A NoSQL (originally referring to "non SQL" or "non relational")[1] database provides a mechanism for storage and retrieval of data that is modeled in means other than the tabular relations used in relational databases. Such databases have existed since the late 1960s, but did not obtain the "NoSQL" moniker until a surge of popularity in the early 21st century,[2] triggered by the needs of Web 2.0 companies.[3][4] NoSQL databases are increasingly used in big data and real-time web applications.[5] NoSQL systems are also sometimes called "Not only SQL" to emphasize that they may support SQL-like query languages, or sit alongside SQL databases in polyglot persistent architectures.[6][7]

Motivations for this approach include: simplicity of design, simpler "horizontal" scaling to clusters of machines (which is a problem for relational databases),[2] finer control over availability and limiting the object-relational impedance mismatch.[8] The data structures used by NoSQL databases (e.g. key-value, wide column, graph, or document) are different from those used by default in relational databases, making some operations faster in NoSQL. The particular suitability of a given NoSQL database depends on the problem it must solve. Sometimes the data structures used by NoSQL databases are also viewed as "more flexible" than relational database tables.[9]

Many NoSQL stores compromise consistency (in the sense of the CAP theorem) in favor of availability, partition tolerance, and speed. Barriers to the greater adoption of NoSQL stores include the use of low-level query languages (instead of SQL, for instance the lack of ability to perform ad-hoc joins across tables), lack of standardized interfaces, and huge previous investments in existing relational databases.[10] Most NoSQL stores lack true ACID transactions, although a few databases have made them central to their designs.

Instead, most NoSQL databases offer a concept of "eventual consistency" in which database changes are propagated to all nodes "eventually" (typically within milliseconds) so queries for data might not return updated data immediately or might result in reading data that is not accurate, a problem known as stale reads.[11] Additionally, some NoSQL systems may exhibit lost writes and other forms of data loss.[12] Some NoSQL systems provide concepts such as write-ahead logging to avoid data loss.[13] For distributed transaction processing across multiple databases, data consistency is an even bigger challenge that is difficult for both NoSQL and relational databases. Relational databases "do not allow referential integrity constraints to span databases".[14] Few systems maintain both ACID transactions and X/Open XA standards for distributed transaction processing.[15] Interactive relational databases share conformational relay analysis techniques as a common feature.[16] Limitations within the interface environment are overcome using semantic virtualization protocols, such that NoSQL services are accessible to most operating systems.[17]


The term NoSQL was used by Carlo Strozzi in 1998 to name his lightweight Strozzi NoSQL open-source relational database that did not expose the standard Structured Query Language (SQL) interface, but was still relational.[18] His NoSQL RDBMS is distinct from the circa-2009 general concept of NoSQL databases. Strozzi suggests that, because the current NoSQL movement "departs from the relational model altogether, it should therefore have been called more appropriately 'NoREL',[19] referring to 'No Relational'.

Johan Oskarsson, then a developer at, reintroduced the term NoSQL in early 2009 when he organized an event to discuss "open source distributed, non relational databases".[20] The name attempted to label the emergence of an increasing number of non-relational, distributed data stores, including open source clones of Google's Bigtable/MapReduce and Amazon's DynamoDB.

Types and examples

There are various ways to classify NoSQL databases, with different categories and subcategories, some of which overlap. What follows is a basic classification by data model, with examples:

A more detailed classification is the following, based on one from Stephen Yen:[21][22]

TypeNotable examples of this type
Key-Value CacheApache Ignite, Couchbase, Coherence, eXtreme Scale, Hazelcast, Infinispan, Memcached, Redis, Velocity
Key-Value StoreArangoDB, Aerospike, Couchbase, Redis
Key-Value Store (Eventually-Consistent)Oracle NoSQL Database, Dynamo, Riak, Voldemort
Key-Value Store (Ordered)FoundationDB, InfinityDB, LMDB, MemcacheDB
Tuple StoreApache River, GigaSpaces
Object DatabaseObjectivity/DB, Perst, ZopeDB
Document StoreArangoDB, BaseX, Clusterpoint, Couchbase, CouchDB, DocumentDB, IBM Domino, MarkLogic, MongoDB, Qizx, RethinkDB, Elasticsearch
Wide Column StoreAmazon DynamoDB, Bigtable, Cassandra, Scylla, HBase, Hypertable
Native Multi-model DatabaseArangoDB, Cosmos DB, OrientDB, MarkLogic

Correlation databases are model-independent, and instead of row-based or column-based storage, use value-based storage.

Key-value store

Key-value (KV) stores use the associative array (also known as a map or dictionary) as their fundamental data model. In this model, data is represented as a collection of key-value pairs, such that each possible key appears at most once in the collection.[23][24]

The key-value model is one of the simplest non-trivial data models, and richer data models are often implemented as an extension of it. The key-value model can be extended to a discretely ordered model that maintains keys in lexicographic order. This extension is computationally powerful, in that it can efficiently retrieve selective key ranges.[25]

Key-value stores can use consistency models ranging from eventual consistency to serializability. Some databases support ordering of keys. There are various hardware implementations, and some users maintain data in memory (RAM), while others employ solid-state drives (SSD) or rotating disks (aka Hard Disk Drive (HDD)).

Document store

The central concept of a document store is the notion of a "document". While each document-oriented database implementation differs on the details of this definition, in general, they all assume that documents encapsulate and encode data (or information) in some standard formats or encodings. Encodings in use include XML, YAML, and JSON as well as binary forms like BSON. Documents are addressed in the database via a unique key that represents that document. One of the other defining characteristics of a document-oriented database is that in addition to the key lookup performed by a key-value store, the database also offers an API or query language that retrieves documents based on their contents.

Different implementations offer different ways of organizing and/or grouping documents:

  • Collections
  • Tags
  • Non-visible metadata
  • Directory hierarchies

Compared to relational databases, for example, collections could be considered analogous to tables and documents analogous to records. But they are different: every record in a table has the same sequence of fields, while documents in a collection may have fields that are completely different.


This kind of database is designed for data whose relations are well represented as a graph consisting of elements interconnected with a finite number of relations between them. The type of data could be social relations, public transport links, road maps, network topologies, etc.

Graph databases and their query language
AllegroGraphSPARQLRDF triple store
Amazon NeptuneGremlin, SPARQLGraph database
ArangoDBAQL, JavaScript, GraphQLMulti-model DBMS Document, Graph database and Key-value store
DEX/SparkseeC++, Java, .NET, PythonGraph database
FlockDBScalaGraph database
IBM DB2SPARQLRDF triple store added in DB2 10
InfiniteGraphJavaGraph database
MarkLogicJava, JavaScript, SPARQL, XQueryMulti-model document database and RDF triple store
Neo4jCypherGraph database
OpenLink VirtuosoC++, C#, Java, SPARQLMiddleware and database engine hybrid
OracleSPARQL 1.1RDF triple store added in 11g
OrientDBJava, SQLMulti-model document and graph database
OWLIMJava, SPARQL 1.1RDF triple store
Profium SenseJava, SPARQLRDF triple store
Sqrrl EnterpriseJavaGraph database

Object database


Tuple store

Triple/quad store (RDF) database


Multivalue databases

Multimodel database


Ben Scofield rated different categories of NoSQL databases as follows:[28]

Data modelPerformanceScalabilityFlexibilityComplexityFunctionality
Key–value storehighhighhighnonevariable (none)
Column-oriented storehighhighmoderatelowminimal
Document-oriented storehighvariable (high)highlowvariable (low)
Graph databasevariablevariablehighhighgraph theory
Relational databasevariablevariablelowmoderaterelational algebra

Performance and scalability comparisons are sometimes done with the YCSB benchmark.

Handling relational data

Since most NoSQL databases lack ability for joins in queries, the database schema generally needs to be designed differently. There are three main techniques for handling relational data in a NoSQL database. (See table Join and ACID Support for NoSQL databases that support joins.)

Multiple queries

Instead of retrieving all the data with one query, it is common to do several queries to get the desired data. NoSQL queries are often faster than traditional SQL queries so the cost of having to do additional queries may be acceptable. If an excessive number of queries would be necessary, one of the other two approaches is more appropriate.

Caching, replication and non-normalized data

Instead of only storing foreign keys, it is common to store actual foreign values along with the model's data. For example, each blog comment might include the username in addition to a user id, thus providing easy access to the username without requiring another lookup. When a username changes however, this will now need to be changed in many places in the database. Thus this approach works better when reads are much more common than writes.[29]

Nesting data

With document databases like MongoDB it is common to put more data in a smaller number of collections. For example, in a blogging application, one might choose to store comments within the blog post document so that with a single retrieval one gets all the comments. Thus in this approach a single document contains all the data you need for a specific task.

ACID and join support

A database is marked as supporting ACID properties (Atomicity, Consistency, Isolation, Durability) or join operations if the documentation for the database makes that claim. The degree to which the capability is fully supported in a manner similar to most SQL databases or the degree to which it meets the needs of a specific application is left up to the reader to assess.

Apache IgniteYesYes
MarkLogicYesYes[nb 1]
OrientDBYesYes[nb 2]
  1. Joins do not necessarily apply to document databases, but MarkLogic can do joins using semantics.[30]
  2. OrientDB can resolve 1:1 joins using links by storing direct links to foreign records.[31]

See also


  1. "NoSQL DEFINITION: Next Generation Databases mostly addressing some of the points: being non-relational, distributed, open-source and horizontally scalable"
  2. Leavitt, Neal (2010). "Will NoSQL Databases Live Up to Their Promise?" (PDF). IEEE Computer.
  3. Mohan, C. (2013). History Repeats Itself: Sensible and NonsenSQL Aspects of the NoSQL Hoopla (PDF). Proc. 16th Int'l Conf. on Extending Database Technology.
  4. "Amazon Goes Back to the Future With 'NoSQL' Database". WIRED. 19 January 2012. Retrieved 6 March 2017.
  5. "RDBMS dominate the database market, but NoSQL systems are catching up". 21 November 2013. Retrieved 24 November 2013.
  6. "NoSQL (Not Only SQL)". NoSQL database, also called Not Only SQL
  7. Fowler, Martin. "NosqlDefinition". many advocates of NoSQL say that it does not mean a "no" to SQL, rather it means Not Only SQL
  8. [NoSQL Distilled: A Brief Guide to the Emerging World of Polyglot Persistence. Addison-Wesley Educational Publishers Inc, 2009] ISBN 978-0321826626
  9. Vogels, Werner (18 January 2012). "Amazon DynamoDB – a Fast and Scalable NoSQL Database Service Designed for Internet Scale Applications". All Things Distributed. Retrieved 6 March 2017.
  10. Grolinger, K.; Higashino, W. A.; Tiwari, A.; Capretz, M. A. M. (2013). "Data management in cloud environments: NoSQL and NewSQL data stores" (PDF). Aira, Springer. Retrieved 8 January 2014.
  11. "Jepsen: MongoDB stale reads". 20 April 2015. Retrieved 6 March 2017.
  12. "Large volume data analysis on the Typesafe Reactive Platform". Retrieved 6 March 2017.
  13. Fowler, Adam. "10 NoSQL Misconceptions". Retrieved 6 March 2017.
  14. "No! to SQL and No! to NoSQL | So Many Oracle Manuals, So Little Time". Retrieved 6 March 2017.
  15. Chapple, Mike. "The ACID Model".
  16. Fiore, S (2011). Grid and cloud database management. Springer Science & Business Media. p. 210.
  17. Lawrence, Integration and virtualization of relational SQL and NoSQL systems including MySQL and MongoDB (2014). "Integration and virtualization of relational SQL and NoSQL systems including MySQL and MongoDB". International Conference on Computational Science and Computational Intelligence 1.
  18. Lith, Adam; Mattson, Jakob (2010). "Investigating storage solutions for large data: A comparison of well performing and scalable data storage solutions for real time extraction and batch insertion of data" (PDF). Göteborg: Department of Computer Science and Engineering, Chalmers University of Technology. p. 70. Retrieved 12 May 2011. Carlo Strozzi first used the term NoSQL in 1998 as a name for his open source relational database that did not offer a SQL interface[...]
  19. "NoSQL Relational Database Management System: Home Page". 2 October 2007. Retrieved 29 March 2010.
  20. "NoSQL 2009". 12 May 2009. Retrieved 29 March 2010.
  21. Yen, Stephen. "NoSQL is a Horseless Carriage" (PDF). NorthScale. Retrieved 26 June 2014.
  22. Strauch, Christof. "NoSQL Databases" (PDF). pp. 23–24. Retrieved 27 August 2017.
  23. Sandy (14 January 2011). "Key Value stores and the NoSQL movement". Stackexchange. Retrieved 1 January 2012. Key-value stores allow the application developer to store schema-less data. This data usually consists of a string that represents the key, and the actual data that is considered the value in the "key-value" relationship. The data itself is usually some kind of primitive of the programming language (a string, an integer, or an array) or an object that is being marshaled by the programming language's bindings to the key-value store. This structure replaces the need for a fixed data model and allows proper formatting.
  24. Seeger, Marc (21 September 2009). "Key-Value Stores: a practical overview" (PDF). Marc Seeger. Retrieved 1 January 2012. Key-value stores provide a high-performance alternative to relational database systems with respect to storing and accessing data. This paper provides a short overview of some of the currently available key-value stores and their interface to the Ruby programming language.
  25. Katsov, Ilya (1 March 2012). "NoSQL Data Modeling Techniques". Ilya Katsov. Retrieved 8 May 2014.
  26. Ignite Documentation
  27. fire-up-big-data-processing-with-apache-ignite
  28. Scofield, Ben (14 January 2010). "NoSQL - Death to Relational Databases(?)". Retrieved 26 June 2014.
  29. "Moving From Relational to NoSQL: How to Get Started". Retrieved 11 November 2019.
  30. "Can't do joins with MarkLogic? It's just a matter of Semantics! - General Networks". Retrieved 6 March 2017.
  31. "SQL Reference · OrientDB Manual". Retrieved 24 April 2017.

Further reading

  • Sadalage, Pramod; Fowler, Martin (2012). NoSQL Distilled: A Brief Guide to the Emerging World of Polyglot Persistence. Addison-Wesley. ISBN 978-0-321-82662-6.
  • McCreary, Dan; Kelly, Ann (2013). Making Sense of NoSQL: A guide for managers and the rest of us. ISBN 9781617291074.
  • Wiese, Lena (2015). Advanced Data Management for SQL, NoSQL, Cloud and Distributed Databases. DeGruyter/Oldenbourg. ISBN 978-3-11-044140-6.
  • Strauch, Christof (2012). "NoSQL Databases" (PDF).
  • Moniruzzaman, A. B.; Hossain, S. A. (2013). "NoSQL Database: New Era of Databases for Big data Analytics - Classification, Characteristics and Comparison". arXiv:1307.0191. Bibcode:2013arXiv1307.0191M. Cite journal requires |journal= (help)
  • Orend, Kai (2013). "Analysis and Classification of NoSQL Databases and Evaluation of their Ability to Replace an Object-relational Persistence Layer". CiteSeerX Cite journal requires |journal= (help)
  • Krishnan, Ganesh; Kulkarni, Sarang; Dadbhawala, Dharmesh Kirit. "Method and system for versioned sharing, consolidating and reporting information".

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