OVERVIEW OF MAJOR COMPONENTS - Database Design and Development: An Essential Guide for IT Professionals - page 56

Query

ProductA
production
quantities?

Result

Product
quantities
by month.

ProductA
data

DATABASE
ENGINE

DATA
REPOSITORY

OPERATING

QUERY
PROCESSOR

SYSTEM

Structure of
ProductA data
Security Authori-
zations

Security
Module

I/O
Module

DATA
DICTIONARY

DBMS

Figure 2-9 Database engine: data access.

completely. The other components depend on the database engine to store data and
other system information so that their tasks may be carried out.
In the following subsections, review a few of the major features of the database
engine component and note how this component interacts with other components.

Data Access Suppose you, as a programmer, create a query for one of your user
groups that needs production data of products from the database of your organi-
zation. When your users run this query, the DBMS must coordinate the tasks to be
carried out by its various components. Once the structures of the product data and
production data are determined, the DBMS interacts with the operating system to
retrieve the requested data. Database engine component is responsible for storing
data, retrieving data, and updating data in the database.
Figure 2-9 demonstrates how the engine component processes the query request
by coordinating the functions of other components.
Note the interaction with the data dictionary. When your query requests for the
production quantities of Product A, the query need not specify that the quantity for
the product is numeric, is 8 bytes long, and starts in position 24 of the product record.
All of this information is stored in the data dictionary. The query just names the
quantity field and specifies Product A by name. Data dictionary definitions enable
the query to find the location of the required data.
Does the user executing the query have authorization to access the data speci-
fied in the query? The database engine component coordinates the services of the
security module to verify the authorization. When the location of the requested data
is determined from the data dictionary, the engine component coordinates the tasks