We have seen really good performance with frequent writes in Redis (~10k writes per second). The retrievals are also fast. I see no reason why it should not be your first choice. I know Redis doesn't have the image of a persistence database but it is capable of persistence and high availability. Moreover, MongoDB or some other database can be used as a cold storage. Even switching to MongoDB or any other NoSQL database entirely wouldn't be a big problem since Redis is a simple key-value store.