Cache Flipper feature flags in L1 and L2 caches

In https://gitlab.com/gitlab-com/gl-infra/production/issues/928, we saw
a significant amount of network traffic and CPU usage due to Redis
checking feature flags via Flipper. Since these flags are hit with every
request, the overhead becomes significant. To alleviate Redis overhead,
we now cache the data in the following way:

* L1: A thread-local memory store for 1 minute
* L2: Redis for 1 hour

1 files changed, 20 insertions, 3 deletions

diff --git a/lib/feature.rb b/lib/feature.rb
index 22420e95ea2..e28333aa58e 100644
--- a/lib/feature.rb
+++ b/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,
+          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(
-        active_record_adapter,
-        Rails.cache,
-        expires_in: 1.hour)
+        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