Oracle Real Application Clusters One Node (Oracle RAC One Node) is a single database instance of Oracle Real Application Clusters (Oracle RAC) that runs on one node in a cluster. RAC One Node provides high availability by eliminating the database server as a single point of failure and takes further advantage of clustering to apply rolling patches and database service relocation without incurring significant downtime. Oracle RAC One Node is easily upgradable to a full multi-instance Oracle Real Application Clusters configuration.
The database instance can be relocated either manually or automatically to another node in the cluster while maintaining application continuity. RAC One node instances may co-exist with other Oracle RAC database instances in the same cluster, assuming there are enough servers with sufficient resources to service all the instances.
The main difference between Oracle RAC One Node and Oracle RAC is that with an Oracle RAC One Node Database there is only one instance running at a time under normal operation. Should this one instance be impacted by planned or unplanned downtime, the instance relocates to another server.