Key-value stores
In a key-value store, the data items are keys that have a set of attributes. All data relevant to a key is stored with the key; data is frequently duplicated. Popular key-value stores include Amazon’s Dynamo DB, Riak, and Voldemort. Additionally, many popular caching technologies act as key-value stores, including Oracle Coherence, Redis, and Memcached.
Column stores
In a column store, also known as a wide-column store or column-oriented store, data is stored by column rather than by row. For example, in a column store, all customer addresses might be stored together, allowing them to be retrieved in a single query. Popular column stores include Apache Hadoop’s HBase, Apache Kudu, and Apache Druid.
Document stores
The basic unit of storage in a document database is the complete document, often stored in a format such as JSON, XML, or YAML. Popular document stores include MongoDB, CouchDB, and several public cloud offerings.
Graph databases
Graph databases represent data as a graph—a network of nodes and edges that connect the nodes. Both nodes and edges can have properties. Because they give heightened importance to relationships, graph databases such as Neo4j, Janus‐Graph, and DataStax Graph have proven popular for building social networking and semantic web applications