You need one central Redis. Redis isn’t (just) used as a cache for Discourse; it also acts as a message broker for all the MessageBus traffic, so if you had one Redis per node, things would get… weird. And broken.
You need one central Redis. Redis isn’t (just) used as a cache for Discourse; it also acts as a message broker for all the MessageBus traffic, so if you had one Redis per node, things would get… weird. And broken.