Data warehousing is the process of constructing and using a data warehouse. A data warehouse is constructed by integrating data from multiple heterogeneous sources that support analytical reporting, structured and/or ad hoc queries, and decision making. Data warehousing involves data cleaning, data integration, and data consolidations.
Using Data Warehouse Information
There are decision support technologies that help utilize the data available in a data warehouse. These technologies help executives to use the warehouse quickly and effectively. They can gather data, analyze it, and take decisions based on the information present in the warehouse. The information gathered in a warehouse can be used in any of the following domains −
- Tuning Production Strategies− The product strategies can be well tuned by repositioning the products and managing the product portfolios by comparing the sales quarterly or yearly.
- Customer Analysis− Customer analysis is done by analyzing the customer’s buying preferences, buying time, budget cycles, etc.
- Operations Analysis− Data warehousing also helps in customer relationship management, and making environmental corrections. The information also allows us to analyze business operations.
Integrating Heterogeneous Databases
To integrate heterogeneous databases, we have two approaches −
- Query-driven Approach
- Update-driven Approach
This is the traditional approach to integrate heterogeneous databases. This approach was used to build wrappers and integrators on top of multiple heterogeneous databases. These integrators are also known as mediators.
Process of Query-Driven Approach
- When a query is issued to a client side, a metadata dictionary translates the query into an appropriate form for individual heterogeneous sites involved.
- Now these queries are mapped and sent to the local query processor.
- The results from heterogeneous sites are integrated into a global answer set.
- Query-driven approach needs complex integration and filtering processes.
- This approach is very inefficient.
- It is very expensive for frequent queries.
- This approach is also very expensive for queries that require aggregations.
This is an alternative to the traditional approach. Today’s data warehouse systems follow update-driven approach rather than the traditional approach discussed earlier. In update-driven approach, the information from multiple heterogeneous sources are integrated in advance and are stored in a warehouse. This information is available for direct querying and analysis.
This approach has the following advantages −
- This approach provide high performance.
- The data is copied, processed, integrated, annotated, summarized and restructured in semantic data store in advance.
- Query processing does not require an interface to process data at local sources.
Functions of Data Warehouse Tools and Utilities
The following are the functions of data warehouse tools and utilities −
- Data Extraction− Involves gathering data from multiple heterogeneous sources.
- Data Cleaning− Involves finding and correcting the errors in data.
- Data Transformation− Involves converting the data from legacy format to warehouse format.
- Data Loading− Involves sorting, summarizing, consolidating, checking integrity, and building indices and partitions.
- Refreshing− Involves updating from data sources to warehouse.
Note − Data cleaning and data transformation are important steps in improving the quality of data and data mining results.
Data Warehousing Architecture
Business Analysis Framework
The business analyst get the information from the data warehouses to measure the performance and make critical adjustments in order to win over other business holders in the market. Having a data warehouse offers the following advantages −
- Since a data warehouse can gather information quickly and efficiently, it can enhance business productivity.
- A data warehouse provides us a consistent view of customers and items, hence, it helps us manage customer relationship.
- A data warehouse also helps in bringing down the costs by tracking trends, patterns over a long period in a consistent and reliable manner.
To design an effective and efficient data warehouse, we need to understand and analyze the business needs and construct a business analysis framework. Each person has different views regarding the design of a data warehouse. These views are as follows −
- The top-down view− This view allows the selection of relevant information needed for a data warehouse.
- The data source view− This view presents the information being captured, stored, and managed by the operational system.
- The data warehouse view− This view includes the fact tables and dimension tables. It represents the information stored inside the data warehouse.
- The business query view− It is the view of the data from the viewpoint of the end-user.
Three-Tier Data Warehouse Architecture
Generally a data warehouses adopts a three-tier architecture. Following are the three tiers of the data warehouse architecture.
- Bottom Tier− The bottom tier of the architecture is the data warehouse database server. It is the relational database system. We use the back end tools and utilities to feed data into the bottom tier. These back end tools and utilities perform the Extract, Clean, Load, and refresh functions.
- Middle Tier− In the middle tier, we have the OLAP Server that can be implemented in either of the following ways.
- By Relational OLAP (ROLAP), which is an extended relational database management system. The ROLAP maps the operations on multidimensional data to standard relational operations.
- By Multidimensional OLAP (MOLAP) model, which directly implements the multidimensional data and operations.
- Top-Tier− This tier is the front-end client layer. This layer holds the query tools and reporting tools, analysis tools and data mining tools.
The following diagram depicts the three-tier architecture of data warehouse −
Data Warehouse Models
From the perspective of data warehouse architecture, we have the following data warehouse models −
- Virtual Warehouse
- Data mart
- Enterprise Warehouse
The view over an operational data warehouse is known as a virtual warehouse. It is easy to build a virtual warehouse. Building a virtual warehouse requires excess capacity on operational database servers.
Data mart contains a subset of organization-wide data. This subset of data is valuable to specific groups of an organization.
In other words, we can claim that data marts contain data specific to a particular group. For example, the marketing data mart may contain data related to items, customers, and sales. Data marts are confined to subjects.
Points to remember about data marts −
- Window-based or Unix/Linux-based servers are used to implement data marts. They are implemented on low-cost servers.
- The implementation data mart cycles is measured in short periods of time, i.e., in weeks rather than months or years.
- The life cycle of a data mart may be complex in long run, if its planning and design are not organization-wide.
- Data marts are small in size.
- Data marts are customized by department.
- The source of a data mart is departmentally structured data warehouse.
- Data mart are flexible.
- An enterprise warehouse collects all the information and the subjects spanning an entire organization
- It provides us enterprise-wide data integration.
- The data is integrated from operational systems and external information providers.
- This information can vary from a few gigabytes to hundreds of gigabytes, terabytes or beyond.
This component performs the operations required to extract and load process.
The size and complexity of the load manager varies between specific solutions from one data warehouse to other.
Load Manager Architecture
The load manager performs the following functions −
- Extract the data from source system.
- Fast Load the extracted data into temporary data store.
- Perform simple transformations into structure similar to the one in the data warehouse.
Extract Data from Source
The data is extracted from the operational databases or the external information providers. Gateways is the application programs that are used to extract data. It is supported by underlying DBMS and allows client program to generate SQL to be executed at a server. Open Database Connection(ODBC), Java Database Connection (JDBC), are examples of gateway.
- In order to minimize the total load window the data need to be loaded into the warehouse in the fastest possible time.
- The transformations affects the speed of data processing.
- It is more effective to load the data into relational database prior to applying transformations and checks.
- Gateway technology proves to be not suitable, since they tend not be performant when large data volumes are involved.
While loading it may be required to perform simple transformations. After this has been completed we are in position to do the complex checks. Suppose we are loading the EPOS sales transaction we need to perform the following checks:
- Strip out all the columns that are not required within the warehouse.
- Convert all the values to required data types.
A warehouse manager is responsible for the warehouse management process. It consists of third-party system software, C programs, and shell scripts.
The size and complexity of warehouse managers varies between specific solutions.
Warehouse Manager Architecture
A warehouse manager includes the following −
- The controlling process
- Stored procedures or C with SQL
- Backup/Recovery tool
- SQL Scripts
Operations Performed by Warehouse Manager
- A warehouse manager analyzes the data to perform consistency and referential integrity checks.
- Creates indexes, business views, partition views against the base data.
- Generates new aggregations and updates existing aggregations. Generates normalizations.
- Transforms and merges the source data into the published data warehouse.
- Backup the data in the data warehouse.
- Archives the data that has reached the end of its captured life.
Note − A warehouse Manager also analyzes query profiles to determine index and aggregations are appropriate.
- Query manager is responsible for directing the queries to the suitable tables.
- By directing the queries to appropriate tables, the speed of querying and response generation can be increased.
- Query manager is responsible for scheduling the execution of the queries posed by the user.
Query Manager Architecture
The following screenshot shows the architecture of a query manager. It includes the following:
- Query redirection via C tool or RDBMS
- Stored procedures
- Query management tool
- Query scheduling via C tool or RDBMS
- Query scheduling via third-party software
Detailed information is not kept online, rather it is aggregated to the next level of detail and then archived to tape. The detailed information part of data warehouse keeps the detailed information in the starflake schema. Detailed information is loaded into the data warehouse to supplement the aggregated data.
The following diagram shows a pictorial impression of where detailed information is stored and how it is used.
Note − If detailed information is held offline to minimize disk storage, we should make sure that the data has been extracted, cleaned up, and transformed into starflake schema before it is archived.
Summary Information is a part of data warehouse that stores predefined aggregations. These aggregations are generated by the warehouse manager. Summary Information must be treated as transient. It changes on-the-go in order to respond to the changing query profiles.
The points to note about summary information are as follows −
- Summary information speeds up the performance of common queries.
- It increases the operational cost.
- It needs to be updated whenever new data is loaded into the data warehouse.
- It may not have been backed up, since it can be generated fresh from the detailed information.