Azure Cosmos DB is the globally distributed, multi-model database service for building planet scale apps. You can easily build globally-distributed applications without the hassle of complex, multiple-datacenter configurations. Cosmos DB automatically replicates all of your data to any number of regions of your choice, for fast, responsive access. Cosmos DB guarantees 99.99% high availability.
Only Cosmos DB allows you to use key-value, graph, and document data in one service.
One of the APIs Azure Cosmos DB supports is the SQL (DocumentDB) API.
https://azure.microsoft.com/en-in/blog/dear-documentdb-customers-welcome-to-azure-cosmos-db/ is a good link to read further.