1. The bottom tier is a warehouse database server that is almost always a relational database system. “How are the data extracted from this tier in order to create the data warehouse?” Data from operational databases and external sources (such as customer profile information
Provided by external consultants) 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 OLE – DB (Open Linking and Embedding for Databases), by Microsoft, and JDBC (Java Database Connection).
2. The middle tier is an OLAP server that is typically implemented using either (1) a relational OLAP (ROLAP) model, that is, an extended relational DBMS that maps operations on multidimensional data to standard relational operations, or (2) a multidimensional OLAP (MOLAP) model, that is, a special – purpose server that directly implements multidimensional data and operations.
3. The top tier is a client, which contains query and reporting tools, analysis tools, and / or data mining tools (e.g., trend analysis, prediction, and so on).
From the architecture point of view, there are three data warehouse models: the enterprise warehouse, the data mart, and the virtual warehouse
