From 63ddb2358526a7d051ff20e76d20ba1bdf1c57f5 Mon Sep 17 00:00:00 2001
From: Kevin McConnell <kevin@37signals.com>
Date: Mon, 16 Jun 2025 15:03:45 +0100
Subject: [PATCH] Opt out of writer affinity on readings controller

---
 app/controllers/application_controller.rb          |  2 +-
 app/controllers/cards/readings_controller.rb       |  2 ++
 app/controllers/concerns/writer_affinity.rb        | 14 ++++++++++++++
 test/controllers/cards/readings_controller_test.rb |  6 ++++++
 4 files changed, 23 insertions(+), 1 deletion(-)
 create mode 100644 app/controllers/concerns/writer_affinity.rb

diff --git a/app/controllers/application_controller.rb b/app/controllers/application_controller.rb
index 20385eda0..af52c2fd6 100644
--- a/app/controllers/application_controller.rb
+++ b/app/controllers/application_controller.rb
@@ -1,5 +1,5 @@
 class ApplicationController < ActionController::Base
-  include Authentication, CurrentTimezone, SetPlatform
+  include Authentication, CurrentTimezone, SetPlatform, WriterAffinity
 
   stale_when_importmap_changes
   allow_browser versions: :modern
diff --git a/app/controllers/cards/readings_controller.rb b/app/controllers/cards/readings_controller.rb
index 2334cb8f5..a92c64525 100644
--- a/app/controllers/cards/readings_controller.rb
+++ b/app/controllers/cards/readings_controller.rb
@@ -1,6 +1,8 @@
 class Cards::ReadingsController < ApplicationController
   include CardScoped
 
+  skip_writer_affinity
+
   def create
     @notifications = @card.read_by(Current.user)
   end
diff --git a/app/controllers/concerns/writer_affinity.rb b/app/controllers/concerns/writer_affinity.rb
new file mode 100644
index 000000000..da03e1a42
--- /dev/null
+++ b/app/controllers/concerns/writer_affinity.rb
@@ -0,0 +1,14 @@
+module WriterAffinity
+  extend ActiveSupport::Concern
+
+  class_methods do
+    def skip_writer_affinity(**)
+      before_action :set_writer_affinity_opt_out_header, **
+    end
+  end
+
+  private
+    def set_writer_affinity_opt_out_header
+      response.headers["X-Writer-Affinity"] = "false"
+    end
+end
diff --git a/test/controllers/cards/readings_controller_test.rb b/test/controllers/cards/readings_controller_test.rb
index b6fa6d040..147dcfe4b 100644
--- a/test/controllers/cards/readings_controller_test.rb
+++ b/test/controllers/cards/readings_controller_test.rb
@@ -22,4 +22,10 @@ class Cards::ReadingsControllerTest < ActionDispatch::IntegrationTest
 
     assert_response :success
   end
+
+  test "create opts-out of writer affinity in the load balancer" do
+    post card_reading_path(cards(:logo)), as: :turbo_stream
+
+    assert_equal "false", response.headers["X-Writer-Affinity"]
+  end
 end