Database Reference
In-Depth Information
2.5.1 The DBMS Engine
The
DBMS
engine
is the link between all other subsystems and the physical device
(the computer) via the operating system. Some important functions are as follows:
•
Provision of direct access to operating system utilities and
programs (e.g. I/O requests, data compaction requests,
communication requests etc.)
•
Management of file access (and data management) via the
operating system
•
Management of data transfer between memory and the system
buffer(s) in order to effect user requests
•
Maintenance of overhead data and metadata stored in the data
dictionary (system catalog)
2.5.2 Definition Tools Subsystem
The
data definition subsystem
(DDS or its equivalent) consists of tools and utilities for
defining and changing the structure of the database. The structure includes relational
tables, relationships, constraints, user profiles, overhead data structures, etc.
The DDL (data definition language) is used to define all database objects that make
up the conceptual schema (relations, relationships, constraints, etc.). The DML (data
manipulation language) is used to facilitate manipulation (insert, remove, update, find,
query etc.) of data. The DML usually includes a query language. The DCL (data control
language) is used to set up control environments for data management by the end user.
As mentioned earlier, the DDL, DML and DCL comprise the DSL.
2.5.3 The User Interface Subsystem
The
user interface subsystem
(UIS or its equivalent) allows users and programs to access
the database via an interactive query language such as SQL and/or the host language.
The traditional interface is command based; however in recent times menus and graphical
user interfaces (GUI) have become more prevalent. Of course, it is not uncommon for
a product to provide the user with all three interfaces (for example Oracle). Other more
sophisticated DBMS suites may use
natural language
interface.
The user interface may also include a DBMS-specific programming language
(e.g. FoxPro, Scalable Application Language, and Oracle's PL/SQL). These languages pertain
only to the DBMS in which they are used. Additionally, the DBMS may support multiple
high level languages such as C++, Java, etc., thus making it more flexible and marketable.
