Oracle RAC

What is Oracle RAC?  

Real Application Cluster otherwise knows as RAC is a software component that provides high availability database instances to the consuming applications.

How it works? 

It is as simple as having multiple instance of the same server/nodes with similar configurations.  Oracle RAC takes charge of managing the redirection of requests to the working server.

What it provides?  

Prior version of Oracle 10G RAC only provides failover, if one node goes down, it redirects to the other node and constantly ping the nodes to redirect the request but it does not provide load balancing, i.e. balancing the traffic by sending request  to multiple nodes at the same time.  From Oracle 10 G onwards it supports both failover as well as load balancing - Load  balancing can be done either through round-robin, or resources consumption (mainly CPU and memory).

How to achieve Load balancing?  

There are several software/hardware components that are available to manage Load Balancing on top of RAC, F5 Switch is one that can be used to do the load balancing , Apache web server uses round robin to manage the load balancing but this is done via the Apache web server configuration.

Oracle RAC architecture provides another feature called Cache Fusion, the fundamental concept behind this feature is to provide database cache and share them across the nodes. By doing this, the database changes happening on the primary nodes are replicated across other nodes and in the event of the node failure, other nodes can server with updated data.

Benefits of RAC?

High Availability of database

Scalability

Failover