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The Single Client Access Name (SCAN) is a fully qualified name (hostname+domain) that is configured to resolve to all the addresses allocated for the SCAN. The addresses resolve using Round Robin DNS either on the DNS server, or within the cluster in a GNS configuration. SCAN listeners can run on any node in the cluster.
Oracle Database 11g release 2 and later instances only register with SCAN listeners as remote listeners. Upgraded databases register with SCAN listeners as remote listeners, and also continue to register with all node listeners.
SCANs run on the cluster, not on a cluster node. SCANs provide location independence for the databases, so that client configuration does not have to depend on which nodes are running a particular database. For example, if you configure policy managed server pools in a cluster, then the SCAN allows connections to databases in these server pools regardless of which nodes are allocated to the server pool.
SCANs function like a node virtual IP (VIP) address. However, SCANs are resolved to SCAN VIPs that run on any node in the cluster, so unlike a virtual IP address for a node, clients connecting to the SCAN no longer require updated virtual IP addresses as nodes are added to or removed from the cluster. 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.
During Oracle grid infrastructure installation, SCAN listeners are created for as many IP addresses as there are addresses assigned to resolve to the SCAN. Oracle recommends that the SCAN resolves to three addresses, to provide high availability and scalability. If the SCAN resolves to three addresses, then there are three SCAN listeners created.
Oracle RAC provides failover with the node VIP addresses by configuring multiple listeners on multiple nodes to manage client connection requests for the same database service. If a node fails, then the service connecting to the VIP is relocated transparently to a surviving node, enabling fast notification of the failure to the clients connecting through the VIP. If the application and client are configured with transparent application failover options, then the client is reconnected to the surviving node. Oracle Clusterware routes client application requests from the SCAN to the VIP on the least loaded instance providing the service.
Clients configured to use IP addresses for Oracle Database releases prior to Oracle Database 11g release 2 can continue to use their existing connection addresses; using SCANs is not required. When you upgrade to Oracle Clusterware 11g release 2 (11.2), the SCAN becomes available, and you should use the SCAN for connections to Oracle Database 11g release 2 or later databases. When an earlier version of Oracle Database is upgraded, it is registered with the SCAN, and clients can start using the SCAN to connect to that database.
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