Throttle the number of UPDATEs triggered by touch

This throttles the number of UPDATE queries that can be triggered by
calling "touch" on a Note, Issue, or MergeRequest. For Note objects we
also take care of updating the associated "noteable" relation in a
smarter way than Rails does by default.

1 files changed, 35 insertions, 1 deletions

diff --git a/app/models/note.rb b/app/models/note.rb
index 340fe087f82..733bbbc013f 100644
--- a/app/models/note.rb
+++ b/app/models/note.rb
@@ -15,6 +15,7 @@ class Note < ActiveRecord::Base
   include IgnorableColumn
   include Editable
   include Gitlab::SQL::Pattern
+  include ThrottledTouch
 
   module SpecialRole
     FIRST_TIME_CONTRIBUTOR = :first_time_contributor
@@ -55,7 +56,7 @@ class Note < ActiveRecord::Base
   participant :author
 
   belongs_to :project
-  belongs_to :noteable, polymorphic: true, touch: true # rubocop:disable Cop/PolymorphicAssociations
+  belongs_to :noteable, polymorphic: true # rubocop:disable Cop/PolymorphicAssociations
   belongs_to :author, class_name: "User"
   belongs_to :updated_by, class_name: "User"
   belongs_to :last_edited_by, class_name: 'User'
@@ -118,6 +119,7 @@ class Note < ActiveRecord::Base
   before_validation :set_discussion_id, on: :create
   after_save :keep_around_commit, if: :for_project_noteable?
   after_save :expire_etag_cache
+  after_save :touch_noteable
   after_destroy :expire_etag_cache
 
   class << self
@@ -367,6 +369,38 @@ class Note < ActiveRecord::Base
     Gitlab::EtagCaching::Store.new.touch(key)
   end
 
+  def touch(*args)
+    # We're not using an explicit transaction here because this would in all
+    # cases result in all future queries going to the primary, even if no writes
+    # are performed.
+    #
+    # We touch the noteable first so its SELECT query can run before our writes,
+    # ensuring it runs on a secondary (if no prior write took place).
+    touch_noteable
+    super
+  end
+
+  # By default Rails will issue an "SELECT *" for the relation, which is
+  # overkill for just updating the timestamps. To work around this we manually
+  # touch the data so we can SELECT only the columns we need.
+  def touch_noteable
+    # Commits are not stored in the DB so we can't touch them.
+    return if for_commit?
+
+    assoc = association(:noteable)
+
+    noteable_object =
+      if assoc.loaded?
+        noteable
+      else
+        # If the object is not loaded (e.g. when notes are loaded async) we
+        # _only_ want the data we actually need.
+        assoc.scope.select(:id, :updated_at).take
+      end
+
+    noteable_object&.touch
+  end
+
   private
 
   def keep_around_commit