How is Oracle 11g SCAN implemented?

For high availability purposes the SCAN name should be associated with at least three IP addresses using DNS round-robin resolution. If you opt to use Grid Naming Service then GNS can also be used to manage the SCAN name.

SCAN is configured at a cluster level not at the node level, that’s what makes it so flexible — no mater how many nodes your clusters consists of, your clients can continue to use SCAN to access the services of your cluster utilizing all nodes even if you add or delete them:

The SCAN is a virtual IP name, similar to the names used for virtual IP addresses, such as node1-vip. However, unlike a virtual IP, the SCAN is associated with the entire cluster, rather than an individual node, and associated with multiple IP addresses, not just one address.

SCAN works as an independent handler for the entire cluster — it acts on client’s behalf during connection request since it knows all cluster services and it’s available, least loaded nodes:

The SCAN works by being able to resolve to multiple IP addresses reflecting multiple listeners in the cluster handling public client connections. When a client submits a request, the SCAN listener listening on a SCAN IP address and the SCAN port is contracted on a client’s behalf. Because all services on the cluster are registered with the SCAN listener, the SCAN listener replies with the address of the local listener on the least-loaded node where the service is currently being offered. Finally, the client establishes connection to the service through the listener on the node where service is offered. All of these actions take place transparently to the client without any explicit configuration required in the client.

Bottom line – use SCAN – it simplifies cluster management:

Because the SCAN addresses resolve to the cluster, rather than to a node address in the cluster, nodes can be added to or removed from the cluster without affecting the SCAN address configuration.