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Structural/DataMapper/README.md

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+# Data Mapper
+
+## Purpose
+
+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). The goal of the pattern is to
+keep the in memory representation and the persistent data store independent of
+each other and the data mapper itself. The layer is composed of one or more
+mappers (or Data Access Objects), performing the data transfer. Mapper
+implementations vary in scope. Generic mappers will handle many different domain
+entity types, dedicated mappers will handle one or a few.
+
+The key point of this pattern is, unlike Active Record pattern, the data model follows Single Responsibility Principle.
+
+## Examples
+
+* DB Object Relational Mapper (ORM) : Doctrine2 uses DAO named as "EntityRepository"