Oracle RAC has always had a latency issue when the RAC nodes are geographically distributed.  In these cases, many shops use Oracle Streams from back-and-forth replication. RAC on Extended Distance Clusters seeks to eliminate those latency issues by providing fast (sub-minute) recovery from a site failure using Dark Fiber (DWDM or CDM) technology. 

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In this architecture, all nodes at all sites are allowed to be part of a single database cluster.  Due to the inherent advantages provided by RAC on Extended Distance Clusters, interest in the architecture has been on the rise, however it is important to know what situations Extended Distance Clusters are ideal for, especially considering the impact of distance and latency, as well as the complexity of the implementation. 

The practical limitations for RAC on Extended Distance Clusters stem from the high impact of latency, and thus distance.   The ideal environment for implementing this architecture is where the datacenters are less than 25km apart, and where direct cables with dedicated channels between the sites have already been set up.  Due to the impact of distance on performance, it is unlikely that RAC on Extended Distance Clusters will be implemented on any distances greater than 25km.