PERF: stop querying banner topic on every page hit

app/controllers/application_controller.rb

@@ -7,6 +7,7 @@ require_dependency 'rate_limiter'
 require_dependency 'crawler_detection'
 require_dependency 'json_error'
 require_dependency 'letter_avatar'
+require_dependency 'distributed_cache'
 
 class ApplicationController < ActionController::Base
   include CurrentUser
@@ -271,11 +272,21 @@ class ApplicationController < ActionController::Base
       MultiJson.dump(data)
     end
 
-    def banner_json
-      topic = Topic.where(archetype: Archetype.banner).limit(1).first
-      banner = topic.present? ? topic.banner : {}
+    def self.banner_json_cache
+      @banner_json_cache ||= DistributedCache.new("banner_json")
+    end
 
-      MultiJson.dump(banner)
+    def banner_json
+
+      json = ApplicationController.banner_json_cache["json"]
+
+      unless json
+        topic = Topic.where(archetype: Archetype.banner).limit(1).first
+        banner = topic.present? ? topic.banner : {}
+        ApplicationController.banner_json_cache["json"] = json = MultiJson.dump(banner)
+      end
+
+      json
     end
 
     def render_json_error(obj)

app/models/topic.rb

@@ -172,6 +172,12 @@ class Topic < ActiveRecord::Base
     unless skip_callbacks
       schedule_auto_close_job
     end
+
+    banner = "banner".freeze
+
+    if archetype_was == banner || archetype == banner
+      ApplicationController.banner_json_cache.clear
+    end
   end
 
   def initialize_default_values