Prevent pinning writes in non-primary DCs

config/deploy.production.yml

@@ -42,6 +42,7 @@ env:
       MYSQL_SOLID_CACHE_HOST: fizzy-solidcache-db-01.sc-chi-int.37signals.com
     df_iad:
       MYSQL_SOLID_CACHE_HOST: fizzy-solidcache-db-101.df-iad-int.37signals.com
+      PRIMARY_DATACENTER: true
     df_ams:
       MYSQL_SOLID_CACHE_HOST: fizzy-solidcache-db-401.df-ams-int.37signals.com
 

config/deploy.staging.yml

@@ -42,6 +42,7 @@ env:
       MYSQL_SOLID_CACHE_HOST: fizzy-staging-solidcache-db-01.sc-chi-int.37signals.com
     df_iad:
       MYSQL_SOLID_CACHE_HOST: fizzy-staging-solidcache-db-101.df-iad-int.37signals.com
+      PRIMARY_DATACENTER: true
     df_ams:
       MYSQL_SOLID_CACHE_HOST: fizzy-staging-solidcache-db-401.df-ams-int.37signals.com
 

config/initializers/multi_db.rb

@@ -1,5 +1,7 @@
+require "deployment"
+
 Rails.application.configure do
   config.active_record.database_selector = { delay: 1.second }
-  config.active_record.database_resolver = ActiveRecord::Middleware::DatabaseSelector::Resolver
+  config.active_record.database_resolver = Deployment::DatabaseResolver
   config.active_record.database_resolver_context = ActiveRecord::Middleware::DatabaseSelector::Resolver::Session
 end

lib/deployment.rb

@@ -0,0 +1 @@
+require_relative "deployment/database_resolver.rb"

lib/deployment/database_resolver.rb

@@ -0,0 +1,18 @@
+module Deployment
+  class DatabaseResolver < ActiveRecord::Middleware::DatabaseSelector::Resolver
+    def self.in_primary_datacenter?
+      ENV["PRIMARY_DATACENTER"].present? || Rails.env.local?
+    end
+
+    def reading_request?(request)
+      # Disables writes (and so primary-DB stickiness) in non-primary datacenters
+      super || !DatabaseResolver.in_primary_datacenter?
+    end
+
+    private
+      def read_from_primary?
+        # Only the primary datacenter can read from the primary database, non-primary DCs have local DB replicas
+        super && DatabaseResolver.in_primary_datacenter?
+      end
+  end
+end