You can add it in your DNS settings of your domain.
Using redis as a cache
I run a forum with currently 4 nodebb processes which are proxied to from nginx, with a mongodb database backend and redis enabled (which from my understanding is used for sessions only in this configuration).
In this configuration every nodebb process eats a fair amount of memory (to note it was reduced significantly with one of the recent versions. 1.15.x I think - I did see some cache related changes).
Please correct me if I'm wrong, but is each nodebb process in this configuration handling its own post cache etc?
It would seem like a good idea to be able to use redis as a cache backend on top of mongodb - but perhaps I am way off how it works. My knowledge of the codebase is very limited.
[Edit] I forgot to say, thanks very much for your continued work on this excellent forum software. Is it possible to make a donation as I've been running it for 4 years for our project and am not on a paid plan etc.
In your setup redis will be used for sessions, pubsub and socket.io will use it to pass around messages.
Each nodejs process has it's own post cache, you can see the caches at /admin/advanced/cache.
Redis pubsub is also used to clear cache on the other nodejs process when it is cleared on one nodejs process. For example when a post is edited it's cache will be cleared on the other nodes.