What is Sharding?

Sharding can be considered as a horizontal partitioning in which a large database is partitioned into smaller, faster, more easily managed parts called data shards.

The basic idea is as the size of database and number of transactions increase linearly, the response time to query the database increases exponentially. And maintaining large database is also costly as it requires high-end computers. Contrast to this data shards can be distributed across a number of less expensive servers.

Tell about MongoDB?

MongoDB is a document-oriented database, not a relational one. Non relational means it does not store data in tables but in the form of JSON document.  Here in MongoDB row is replaced with document and table is replaced with collection. Collection can be considered as a group of documents.

Document is the basic unit of data for MongoDB, roughly equivalent to a row. It is a data structure composed of field and value pairs. MongoDB documents are similar to JSON objects. The values maybe documents, arrays, and arrays of documents.

Tell about RethinkDb ?

RethinkDb is a NoSQL, distributed document based database. It is free and open sourced, we write queries in ReQL query language. The interface also looks amazing, and we can view our results in a tree , a table, or a JSONView. RethinkDb is fast, its very scalable and easy to administer as it ships with a web based interface that comes online immediately. Using this we can do many things, from creating databases and tables to sharding, or replicating a selected table.

RethinkDb is considered as MongoDb done right. RethinkDb uses all the best features of MongoDb and corrects the things gone wrong. RethinkDb stands between Riak,Cassandra and CouchDb, MongoDb. RethinkDb supports "joins" which is not there in many document based databases.