1 files changed, 100 insertions, 0 deletions
diff --git a/spec/initializers/load_balancing_spec.rb b/spec/initializers/load_balancing_spec.rb
new file mode 100644
index 00000000000..d9162acd2cd
--- /dev/null
+++ b/spec/initializers/load_balancing_spec.rb
@@ -0,0 +1,100 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'load_balancing', :delete, :reestablished_active_record_base do
+  subject(:initialize_load_balancer) do
+    load Rails.root.join('config/initializers/load_balancing.rb')
+  end
+
+  before do
+    # Stub out middleware call, as not idempotent
+    allow(Gitlab::Application.instance.middleware).to receive(:use)
+  end
+
+  context 'with replica hosts configured' do
+    before do
+      # Setup host-based load balancing
+      # Patch in our load balancer config, simply pointing at the test database twice
+      allow(Gitlab::Database::LoadBalancing::Configuration).to receive(:for_model) do |base_model|
+        db_host = base_model.connection_pool.db_config.host
+
+        Gitlab::Database::LoadBalancing::Configuration.new(base_model, [db_host, db_host])
+      end
+    end
+
+    after do
+      # reset load balancing to original state
+      allow(Gitlab::Database::LoadBalancing::Configuration).to receive(:for_model).and_call_original
+      allow(Gitlab::Cluster::LifecycleEvents).to receive(:in_clustered_puma?).and_call_original
+
+      load Rails.root.join('config/initializers/load_balancing.rb')
+    end
+
+    it 'configures load balancer with two replica hosts' do
+      expect(ApplicationRecord.connection.load_balancer.configuration.hosts.size).to eq(0)
+      expect(Ci::ApplicationRecord.connection.load_balancer.configuration.hosts.size).to eq(0)
+
+      initialize_load_balancer
+
+      expect(ApplicationRecord.connection.load_balancer.configuration.hosts.size).to eq(2)
+      expect(Ci::ApplicationRecord.connection.load_balancer.configuration.hosts.size).to eq(2)
+    end
+
+    context 'for a clustered puma worker' do
+      let!(:group) { create(:group, name: 'my group') }
+
+      before do
+        # Pretend we are in clustered environment
+        allow(Gitlab::Cluster::LifecycleEvents).to receive(:in_clustered_puma?).and_return(true)
+      end
+
+      it 'configures load balancer to have two replica hosts' do
+        initialize_load_balancer
+
+        simulate_puma_worker do
+          expect(ApplicationRecord.connection.load_balancer.configuration.hosts.size).to eq(2)
+          expect(Ci::ApplicationRecord.connection.load_balancer.configuration.hosts.size).to eq(2)
+        end
+      end
+
+      # We tried using Process.fork for a more realistic simulation
+      # but run into bugs where GPRC cannot be used before forking processes.
+      # See https://gitlab.com/gitlab-org/gitlab/-/issues/333184#note_1081658113
+      def simulate_puma_worker
+        # Called in https://github.com/rails/rails/blob/6-1-stable/activerecord/lib/active_record/connection_adapters/pool_config.rb#L73
+        ActiveRecord::ConnectionAdapters::PoolConfig.discard_pools!
+
+        # Called in config/puma.rb
+        Gitlab::Cluster::LifecycleEvents.do_worker_start
+
+        yield
+      end
+
+      it 'makes a read query successfully' do
+        # Clear any previous sticky writes
+        ::Gitlab::Database::LoadBalancing::Session.clear_session
+
+        initialize_load_balancer
+
+        group_name = simulate_puma_worker do
+          Group.find_by_name('my group').name
+        end
+
+        expect(group_name).to eq(group.name)
+      end
+
+      it 'makes a write query successfully' do
+        initialize_load_balancer
+
+        expect do
+          simulate_puma_worker do
+            Group.touch_all
+          end
+
+          group.reload
+        end.to change(group, :updated_at)
+      end
+    end
+  end
+end