In database computing, Oracle Real Application Clusters (RAC) — an option  for the Oracle Database software produced by Oracle Corporation and introduced in 2001 with Oracle9i — provides software for clustering and high availability in Oracle database environments. Oracle Corporation includes RAC with the Standard Edition of Oracle Database (aka Baby RAC), but makes it an extra-charge option for the Enterprise Edition.
In a non-RAC Oracle database, a single instance accesses a single database. 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 Oracle RDBMS 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.
The main aim of Oracle RAC is to implement a clustered database to provide performance, scalability and resilience.
Oracle RAC depends on the infrastructure component Oracle Clusterware to coordinate multiple servers and their sharing of data storage. The FAN (Fast Application Notification) technology detects down-states.
Cache Fusion 
Prior to Oracle 9, network-clustered Oracle databases used a storage device as the data-transfer medium (meaning that one node would write a data block to disk and another node would read that data from the same disk), which had the inherent disadvantage of lackluster performance. Oracle 9i addressed this issue: RAC uses a dedicated network connection for communications internal to the cluster.
Since all computers/instances in a RAC access the same database, the overall system must guarantee the coordination of data changes on different computers such that whenever a computer queries data, it receives the current version — even if another computer recently modified that data. Oracle RAC refers to this functionality as Cache Fusion. Cache Fusion involves the ability of Oracle RAC to "fuse" the in-memory data cached physically separately on each computer into a single, global cache.
- Oracle Real Application Clusters One Node (RAC One Node) applies RAC to single-node installations running Oracle Database 11g Release 2 Enterprise Edition.
Relative to the single-instance Oracle database, Oracle RAC adds additional complexity. While database automation makes sense for single-instance databases, it becomes even more necessary for clustered databases because of their increased complexity.
Oracle Real Application Clusters (RAC), introduced with Oracle9i in 2001, supersedes the Oracle Parallel Server (OPS) database option. Whereas Oracle9i required an external clusterware (known as vendor clusterware like Veritas or Sun Cluster) for most of the Unix flavors (except for Linux and Windows where Oracle provided free clusterware called Cluster Ready Services or CRS), as of Oracle 10g, Oracle's clusterware product was available for all operating systems. With the release of Oracle Database 10g Release 2 (10.2), Cluster Ready Services was renamed to Oracle Clusterware. When using Oracle 10g or higher, Oracle Clusterware is the only clusterware that you need for most platforms on which Oracle RAC operates (except for Tru cluster, in which case you need vendor clusterware). You can still use clusterware from other vendors, if the clusterware is certified for Oracle RAC.
In RAC, the write-transaction must take ownership of the relevant area of the database: typically, this involves a request across the cluster interconnection (local IP network) to transfer the data-block ownership from another node to the one wishing to do the write. This takes a relatively long time (from a few to tens of milliseconds) compared to single database-node using in-memory operations. For many types of applications, the time spent coordinating block access across systems is low relative to the many operations on the system, and RAC will scale comparably to a single system. Moreover, high read-transactional databases (such as data-warehousing applications) work very well under RAC, as no need for ownership-transfer exists. (Oracle 11g has made many enhancements in this area and performs a lot better than earlier versions for read-only workloads.)
The overhead on the resource mastering (or ownership-transfer) is minimal for fewer than three nodes, as the request for any resource in the cluster can be obtained in a maximum of three hops (owner-master-requestor). This makes Oracle RAC horizontally scalable with many nodes. Application vendors (such as SAP) use Oracle RAC to demonstrate the scalability of their application. Most of the biggest OLTP benchmarks are on Oracle RAC. Oracle RAC 11g supports up to 100 nodes.
For some[which?] applications, RAC may require careful application partitioning to enhance performance. An application that scales linearly on an SMP machine may scale linearly under RAC. However, if the application cannot scale linearly on SMP, it will not scale when ported to RAC. In short, the application scalability is based on how well the application scales in a single instance.
Competitive context 
Shared-nothing and shared-everything architectures each have advantages over the other. DBMS vendors and industry analysts regularly debate the matter; for example, Microsoft touts a comparison of its SQL Server 2005 with Oracle 10g RAC.
Oracle Corporation offered a Shared Nothing architecture RDBMS with the advent of the IBM SP and SP2 with the release of 7.x MPP editions, in which virtual shared drives (VSD) were used to create a Shared Everything implementation on a Shared Nothing architecture.
"Shared-everything" architectures share both disk and memory between nodes in the cluster. This is in contrast to "shared-nothing" architectures that share none of them.
Some commercially-available databases offer a "shared-everything" architecture. IBM DB2 for z/OS (the IBM mainframe operating-system) has provided a high-performance data-sharing option since the mid-1990s when IBM released its mainframe hardware and software-clustering infrastructure. In late 2009, IBM announced DB2 pureScale, a shared-disk clustering scheme for DB2 9.8 on AIX that mimics the parallel sysplex implementation behind DB2 data sharing on the mainframe.
Although technically not shared-everything, Sybase also provides a column-based relational database focused on analytic and datawarehouse applications called Sybase IQ that can be configured to run in a shared disk mode.
"Shared-nothing" architectures share neither disk nor memory between nodes in the cluster. This is in contrast to "shared-everything" architectures, which share both.
Competitive products offering shared-nothing architectures include:
- MySQL Cluster (Oracle Corporation has owned MySQL since 2009) 
- HP NonStop
- IBM InfoSphere Warehouse editions that include the Database Partitioning Feature (formerly known as DB2 Extended Enterprise Edition)
- Oracle NoSQL Database
- Netezza (aka. Netezza Performance Server)
- Lexst Database Cluster
- Couchbase Server
- SAP HANA
See also 
- Oracle RAC Documentation
- Oracle Real Application Clusters (official website from Oracle Corporation)
- A Step-By-Step Project Guide for Implementing Oracle RAC
- Oracle Real Application Clusters and Industry Trends in Cluster Parallelism and Availability
- Gopalakrishnan, K : Oracle Database 10g Real Application Clusters Handbook. McGraw-Hill Osborne Media (Oracle Press): 2006.
- Options and Packs
- Oracle Database 11g: Product Editions & Features
- Introduction to Oracle Real Application Clusters
- Mensah, Kuassi (2006). Oracle database programming using Java and Web services. Digital Press. p. 1087. ISBN 978-1-55558-329-3. Retrieved 2011-09-11. "The Fast Application Notification (FAN) mechanism [...] allows the rapid detection of "
Instance DOWN" or "
Node DOWNevents [...]"
- "Oracle Real Application Clusters One Node: Better Virtualization for Databases". Find White Papers. 2009-12-09. Retrieved 2010-04-19. "Oracle RAC One Node provides: . Always on single-instance database services . Better consolidation for database servers . Enhanced server virtualization . [,,,] should the need arise, upgrade to a full multi-node Oracle RAC database without downtime or disruption. [...] Oracle Real Application Clusters (RAC) One Node is a new option to Oracle Database 11g Release 2 Enterprise Edition. It provides enhanced high availability for singleinstance databases,"
- "clustering". Oracle.com. Retrieved 2012-11-07.
- Thomas, Bryan (2006-05-30). "Solutions for Highly Scalable Database Applications: An analysis of architectures and technologies" (PDF). Microsoft. Retrieved 2007-09-09.
- "Sybase.com". Sybase.com. Retrieved 2012-11-07.
- "Oracle buys Finnish open-source developer". InfoWorld. October 7, 2005. "Oracle Buys SUN; MySQL is Forked". Linux Magazine. April 20, 2009.