Oracle RAC
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Oracle Real Application Clusters (RAC) is software option to the Oracle RDBMS produced by Oracle Corporation.
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[edit] Introduction
Oracle RAC allows multiple computers to run the Oracle RDBMS software simultaneously while accessing a single database. This is called a clustered database.
In a non-RAC Oracle database, a single database is accessed by a single instance. The database is considered the collection of data files, control files, and redo log files located on disk. The instance is considered the collection of Oracle-related memory and operating system processes that are running on the computer.
In Oracle RAC, 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 the same data.
[edit] Cache Fusion
Since all computers/instances access the same database, the Oracle software must guarantee that data changes on different computers are coordinated and that when a computer queries data it receives the current version even if that data was recently modified by another computer. This functionality of Oracle RAC is called Cache Fusion. Cache Fusion is the name given to the ability of Oracle RAC to treat the In-memory data caches on each computer as a single, global cache. Cache Fusion essentially fuses the physically separate caches into a single, global cache.
[edit] Benefits
Since Oracle RAC allows multiple computers to access a single database, it can be used to address several areas of database management. These areas include: High Availability, Scalability, Incremental Growth, and Database Consolidation.
[edit] Competitors
Oracle is currently the only commercially available database that offers a shared-everything architecture where all computers share the same disks. Oracle's competitors who offer shared-nothing architectures include: IBM DB2, Microsoft SQL Server, and MySQL Cluster.
It should be noted that both shared-nothing and shared-everything architectures have advantages over each other. Database vendors are often heard debating that their architecture is superior to the other.
