NoSQL databases (also known as “not only SQL”) are databases that do not store data in traditional row-column format. These databases can store huge amounts of data easily and have flexible schemas.

But what was the need to have NoSQL databases?

NoSQL databases gained momentum in the 2000s when the size of the data all around the world started increasing at an exponential level. Not only the size but its type too. Data started arriving in all kinds of formats, PDF, audio files, video files, emails, text messages and so on. It became a task to store and process this data in relational databases. Hence, many NoSQL databases were introduced like MongoDB, HBase, Cassandra, Neo4j, Redis and so on.

What Are the Key Types of NoSQL Databases?

Each of the above databases is stored and processed differently from each other. In other words, NoSQL databases can be distinguished into these types:

1. Document Databases: These databases store data in documents like JSON files. The document is a collection of key-value pairs (equivalent to a row in relational databases). An example of a document database can be MongoDB.
2. Key-Value Databases: These databases are the simplest ones as it has the data stored in key-value format. You can use this database when you need to store huge amounts of data but do not want to perform complex operations for its retrieval. Redis and Riak are examples of key-value databases.
3. Columnar Databases: The data is stored in column-fashion as opposed to row-fashion in traditional databases. HBase is an example of a columnar NoSQL database. It has rows and each row does not maintain the same number or types of columns as the previous ones. It has a flexible schema. Some other examples of columnar databases are Cassandra and Amazon’s Redshift.
4. Graph Databases: These databases have data stored in the form of nodes and edges of a graph. Nodes contain the data and edges show the relationship between the data stored in the nodes. An example of this type of database would be Neo4j.

In theoretical computer science, there is a theorem called CAP theorem which states that in a distributed data store, only two of the following can be guaranteed:

• Consistency
• Availability
• Partition Tolerance

The Future of NoSQL Databases

Many of the NoSQL databases compromise consistency in favour of availability and partition tolerance. Most of these databases also lack true ACID properties (Atomicity, Consistency, Isolation, and Durability) transactions.

These NoSQL databases are increasingly used in Big Data and real-time web applications.

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