Three tier Data Warehouse Architecture Find a simpler diagram for understanding at the bottom Bottom tier : The bottom tier is a warehouse database server that is almost always a relational database system Back-end tools and utilities are used to feed data into the bottom tier from operational databases or other external sources These tools and utilities perform data extraction, cleaning, and transformation, as well as load and refresh functions to update the data warehouse The data are extracted using application program interfaces known as gateways A gateway is supported by the underlying DBMS and allows client programs to generate SQL code to be executed at a server Examples of gateways include ODBC (Open Database Connection) and OLEDB (Open Linking and Embedding for Databases) by Microsoft and JDBC (Java Database Connection This tier also contains a metadata repository, which stores information about the data warehouse and its contents. Middle tier : The middle tier is an OLAP server that is typically implemented using either A relational OLAP (ROLAP) model, that is, an extended relational DBMS that maps operations on multidimensional data to standard relational operations or A multidimensional OLAP (MOLAP) model, that is, a special-purpose server that directly implements multidimensional data and operations. Top tier : The top tier is a front-end client layer, which contains query and reporting tools, analysis tools, and/or data mining tools
