This module is about the processes that make a running Oracle database work. After completing this module, you will be able to:
- Explain the difference between a database and an instance
- Identify the major background processes of an Oracle instance, and explain their purpose
- Display a list of the Oracle background processes that are currently running on your server
- Identify the instances running on your system based on a listing of process names
The first thing to understand is the difference between a database and an instance.
When an instance is started, Oracle Database allocates a memory area called the system global area (SGA) and starts one or more background processes. The SGA serves various purposes, including the following:
- Maintaining internal data structures that are accessed by many processes and threads concurrently
- Caching data blocks read from disk
- Buffering redo data before writing it to the online redo log files
- Storing SQL execution plans
The SGA is shared by the Oracle processes, which include server processes and background processes, running on a single computer. The way in which Oracle processes are associated with the SGA is dependent on the operating system.
A database instance includes
- background processes,
- server processes, and
- the process memory
allocated in these processes
The instance continues to function when server processes terminate.