NoSQL databases provide the performance,scalability and stability that’s required by the modern data-driven apps we interact with these days.But that is where the similarity between NoSQL systems end.In fact,it wouldn’t be wrong to say that the only thing most NoSQLdatabases have in common is that they do not follow the traditional relational data model.Broadly speaking,NoSQLdatabases typically fall into one of four categories:
Key-valueThese function in heavy read environments.Key-value stores are the simplest form of NoSQLdatabases and won’t be of much help when there are complex relationships between data elements.That’s because all access to the database is done using a primary key. Redis and Memcached are frequently used solutions.
Columnar These are optimised for reading and writing columns of data as opposed to rows of data. Also known as wide-column store databases, they are well suited for analysing huge data sets. Apache Cassandra, HBase and Accumulo are the best-known ones.
Document These store, manage and retrieve data as semi-structured documents such as JSON and XML. These document-orientated databases store all information for a given object, and each object can be quite different from the others. MongoDB, Couchbase and CouchDB are the best known and most widely used.
Graph Focus on how data relates to other data points. These databases explore the relationships that link data. Data from graph databases stores directed link between data sets called edges. Neo4j,OrientDB and JanusGraph are popular examples.
NoSQL數據庫提供了我們現在與之交互的現代數據驅動應用程序所需的性能，可伸縮性和穩定性。但這就是NoSQL系統之間的相似性結束的地方。事實上，說這是唯一的事情並不是錯的。大多數NoSQL數據庫的共同之處在於它們不遵循傳統的關係數據模型。總的來說，NoSQL數據庫通常屬於以下四類之一：
鍵值這些重讀值環境中的函數。鍵值存儲是NoSQL數據庫的最簡單形式並且贏了當數據元素之間存在複雜關係時，會有很大幫助。這是因爲所有對數據庫的訪問都是使用主鍵完成的。 Redis和Memcached是經常使用的解決方案。
Columnar這些針對讀取和寫入數據列進行了優化，而不是數據行。它們也稱爲寬列存儲數據庫，非常適合分析大量數據集。 Apache Cassandra，HBase和Accumulo是最知名的。
Document這些以半結構化文檔（如JSON和XML）存儲，管理和檢索數據。這些面向文檔的數據庫存儲給定對象的所有信息，每個對象可能與其他對象完全不同。 MongoDB，Couchbase和CouchDB是最知名和最廣泛使用的。
圖表關注數據與其他數據點的關係。這些數據庫探索鏈接數據的關係。來自圖數據庫的數據存儲稱爲邊的數據集之間的有向鏈接。 Neo4j，OrientDB和JanusGraph是很受歡迎的例子。
https://db-engines.com/en/ranking
