Page 105 - DCAP516_COMPUTER_SECURITY

Unit 8: Introduction to Databases

associated, ownership, etc.). Meta-data may include descriptive information about the context, Notes
quality and condition, or characteristics of the data.

8.16 Data Independence

The ability to modify a scheme definition in one level without affecting a scheme definition in
a higher level is called data independence. These are of two kinds:
1. Physical data independence

(i) The ability to modify the physical scheme without causing application programs to
be rewritten.
(ii) Modifications at this level are usually to improve performance.

2. Logical data independence
(i) The ability to modify the conceptual scheme without causing application programs
to be rewritten.

(ii) Usually done when logical structure of database is altered.
Logical data independence is harder to achieve as the application programs are usually heavily
dependent on the logical structure of the data. An analogy is made to abstract data types in
programming languages.

8.17 Database Manager

The database manager is a program module which provides the interface between the low-level
data stored in the database and the application programs and queries submitted to the system.

Databases typically require lots of storage space (gigabytes). This must be stored on disks. Data
is moved between disk and main memory (MM) as needed. The goal of the database system is to
simplify and facilitate access to data.
So the database manager module is responsible for:
1. Interaction with the file manager: Storing raw data on disk using the file system usually
provided by a conventional operating system. The database manager must translate DML
statements into low-level file system commands (for storing, retrieving and updating
data in the database).
2. Integrity enforcement: Checking that updates in the database do not violate consistency
constraints (e.g. no bank account balance below ` 25).

3. Security enforcement: Ensuring that users only have access to information they are
permitted to see.
4. Backup and recovery: Detecting failures due to power failure, disk crash, software errors,
etc., and restoring the database to its state before the failure.
5. Concurrency control: Preserving data consistency when there are concurrent users.
Some small database systems may miss some of these features, resulting in simpler database
managers (For example, no concurrency is required on a PC running MS DOS).
These features are necessary on larger systems.

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