is an open-source, document-oriented database, where data is stored in the form of documents or key-value pairs using the unstructured flexible schema. It allows various forms of queries on the database, such as search by key, range queries or regular expression based search. MongoDB offers redundancy and high availability by means of replication and automatic failover. For horizontal scaling, MongoDB divides and stores data across multiple servers (so-called "sharding"), allowing high throughput operations over large data sets. MongoDB's storage engine provides multi-granular locking (global, database, collection, document-level) and optimistic con currency control to support multiple clients efficiently.