An Oracle Instance
- Every running Oracle database is associated with an Oracle instance.
- Every time a database is started on a database server, Oracle allocates a memory area called the System Global Area (SGA) and starts one or more Oracle processes.
- The combination of the SGA and the Oracle processes is called an Oracle database instance.
- The memory and processes of an instance work to manage the database’s data efficiently and serve the one or multiple users of the associated database.
- Oracle starts an instance, then mounts a database to the instance.
- Multiple instances can execute concurrently on the same machine, each
accessing its own physical database.
Single-Process Oracle Instance
- A single-user Oracle instance is a database system in which all Oracle code is executed by one process.
- All code of Oracle and the lone user’s database application is executed by a single process.
For example, Oracle running under the MS DOS operating system on a PC can only be accessed by a single user because MS DOS is not capable of running multiple processes.
Multiple-Process Oracle Instance
Multi-user Oracle uses several processes to execute different parts of oracle and a separate process for each connected user.
- In a multiple-process system, processes can be categorized into two groups: user processes and Oracle processes.
- Each process in a multiple-process Oracle instance performs a specific job.
- By dividing the work of Oracle and database applications into several processes, multiple users and applications can simultaneously connect to a single database instance while the system maintains adequate performance.
- Most database systems are multi-user, because one of the primary benefits of a database is managing data needed by multiple users at the same time.