When an editor leaves a note on another editor's edit, we set a "edit_notes_received_last_updated:$edit_author" key in our Redis store. When the edit's author next loads a page on the website, we read this key to check if any new notes have been left since they last viewed the /edit/notes-received page.

The problem is, our production & beta servers use separate Redis stores, so the edit author and edit note leaver have to be using the same server for them to be reading from and setting the same Redis keys. Merging the stores is not a simple task, since login sessions are kept there, so an easier solution is probably to use the editor_preferences table.