Allow caching feature flags for a given duration (#14668)

This duration is specified in the `CACHE_FEATURE_FLAGS_SECONDS`
environment variable. It can't be updated without restarting the
application.

Gemfile

@@ -36,6 +36,7 @@ gem "feedjira", "~> 3.2" # A feed fetching and parsing library
 gem "field_test", "~> 0.4" # A/B testing
 gem "flipper", "~> 0.22.1" # Feature flipping / flags for Ruby
 gem "flipper-active_record", "~> 0.22.1" # Store Flipper flags in ActiveRecord
+gem "flipper-active_support_cache_store", "~> 0.22.1" # Cache feature flags for a short time
 gem "flipper-ui", "~> 0.22.1"
 gem "fog-aws", "~> 3.12" # 'fog' gem to support Amazon Web Services
 gem "front_matter_parser", "~> 1.0" # Parse a front matter from syntactically correct strings or files

Gemfile.lock

@@ -278,6 +278,9 @@ GEM
     flipper-active_record (0.22.1)
       activerecord (>= 4.2, < 7)
       flipper (~> 0.22.1)
+    flipper-active_support_cache_store (0.22.1)
+      activesupport (>= 4.2, < 7)
+      flipper (~> 0.22.1)
     flipper-ui (0.22.1)
       erubi (>= 1.0.0, < 2.0.0)
       flipper (~> 0.22.1)
@@ -879,6 +882,7 @@ DEPENDENCIES
   field_test (~> 0.4)
   flipper (~> 0.22.1)
   flipper-active_record (~> 0.22.1)
+  flipper-active_support_cache_store (~> 0.22.1)
   flipper-ui (~> 0.22.1)
   fog-aws (~> 3.12)
   front_matter_parser (~> 1.0)

config/initializers/flipper.rb

@@ -1,7 +1,13 @@
 Flipper.configure do |config|
   config.default do
-    # pick an adapter, this uses memory, any will do
-    adapter = Flipper::Adapters::ActiveRecord.new
+    adapter = if (cache_duration = ENV["CACHE_FEATURE_FLAGS_SECONDS"])
+                Flipper::Adapters::ActiveSupportCacheStore.new(
+                  Flipper::Adapters::ActiveRecord.new,
+                  ActiveSupport::Cache::MemoryStore.new(expires_in: cache_duration.to_i.seconds),
+                )
+              else
+                Flipper::Adapters::ActiveRecord.new
+              end
 
     # pass adapter to handy DSL instance
     Flipper.new(adapter)