Adding content to 101-nosql

src/roadmaps/system-design/content/111-databases/101-nosql/100-key-value-store.md

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-# Key value store
+# Key Value Store
+
+A key-value store generally allows for O(1) reads and writes and is often backed by memory or SSD. Data stores can maintain keys in lexicographic order, allowing efficient retrieval of key ranges. Key-value stores can allow for storing of metadata with a value.
+
+Key-value stores provide high performance and are often used for simple data models or for rapidly-changing data, such as an in-memory cache layer. Since they offer only a limited set of operations, complexity is shifted to the application layer if additional operations are needed.
+
+To learn more, visit the following links:
+
+- [Key–value database](https://en.wikipedia.org/wiki/Key%E2%80%93value_database)
+- [What are the disadvantages of using a key/value table?](https://stackoverflow.com/questions/4056093/what-are-the-disadvantages-of-using-a-key-value-table-over-nullable-columns-or)

src/roadmaps/system-design/content/111-databases/101-nosql/101-document-store.md

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-# Document store
+# Document Store
+
+A document store is centered around documents (XML, JSON, binary, etc), where a document stores all information for a given object. Document stores provide APIs or a query language to query based on the internal structure of the document itself. Note, many key-value stores include features for working with a value's metadata, blurring the lines between these two storage types.
+
+Based on the underlying implementation, documents are organized by collections, tags, metadata, or directories. Although documents can be organized or grouped together, documents may have fields that are completely different from each other.
+
+To learn more, visit the following links:
+
+- [Getting started with Document Store](https://github.com/donnemartin/system-design-primer#document-store)
+- [Document-oriented database](https://en.wikipedia.org/wiki/Document-oriented_database)

src/roadmaps/system-design/content/111-databases/101-nosql/102-wide-column-store.md

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-# Wide column store
+# Wide Column Store
+
+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 as the first wide column store, which influenced the open-source HBase often-used in the Hadoop ecosystem, and Cassandra from Facebook. Stores such as BigTable, HBase, and Cassandra maintain keys in lexicographic order, allowing efficient retrieval of selective key ranges.
+
+Learn more from the following links:
+
+- [A brief of Wide Column Store](https://github.com/donnemartin/system-design-primer#Wide%20column%20store)
+- [Bigtable architecture](https://www.read.seas.harvard.edu/~kohler/class/cs239-w08/chang06bigtable.pdf)

src/roadmaps/system-design/content/111-databases/101-nosql/103-graph-databases.md

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-# Graph databases
+# Graph Databases
+
+In a graph database, each node is a record and each arc is a relationship between two nodes. Graph databases are optimized to represent complex relationships with many foreign keys or many-to-many relationships.
+
+Graphs databases offer high performance for data models with complex relationships, such as a social network. They are relatively new and are not yet widely-used; it might be more difficult to find development tools and resources. Many graphs can only be accessed with REST APIs.
+
+Learn more from the following links:
+
+- [Graph database](https://en.wikipedia.org/wiki/Graph_database)
+- [Introduction to NoSQL](https://www.youtube.com/watch?v=qI_g07C_Q5I)

src/roadmaps/system-design/content/111-databases/101-nosql/index.md

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-# Nosql
+# NoSQL
+
+NoSQL is a collection of data items represented in a key-value store, document store, wide column store, or a graph database. Data is denormalized, and joins are generally done in the application code. Most NoSQL stores lack true ACID transactions and favor eventual consistency.
+
+BASE is often used to describe the properties of NoSQL databases. In comparison with the CAP Theorem, BASE chooses availability over consistency.
+
+- Basically available - the system guarantees availability.
+- Soft state - the state of the system may change over time, even without input.
+- Eventual consistency - the system will become consistent over a period of time, given that the system doesn't receive input during that period.
+
+Learn more from the following links:
+
+- [SQL or noSQL?](https://github.com/donnemartin/system-design-primer#sql-or-nosql)
+- [Brief of noSQL Patterns](http://horicky.blogspot.com/2009/11/nosql-patterns.html)
+- [Introduction to NoSQL](https://www.youtube.com/watch?v=qI_g07C_Q5I)