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please explain Three-Tier Data Warehouse Architecture

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Data Warehouses usually have a three-level (tier) architecture that includes:

  1. Bottom Tier (Data Warehouse Server)
  2. Middle Tier (OLAP Server)
  3. Top Tier (Front end Tools).

bottom-tier that consists of the Data Warehouse server, which is almost always an RDBMS. It may include several specialized data marts and a metadata repository.

Data from operational databases and external sources (such as user profile data provided by external consultants) are extracted using application program interfaces called a gateway. A gateway is provided by the underlying DBMS and allows customer programs to generate SQL code to be executed at a server.

Examples of gateways contain ODBC (Open Database Connection) and OLE-DB (Open-Linking and Embedding for Databases), by Microsoft, and JDBC (Java Database Connection).

Three-Tier Data Warehouse Architecture

middle-tier which consists of an OLAP server for fast querying of the data warehouse.

The OLAP server is implemented using either

(1) A Relational OLAP (ROLAP) model, i.e., an extended relational DBMS that maps functions on multidimensional data to standard relational operations.

(2) A Multidimensional OLAP (MOLAP) model, i.e., a particular purpose server that directly implements multidimensional information and operations.

top-tier that contains front-end tools for displaying results provided by OLAP, as well as additional tools for data mining of the OLAP-generated data.