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4.1 Data Warehouse: Basic Concepts 131

Query/report Analysis Data mining

Top tier:
Front-end tools

Output
OLAP server OLAP server

Middle tier:
OLAP server

Monitoring Administration Data warehouse Data marts
Bottom tier:
Data warehouse
Metadata repository server

Extract
Clean
Transform
Load Data
Refresh

Operational databases External sources

Figure 4.1 A three-tier data warehousing architecture.

1. 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 bot-
tom tier from operational databases or other external sources (e.g., customer profile
information provided by external consultants). These tools and utilities perform data
extraction, cleaning, and transformation (e.g., to merge similar data from different
sources into a unified format), as well as load and refresh functions to update the
data warehouse (see Section 4.1.6). The data are extracted using application pro-
gram 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. Exam-
ples of gateways include ODBC (Open Database Connection) and OLEDB (Object