If you're already built around Redis I'd just keep using it, but if you're doing new development there's not so much a single drop in replacement as a substantially better alternative for any given feature (and not particularly any advantage to having all your data in the "same Redis instance"). That said, 90+% of the time the answer is probably "transactional SQL database" or "message queue"

For Kafka, the answer is probably an object store, a message queue, a specialized logging system, an ordinary transactional database table, or whatever mechanism your chosen analytics DB uses for bulk input (probably S3 or equivalent these days). Or maybe just a REST interface in front of a filesystem. Unless of course you truly need to interface with a Kafka consumer/producer in which case you’re stuck with it (the actual reason I've seen for every Kafka deployment I've personally witnessed in recent history)

> What's the better kafka/redis?

If you are going to leverage caching I’d use the OSS Valkey over Redis. Based on the company’s past behavior, Redis is dead to me now.