Extended Distance Clusters
From Oracle FAQ
Oracle RAC (Real Application Clusters) can be deployed on extended distance or stretched clusters using dark fiber (DWDM or CWM) technology.
Benefits and drawbacks
The main benefit of using RAC on Extended Clusters is that it provides faster recovery from site failure than any other technology on the market - work can simply continue on the remaining site.
However, RAC requires low latency making it unpractical to deploy nodes further than 25km apart. Performance are considered good for nodes up to 25km, mediocre for 50km, and weak for 100km and further.
Another drawback is the high cost of establishing a DWDM network.
- Real Application Clusters - base technology