docs: Data mapper1762 (#2165)

* update README.md

* update README.md

data-mapper/README.md

@@ -3,21 +3,53 @@ title: Data Mapper
 category: Architectural
 language: en
 tags:
- - Decoupling
+- Decoupling
 ---
 
 ## Intent
 A layer of mappers that moves data between objects and a database while keeping them independent of each other and the mapper itself
 
+This layer consists of one or more mappers (or data access objects) that perform data transfer. The scope of mapper implementations varies.
+A generic mapper will handle many different domain entity types, a dedicated mapper will handle one or a few.
+
+## Explanation
+
+Real-world example
+
+> When accessing web resources through a browser, there is generally no need to interact with the server directly;
+> the browser and the proxy server will complete the data acquisition operation, and the three will remain independent.
+
+In plain words
+
+> The data mapper will help complete the bi-directional transfer of persistence layer and in-memory data.
+
+Wikipedia says
+
+> A Data Mapper is a Data Access Layer that performs bidirectional transfer of data between a
+> persistent data store (often a relational database) and an in-memory data representation (the domain layer).
+
+Programmatic example
+
 ## Class diagram
 ![alt text](./etc/data-mapper.png "Data Mapper")
 
 ## Applicability
 Use the Data Mapper in any of the following situations
 
-* when you want to decouple data objects from DB access layer 
+* when you want to decouple data objects from DB access layer
 * when you want to write multiple data retrieval/persistence implementations
 
-## Credits
+## Known uses
+* [SqlSession.getMapper()](https://mybatis.org/mybatis-3/java-api.html)
 
-* [Data Mapper](http://richard.jp.leguen.ca/tutoring/soen343-f2010/tutorials/implementing-data-mapper/)
+## Consequences
+
+> Neatly mapped persistence layer data
+> Data model follows the single function principle
+
+## Related patterns
+* [Active Record Pattern](https://en.wikipedia.org/wiki/Active_record_pattern)
+* [Object–relational Mapping](https://en.wikipedia.org/wiki/Object%E2%80%93relational_mapping)
+
+## Credits
+* [Data Mapper](http://richard.jp.leguen.ca/tutoring/soen343-f2010/tutorials/implementing-data-mapper/)