[lmm]

Honestly all the popular datastores are fine, though they all have their own foibles. Redis is fine if you just want a basic key-value store that's not HA (and not particularly durable). Cassandra is fine if you want HA (Riak had a better design, but is so much less popular that I probably wouldn't use it these days). Kafka is really good but a much bigger conceptual leap. Even MongoDB is pretty much fine - there are plenty of horror stories about data loss in earlier versions, but it pretty much works most of the time, which is the best you can ever really say about any datastore IME (even PostgreSQL).