Beginning Oracle Database 11g Administration From Novice to Professional

CHAPTER 3 ORACLE ARCHITECTURE 53
Database vs. Instance
In Oracle terminology, the word database is used to collectively indicate the data files and
supporting files on the storage disks attached to the host computer, and the word instance
is used to describe the various computer processes resident in computer memory and
memory areas shared by these processes. This is in contrast to database architectures such
as Microsoft SQL Server and IBM DB2, for which the word instance indicates a collection
of databases that share common memory resources—that is, the relationship between
instances and databases is one-to-many. The relationship between Oracle instances and
databases, on the other hand, is either one-to-one (one instance per database) or manyto-one
(multiple instances per database). The many-to-one configuration is called Real
Application Clusters (RAC)—the database lives on shared disks, and instances on multiple
computers attach to the database.
In Figure 3-1, the database is represented by the panel on the far right labeled Disk
Storage. The three blocks in the middle—labeled Server Processes, SGA, and Background
Processes—represent the instance.
Database
The most concrete aspect of a database is the files on the storage disks connected to the
database host. In this section, I briefly discuss each category of file. Placement, sizing, and
other configuration details are discussed in Chapters 5 and 6.
Software
The location of the database software is called the Oracle home and is usually stored in
the environment variable . There are two species of database software: server
software and client software. Server software is necessary to create and manage the database
and is required only on the database host. Client software is necessary to utilize the
database and is required on every user’s computer—the most common example is the
SQL*Plus command-line tool.
Configuration Files
The most important database configuration file is the one containing the settings used
during database startup. It comes in two versions: a text version called a pfile and a binary
version called an spfile. You will see an example in Chapter 6. The pfile and spfile specify
such details as the amount of computer memory that Oracle may use during operation.
The pfile is traditionally referred to as the file.