When should you use a CDN versus just Redis caching?

hey everyone, im wondering how you all handle lots of reads. we have tons of static assets and some api data that gets accessed a lot, and its really getting hammered. right now we use redis for caching a good part of the api data, but we're thinking maybe we should add a cdn too. what do you think about just scaling redis more (like with sharding or replication) versus adding a cdn layer? are there certain traffic patterns or kinds of data that make one choice better than the other? cant wait to hear what youve done!