Key-value databases

The key-value store is the simplest database store. In this database model, the storage, as its name suggests, is based on maps or hash tables. Some key-value databases allow complex values to be stored as lists and hash tables. Key-value pairs are extremely fast for certain scenarios, but lack the support for complex queries and aggregation. Some of the existing open source key-value databases are Riak, Redis, Memebase, and MemcacheDB.