> Redis (and memcache) are memory caches and should be treated like that

If you haven't come across Kvrocks yet, it may be worth a look: https://github.com/apache/kvrocks https://kvrocks.apache.org/ . It's a database with a Redis-compatible wire protocol, but the database is stored on disk. This means your working set is not limited by RAM and can be a few orders of magnitude larger! On modern SSDs this is still very fast. I think it improves the durability story as well. But the big win is the orders of magnitude larger database space.

As I've been improving my side project https://totalrealreturns.com/ recently I've ended up using both Redis and Kvrocks together. Redis is great for small global state that needs to be super fast. Kvrocks is great for larger bulk data storage (large precomputed datasets), but also supports a lot of the Redis data structures as well as Lua scripts.

Redis is used for plenty of things, not just memory caches.

For example if you use it for session storage, you can't have your application read from a random instance that may or may not contain the session.

Redis have many use cases, and acting as a cache is only one of them. One very common usage is as a backend for background worker jobs. That can need HA.

For the project I've been working on for more than 15 years, we make extensive use of the pub/sub functionality for distributing live data. Pub/sub scales well across the cluster. Publish to one, and it goes out to subscribers on any of the nodes that they've connected to.

Will millions of users, high availability is critical for this functionality.

Redis doesn't necessarily have to be used as a cache. Streams, for example, make it a great message queue; but a single-node message queue is a single point of failure and thus not viable for many setups.

Years ago I enabled durability on redis & used it as database for an online card game