Oracle RAC(Real Application Clusters)
DEFINITION - Real Application Cluster (RAC) is a component of the Oracle 9i database product that allows multiple computers to run Oracle RDBMS software simultaneously while accessing a single database, thus providing a clustered database.
According to Oracle, RAC's shared disk method of clustering databases:
increases scalability: because servers can easily be added or subtracted to meet current needs
lowers costs: because companies don't have to buy high-end servers
improves availability: because if one server fails, another can assume its workload.
In a non-RAC Oracle database, a single instance accesses a single database. Where the "database" consists of a collection of data files, control files, and redo logs located on disk; the "instance" comprises the collection of Oracle-related memory and operating system processes that run on a computer system.
In an Oracle RAC environment, two or more computers (each with an instance) concurrently access a single database. This allows an application or user to connect to either computer and have access to a single coordinated set of data.
RAC's shared disk architecture is an unusual approach to database clustering. Most competing database products (such as Microsoft's SQL Server and IBM's DB2 for Windows and Unix environments) use the alternative, which is known as "shared nothing" architecture. Shared nothing architecture partitions data and only gives each server access to its own disk subsystem, while shared disk architecture gives all servers access to the entire database. This adds failover capacity to the database, because all servers have access to the whole database.
Cluster
A computer cluster is a group of linked computers, working together closely so that in many respects they form a single computer. The components of a cluster are commonly, but not always, connected to each other through fast local area networks. Clusters are usually deployed to improve performance and/or availability over that provided by a single computer, while typically being much more cost-effective than single computers of comparable speed or availability.
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