1 files changed, 66 insertions, 0 deletions
diff --git a/lib/gitlab/etag_caching/middleware.rb b/lib/gitlab/etag_caching/middleware.rb
new file mode 100644
index 00000000000..0f24f9bbfde
--- /dev/null
+++ b/lib/gitlab/etag_caching/middleware.rb
@@ -0,0 +1,66 @@
+module Gitlab
+  module EtagCaching
+    class Middleware
+      RESERVED_WORDS = ProjectPathValidator::RESERVED.map { |word| "/#{word}/" }.join('|')
+      ROUTE_REGEXP = Regexp.union(
+        %r(^(?!.*(#{RESERVED_WORDS})).*/noteable/issue/\d+/notes\z)
+      )
+
+      def initialize(app)
+        @app = app
+      end
+
+      def call(env)
+        return @app.call(env) unless enabled_for_current_route?(env)
+        Gitlab::Metrics.add_event(:etag_caching_middleware_used)
+
+        etag, cached_value_present = get_etag(env)
+        if_none_match = env['HTTP_IF_NONE_MATCH']
+
+        if if_none_match == etag
+          Gitlab::Metrics.add_event(:etag_caching_cache_hit)
+          [304, { 'ETag' => etag }, ['']]
+        else
+          track_cache_miss(if_none_match, cached_value_present)
+
+          status, headers, body = @app.call(env)
+          headers['ETag'] = etag
+          [status, headers, body]
+        end
+      end
+
+      private
+
+      def enabled_for_current_route?(env)
+        ROUTE_REGEXP.match(env['PATH_INFO'])
+      end
+
+      def get_etag(env)
+        cache_key = env['PATH_INFO']
+        store = Store.new
+        current_value = store.get(cache_key)
+        cached_value_present = current_value.present?
+
+        unless cached_value_present
+          current_value = store.touch(cache_key, only_if_missing: true)
+        end
+
+        [weak_etag_format(current_value), cached_value_present]
+      end
+
+      def weak_etag_format(value)
+        %Q{W/"#{value}"}
+      end
+
+      def track_cache_miss(if_none_match, cached_value_present)
+        if if_none_match.blank?
+          Gitlab::Metrics.add_event(:etag_caching_header_missing)
+        elsif !cached_value_present
+          Gitlab::Metrics.add_event(:etag_caching_key_not_found)
+        else
+          Gitlab::Metrics.add_event(:etag_caching_resource_changed)
+        end
+      end
+    end
+  end
+end