1 files changed, 45 insertions, 0 deletions
diff --git a/activerecord/lib/active_record/middleware/database_selector/resolver/session.rb b/activerecord/lib/active_record/middleware/database_selector/resolver/session.rb
new file mode 100644
index 0000000000..df7af054b7
--- /dev/null
+++ b/activerecord/lib/active_record/middleware/database_selector/resolver/session.rb
@@ -0,0 +1,45 @@
+# frozen_string_literal: true
+
+module ActiveRecord
+  module Middleware
+    class DatabaseSelector
+      class Resolver
+        # The session class is used by the DatabaseSelector::Resolver to save
+        # timestamps of the last write in the session.
+        #
+        # The last_write is used to determine whether it's safe to read
+        # from the replica or the request needs to be sent to the primary.
+        class Session # :nodoc:
+          def self.call(request)
+            new(request.session)
+          end
+
+          # Converts time to a timestamp that represents milliseconds since
+          # epoch.
+          def self.convert_time_to_timestamp(time)
+            time.to_i * 1000 + time.usec / 1000
+          end
+
+          # Converts milliseconds since epoch timestamp into a time object.
+          def self.convert_timestamp_to_time(timestamp)
+            timestamp ? Time.at(timestamp / 1000, (timestamp % 1000) * 1000) : Time.at(0)
+          end
+
+          def initialize(session)
+            @session = session
+          end
+
+          attr_reader :session
+
+          def last_write_timestamp
+            self.class.convert_timestamp_to_time(session[:last_write])
+          end
+
+          def update_last_write_timestamp
+            session[:last_write] = self.class.convert_time_to_timestamp(Time.now)
+          end
+        end
+      end
+    end
+  end
+end