Databases Reference
In-Depth Information
Figure 2-11
Utility
Tools
Commn.
Interface
Data
Dictionary
Application
Developer
Forms
Generator
Database
Engine
Query
Processor
Report
Writer
DBMS components.
needed to determine where the data is located and how it is structured. Once
the structures and data location are determined, the DBMS interacts with the
operating system to retrieve the requested data.
The database engine is also responsible for security, sometimes working
together with the operating system. Does the user executing the query have
authorization to access the data specified in the query? The database engine
component coordinates the services of the security module to verify the
authorization.
Some systems include an automated agent service or agent and alert system
that works in close cooperation with the database engine. This service is known
by various names, depending on the DBMS, but performs the same function in
each case. Some descriptions of DBMS components might not identify this as a
separate component, instead including it with database tools and utilities. How-
ever, its role in the DBMS and the fact that it is often implemented as a sepa-
rate service or application justifies listing it as a unique component in its own
right.
An agent and alert system performs background tasks that support the data-
base service. Key among these is managing periodic activities that are scheduled
for automatic execution. It ensures that these activities run when scheduled and
if not, reports the error and takes appropriate corrective actions. Other tasks for
which the service might take responsibility include background monitoring and
managing activity logs. Background monitoring takes different forms, but is most
often related to performance monitoring and detecting and reporting errors. It
usually falls on the database administrator to define and configure performance
thresholds and identify the error conditions the service should report. Log man-
agement includes writing error and status messages to database log files. Most
