Page 64 - DLIS408_INFORMATION_TECHNOLOGY-APPLICATIONSL SCIENCES

Unit 5: Types of Database

repair it without special measures. The DBMS should provide automatic recovery from failure Notes
procedures that repair the DB and return it to a well defined state.
A different type of failure is due to a disaster, either by Nature (e.g., Earthquake, Flood, and
Tornado) or by Man (e.g., intentional physical systems’ sabotage, destructive acts of war). Recovery
from disasters (Disaster recovery), which typically incapacitate whole computer systems beyond
repair (and different from software failure or hardware component failure) requires special
protecting means.
Backup and Restore

Sometimes it is desired to bring a database back to a previous state (for many reasons, e.g., cases
when the database is found corrupted due to a software error, or if it has been updated with
erroneous data). To achieve this a backup operation is done occasionally or continuously, where
each desired database state (i.e., the values of its data and their embedding in database’s data
structures) is kept within dedicated backup files (many techniques exist to do this effectively).
When this state is needed, i.e., when it is decided by a database administrator to bring the database back
to this state (e.g., by specifying this state by a desired point in time when the database was in this state),
these files are utilized to restore that state.

Self Assessment
Fill in the blanks:
1. .................... Archive data from operational databases and often from external sources such
as market research firms.
2. Databases store detailed data about the operations of an organization is called ............. .
3. The database needs .................... to protect its content from dangers of unauthorized users.

5.2 Database Architecture

Database architecture may be viewed, to some extent, as an extension of Data modeling. It is used
to conveniently answer different end-user requirements from a same database. For example, a
financial department of a company needs the payment details of all employees as part of the
company’s expenses, but not other many details about employees, that are the interest of the
human resources department. Thus different departments need different views of the company’s
database that both include the employees’ payments, possibly in a different level of detail (and
presented in different visual forms). To meet such requirement effectively database architecture
consists of three levels: external, conceptual and internal. Clearly separating the three levels was
a major feature of the relational database model implementations that dominates 21st century
databases.
The external level defines how each end-user type understands the organization of its
respective relevant data in the database, i.e., the different needed end-user views. A single
database can have any number of views at the external level.
The conceptual level unifies the various external views into a coherent whole, global view.
It provides the common-denominator of all the external views. It comprises all the end-
user needed generic data, i.e., all the data from which any view may be derived/calculated.
It is provided in the simplest possible way of such generic data, and comprises the back-
bone of the database. It is out of the scope of the various database end-users, and serves
database application developers and defined by database administrators that build the
database.
The Internal level (or Physical level) is as a matter of fact part of the database implementa-
tion (see section below). It is concerned with cost, performance, scalability and other

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