Add Wide column store section

README.md

@@ -1113,3 +1113,26 @@ Document stores provide high flexibility and are often used for working with occ
 * [MongoDB architecture](https://www.mongodb.com/mongodb-architecture)
 * [CouchDB architecture](https://blog.couchdb.org/2016/08/01/couchdb-2-0-architecture/)
 * [Elasticsearch architecture](https://www.elastic.co/blog/found-elasticsearch-from-the-bottom-up)
+
+#### Wide column store
+
+<p align="center">
+  <img src="http://i.imgur.com/n16iOGk.png">
+  <br/>
+  <i><a href=http://blog.grio.com/2015/11/sql-nosql-a-brief-history.html>Source: SQL & NoSQL, a brief history</a></i>
+</p>
+
+> Abstraction: nested map `ColumnFamily<RowKey, Columns<ColKey, Value, Timestamp>>`
+
+A wide column store's basic unit of data is a column (name/value pair).  A column can be grouped in column families (analogous to a SQL table).  Super column families further group column families.  You can access each column independently with a row key, and columns with the same row key form a row.  Each value contains a timestamp for versioning and for conflict resolution.
+
+Google introduced [Bigtable](http://www.read.seas.harvard.edu/~kohler/class/cs239-w08/chang06bigtable.pdf) as the first wide column store, which influenced the open-source [HBase](https://www.mapr.com/blog/in-depth-look-hbase-architecture) often-used in the Hadoop ecosystem, and [Cassandra](http://docs.datastax.com/en/archived/cassandra/2.0/cassandra/architecture/architectureIntro_c.html) from Facebook.  Stores such as BigTable, HBase, and Cassandra maintain keys in lexicographic order, allowing efficient retrieval of selective key ranges.
+
+Wide column stores offer high availability and high scalability.  They are often used for very large data sets.
+
+##### Source(s) and further reading: wide column store
+
+* [SQL & NoSQL, a brief history](http://blog.grio.com/2015/11/sql-nosql-a-brief-history.html)
+* [Bigtable architecture](http://www.read.seas.harvard.edu/~kohler/class/cs239-w08/chang06bigtable.pdf)
+* [HBase architecture](https://www.mapr.com/blog/in-depth-look-hbase-architecture)
+* [Cassandra architecture](http://docs.datastax.com/en/archived/cassandra/2.0/cassandra/architecture/architectureIntro_c.html)