I suspect:

- memcached if you don't need to persist the data

- Redis if you don't know whether you need to use Redis or FoundationDB

- FoundationDB if you learn that Redis doesn't do what you need

I don't mean this in any kind of a derogatory way but I suspect that if you need to ask then you probably don't need FDB.

The principle of keeping tech stacks boring and using well established components is less exciting as an Engineer but is usually the best choice.

FDB is for permanent data storage, Redis is for temporary data. It does not matter that you can configure Redis to persist data to disk because the performance most Redis use cases need makes it less usable that way.

If you need transactions and high availability, then use FDB. But if you also need low latency / high throughput, then you should consider RonDB

Disclaimer: i am involved with RonDB