Object databases
Object databases store data not in terms of relations and columns and rows, but in terms of objects as understood from the discipline of object-oriented programming. This makes it straightforward to use these databases from object-oriented applications. Object databases such as db4o and InterSystems Cache allow you to avoid techniques like stored procedures and object-relational mapping (ORM) tools. The most widely used object database is Amazon Web Services’ Simple Storage Service (S3).
XML databases
XML databases are a special form of document databases, optimized specifically for working with data described in the eXtensible Markup Language (XML). So called “XML native” databases include BaseX and eXist.
Multimodel databases
Databases that support more than one of these styles have been growing in popularity. These “multimodel” databases are based on a primary underlying database (most often a relational, key-value, or column store) and expose additional models as APIs on top of that underlying database. Examples of these include Microsoft Azure Cosmos DB, which exposes document, wide column, and graph APIs on top of a key-value store, and DataStax Enterprise, which offers a graph API on top of Cassandra’s wide column model. Multimodel databases are often touted for their ability to support an approach known as polyglot persistence, in which different microservices or components of an application can interact with data using more than one of the models we’ve described here