Oracle Instance

An Oracle Instance

1. Every running Oracle database is associated with an Oracle instance.
2. 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.
3. The combination of the SGA and the Oracle processes is called an Oracle database instance.
4. 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.

5. Oracle starts an instance, then mounts a database to the instance.
6. Multiple instances can execute concurrently on the same machine, each
   accessing its own physical database. 
   
   Single-Process Oracle Instance
   
   1. A single-user Oracle instance is a database system in which all Oracle code is executed by one process.
   2. 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.
      

1. In a multiple-process system, processes can be categorized into two groups: user processes and Oracle processes.
2. Each process in a multiple-process Oracle instance performs a specific job.
3. 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.
4. 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.