Describe principle~
The real application clusters database highly available and extensible.Failure of one node in the cluster does not affect customer usage until the last node in cluster fails.
Defects~
The high cost.Each node has a separate oracle license and also needs a rac license.
The high speed interconnection requirements make the nodes close together, which means that a natural disaster may destroy the whole cluster.

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The heardware configuration：
For RAC database,include two or more nodes. Redundant power supply,network card,dual CPU and error correction memory.
Shared disks should also have built-in hardware redundancy -multiple power supplies,raid-enabled disks,and so on.

The software configuration:
Cluster Ready Service,CRS

The network configuration:
Each node in RAC has at least three IP addresses.
Public network:All normal connections between the node and the server.
Private network:Intra-cluster node communication.
Virtual IP:Listen for addresses,and support rapid connect-time failover.
Public and private networks are best bond,providing redundancy and bandwidth.

Summary:This blog birefly describes RAC.
NOTE:The next blog will describe the RAC process.