CouchDB fits the bill. It's all about documents, persistence and defining indexes for range-queries. Keep in mind two things: 1. It's all on the disk. So, whil

CouchDB fits the bill. It's all about documents, persistence and defining indexes for range-queries. Keep in mind two things: 1. It's all on the disk. So, while its throughput is excellent (thousands or more requests per second), each individual request has a latency of ~10ms 2. You define your indexes beforehand (called 'views' in couch terminology), and then you can only make simple queries on them - like by a specific key or by a range of keys. It takes some learning. If you and your app are ok with both, go for Couch.