What is RAC? What is the benefit of RAC over single instance database?In Real Application Clusters environments, all nodes concurrently execute transactions against the same database. Real Application Clusters coordinates each node's access to the shared data to provide consistency and integrity.Benefits:Improve response timeImprove throughputHigh availabilityTransparencyWhat is Oracle RAC One Node?Oracle RAC one Node is a single instance running on one node of the cluster while the 2nd node is in cold standby mode. If the instance fails for some reason then RAC one node detect it and restart the instance on the same node or the instance is relocate to the 2nd node incase there is failure or fault in 1st node. The benefit of this feature is that it provides a cold failover solution and it automates the instance relocation without any downtime and does not need a manual intervention. Oracle introduced this feature with the release of 11gR2 (available with Enterprise Edition).Real Application ClustersOracle RAC is a cluster database with a shared cache architecture that overcomes the limitations of traditional shared-nothing and shared-disk approaches to provide a highly scalable and available database solution for all your business applications. Oracle RAC provides the foundation for enterprise grid computing.Oracle's Real Application Clusters (RAC) option supports the transparent deployment of a single database across a cluster of servers, providing fault tolerance from hardware failures or planned outages. Oracle RAC running on clusters provides Oracle's highest level of capability in terms of availability, scalability, and low-cost computing.One DB opened by multipe instances so the the db ll be Highly Available if an instance crashes.Cluster Software. Oracles Clusterware or products like Veritas Volume Manager are required to provide the cluster support and allow each node to know which nodes belong to the cluster and are available and with Oracle Cluterware to know which nodes have failed and to eject then from the cluster, so that errors on that node can be cleared.Oracle Clusterware has two key components Cluster Registry OCR and Voting Disk.The cluster registry holds all information about nodes, instances, services and ASM storage if used, it also contains state information ie they are available and up or similar.The voting disk is used to determine if a node has failed, i.e. become separated from the majority. If a node is deemed to no longer belong to the majority then it is forcibly rebooted and will after the reboot add itself again the the surviving cluster nodes.Advantages of RAC (Real Application Clusters)Reliability if one node fails, the database won't failAvailability nodes can be added or replaced without having to shutdown the databaseScalability more nodes can be added to the cluster as the workload increasesWhat is a virtual IP address or VIP?A virtual IP address or VIP is an alternate IP address that the client connections use instead of the standard public IP address. To configure VIP address, we need to reserve a spare IP address for each node, and the IP addresses must use the same subnet as the public network.What is the use of VIP?If a node fails, then the node's VIP address fails over to another node on which the VIP address can accept TCP connections but it cannot accept Oracle connections.Give situations under which VIP address failover happens:-VIP addresses failover happens when the node on which the VIP address runs fails, all interfaces for the VIP address fails, all interfaces for the VIP address are disconnected from the network.Using virtual IP we can save our TCP/IP timeout problem because Oracle notification service maintains communication between each nodes and listeners.What is the significance of VIP address failover?When a VIP address failover happens, Clients that attempt to connect to the VIP address receive a rapid connection refused error .They don't have to wait for TCP connection timeout messages.What is voting disk?Voting Disk is a file that sits in the shared storage area and must be accessible by all nodes in the cluster. All nodes in the cluster registers their heart-beat information in the voting disk, so as to confirm that they are all operational. If heart-beat information of any node in the voting disk is not available that node will be evicted from the cluster. The CSS (Cluster Synchronization Service) daemon in the clusterware maintains the heart beat of all nodes to the voting disk. When any node is not able to send heartbeat to voting disk, then it will reboot itself, thus help avoiding the split-brain syndrome.For high availability, Oracle recommends that you have a minimum of three or odd number (3 or greater) of votingdisks.Voting Disk is file that resides on shared storage and Manages cluster members. Voting disk reassigns cluster ownership between the nodes in case of failure.The Voting Disk Files are used by Oracle Clusterware to determine which nodes are currently members of the cluster. The voting disk files are also used in concert with other Cluster components such as CRS to maintain the clusters integrity.