Database Systems - Document Databases - NoSQL Overview - Appficial

A document database is a type of NoSQL database that stores and queries document data as JSON or XML. Document databases make it easy for developers to store and query data in a database by using the same document-model format they use in their application code.

Document databases, like other noSQL databases, support replication, horizontal scaling, sharding, and eventual consistency. However, document databases also support indexes on non-key values if the value is located in every documents of a collection. Documents are assigned to a shard using a shard key, which identifies the document identifier or another value. If the shard key is a value, then an index of shard key values is created to help locate database documents quickly. Documents are assigned to a shard with either a hash or range function.

A document database may support strict consistency, where all updates affect the primary replica first, and then make their way to the secondary replicas. All read queries read the primary replica, in order to get the most current document. In case it fails, the secondary replica takes over as the primary. Strict consistency is slower and not as available as eventual consistency. If the document database has eventual consistency, no replicated primary is specified. Replicas handle the updates, and make their way to the other replicas asynchronously. A read is just sent to any replica and may return something out of date.

Popular document databases include:

MongoDB - the most popular, and highest-ranked document database. Data is stored in binary JSON (BSON) format, which is optimized for speed and is not human-readable. MongoDB can be set up to be either strict or eventual consistency. MongoDB supports SQL read queries, but does not support SQL create, update, or delete statements.

CouchDB - an open source database by Apache. CouchDB was the very first document database, and it stores data in JSON format and only supports eventual but not strict consistency. CouchDB was initially crated for offline mobile device support.

Couchbase - open source, and derived from CouchDB which is a document database and Membase, which is a key-value database. Therefore, Couchbase supports both document and key-value models. Couchbase can be configured to both eventual and strict consistency.

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