Merge branch 'sh-cache-flipper-checks-in-memory' into 'master'

Cache Flipper feature flags in L1 and L2 caches

See merge request gitlab-org/gitlab-ce!30276

changelogs/unreleased/sh-cache-flipper-checks-in-memory.yml

+---
+title: Cache Flipper feature flags in L1 and L2 caches
+merge_request: 30276
+author:
+type: performance

lib/feature.rb

@@ -103,10 +103,27 @@ class Feature
         feature_class: FlipperFeature,
         gate_class: FlipperGate)
 
+      # Redis L2 cache
+      redis_cache_adapter =
         Flipper::Adapters::ActiveSupportCacheStore.new(
           active_record_adapter,
-        Rails.cache,
+          l2_cache_backend,
           expires_in: 1.hour)
+
+      # Thread-local L1 cache: use a short timeout since we don't have a
+      # way to expire this cache all at once
+      Flipper::Adapters::ActiveSupportCacheStore.new(
+        redis_cache_adapter,
+        l1_cache_backend,
+        expires_in: 1.minute)
+    end
+
+    def l1_cache_backend
+      Gitlab::ThreadMemoryCache.cache_backend
+    end
+
+    def l2_cache_backend
+      Rails.cache
     end
   end
 

spec/lib/feature_spec.rb

@@ -144,6 +144,68 @@ describe Feature do
       expect(described_class.enabled?(:enabled_feature_flag)).to be_truthy
     end
 
+    it { expect(described_class.l1_cache_backend).to eq(Gitlab::ThreadMemoryCache.cache_backend) }
+    it { expect(described_class.l2_cache_backend).to eq(Rails.cache) }
+
+    it 'caches the status in L1 and L2 caches',
+       :request_store, :use_clean_rails_memory_store_caching do
+      described_class.enable(:enabled_feature_flag)
+      flipper_key = "flipper/v1/feature/enabled_feature_flag"
+
+      expect(described_class.l2_cache_backend)
+        .to receive(:fetch)
+        .once
+        .with(flipper_key, expires_in: 1.hour)
+        .and_call_original
+
+      expect(described_class.l1_cache_backend)
+        .to receive(:fetch)
+        .once
+        .with(flipper_key, expires_in: 1.minute)
+        .and_call_original
+
+      2.times do
+        expect(described_class.enabled?(:enabled_feature_flag)).to be_truthy
+      end
+    end
+
+    context 'cached feature flag', :request_store do
+      let(:flag) { :some_feature_flag }
+
+      before do
+        described_class.flipper.memoize = false
+        described_class.enabled?(flag)
+      end
+
+      it 'caches the status in L1 cache for the first minute' do
+        expect do
+          expect(described_class.l1_cache_backend).to receive(:fetch).once.and_call_original
+          expect(described_class.l2_cache_backend).not_to receive(:fetch)
+          expect(described_class.enabled?(flag)).to be_truthy
+        end.not_to exceed_query_limit(0)
+      end
+
+      it 'caches the status in L2 cache after 2 minutes' do
+        Timecop.travel 2.minutes do
+          expect do
+            expect(described_class.l1_cache_backend).to receive(:fetch).once.and_call_original
+            expect(described_class.l2_cache_backend).to receive(:fetch).once.and_call_original
+            expect(described_class.enabled?(flag)).to be_truthy
+          end.not_to exceed_query_limit(0)
+        end
+      end
+
+      it 'fetches the status after an hour' do
+        Timecop.travel 61.minutes do
+          expect do
+            expect(described_class.l1_cache_backend).to receive(:fetch).once.and_call_original
+            expect(described_class.l2_cache_backend).to receive(:fetch).once.and_call_original
+            expect(described_class.enabled?(flag)).to be_truthy
+          end.not_to exceed_query_limit(1)
+        end
+      end
+    end
+
     context 'with an individual actor' do
       CustomActor = Struct.new(:flipper_id)