Oracle Database Architecture consists of following three main structures
1- Storage Structure
2- Memory Structure
3- Process Structure
Two of these constitute in primary storage and one in secondary storage.
When a database is created, database files are installed on secondary storage. These files include Control files, Data files, online redo log files, parameter file and password file. All these files formulate “Storage Structure”.
When oracle Database is started on Database Server, oracle software allocates shared memory area and also starts several processes to run database. The shared memory area is called System Global Area (SGA). The combination of System Global Area and Processes is known as Oracle Instance. Oracle Instance formulates Memory Structure and Process Structure.
Each instance in oracle server is associated with a specific database. The process of assigning an instance to a database is called Mounting. After mounting, database is opened and becomes accessible for users. Multiple instances can run on single computer.