ORACLE SERVER

The oracle server mainly consists oracle instance and database

ORACLE INSTANCE

To enable users and applications to view or update data in the database, Oracle must start a set of processes, called background processes, and must allocate some memory to be used during database operation. The background processes and memory allocated by Oracle (SGA) together make up an oracle instance. An instance must be started to read and write information to the database. The SGA (Shared Global Area) is mainly used to cache some data blocks frequently queried by the application thererby improving the overall performance of the database.Oracle creates a set of background processes for an instance to manage memory structure, asynchronously perform I/O to write data to disk, and do general maintenance tasks. They monitor other Oracle processes to provide increased parallelism for better performance and reliability.RAC (Real Application Cluster) Setup which is the high availability solution provided by oracle consists of 2 instances ,1 instance per each node acessing the same database placed on shared storage .Such a setup provides HIGH AVAILIBILTY since database can be accessed by 2 instance and loss of any one of instance still makes the database accessible through another instance