Real application clustering is a method where multiple instances of oracle accessing a single data store simultaneously, with cache synchronization according to oracle global cache management technology (cache fusion).
Why actually oracle RAC is needed is worth considering. Without RAC in an environment where applications busy accessing the physical database, there is a higher chance of failure. And the reliability factor as well as the scalability factor are also considerably low. Because of these needs, RAC emerged where it could bring about a solution to the above mentioned.
Architecture is arranged in such a way that multiple nodes access the physical data sore concurrently, which can be exceeded up to eight nodes. This provides a complete clustering solution where data between applications are synced, monitored and manged. And it also restart the oracle instances.
In the oracle cluster any node is allowed to update data on the single physical database, SAN (Storage Area Network). The real advantage of this is all the nodes access the same set of data which is updated and changed live. In almost all the value critical applications such as bank systems, this is really important feature since live data is needed for processing.
Since maintaining a single database, it makes the management and administration of the database easy. Even though it shares the data store, in each database servers, it has it's own, application, logs, operating system, and database instance. What is shared here is the all the data and meta data which affects the real data.
RAC is capable of handling fail overs and load balancing as well. In an situation of failure, it can balance the load among the other servers and users won't feel any difference in the service offered to them.
Cache fusion is also another important feature associated with RAC in which instance's cache is copied to a global cache as any node can access the information fast. This allows any server to get the most updated data without actually going to the physical disk. This is like information having in cache memory of a computer.
Benefits of this RAC technology includes, reliability, recoverability, error detection and continuous operation which comes under the main category of high availability and also it supports scalability.
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