Oracle Essentials, 5th Edition

Chapter 2. Oracle Architecture

This chapter focuses on the concepts and structures at the core of the Oracle Database. When you understand the architecture of the Oracle Database server, you’ll have a context for understanding the rest of the features of Oracle described in this book.

An Oracle Database consists of both physical and logical components. The first section of this chapter covers the difference between an Oracle Database and an instance, and subsequent sections describe physical components, the instance, and the data dictionary.

Databases and Instances

Many Oracle practitioners use the terms instance and database interchangeably. In fact, an instance and a database are different (but related) entities. This distinction is important because it provides insight into Oracle’s architecture.

In Oracle, the term database refers to the physical storage of information, and the term instance refers to the software executing on the server that provides access to the information in the database and the resources that software uses. The instance runs on the computer or server; the database is stored on the disks attached to the server. Figure 2-1 illustrates this relationship.

The database is physical: it consists of files stored on disks. The instance is logical: it consists of in-memory structures and processes on the server. For example, Oracle uses an area of shared memory called the System Global Area (SGA) and a private memory area for each process called the Program Global Area ...