature_flags(load_balancing_for_export_workers: true)
+        end
+
+        context 'when enabled', :db_load_balancing do
+          it 'reads from replica' do
+            expect(Gitlab::Database::LoadBalancing::Session.current).to receive(:use_replicas_for_read_queries).and_call_original
+
+            subject.execute
+          end
+        end
+
+        context 'when disabled' do
+          it 'reads from primary' do
+            allow(Gitlab::Database::LoadBalancing).to receive(:enable?).and_return(false)
+            expect(Gitlab::Database::LoadBalancing::Session.current).not_to receive(:use_replicas_for_read_queries)
+
+            subject.execute
+          end
+        end
+      end
+
+      context 'when feature flag load_balancing_for_export_workers is disabled' do
+        it 'reads from primary' do
+          stub_feature_flags(load_balancing_for_export_workers: false)
+
+          expect(Gitlab::Database::LoadBalancing::Session.current).not_to receive(:use_replicas_for_read_queries)
+
+          subject.execute
+        end
+      end
+    end
   end
 
   describe '.batch_size' do