About Connecting to an Oracle RAC Database Using SCANs
Oracle recommends that you configure Oracle RAC database clients to use the SCAN to connect to the database instead of configuring the tnsnames.ora file.
Clients configured to connect to the cluster using node VIP addresses for Oracle RAC releases earlier than Oracle Database 11g Release 2 can continue to use their existing connection addresses. Using the SCAN is not required. When an earlier release of Oracle Database is upgraded, the database is not only registered with the local listeners, but is also registered with the SCAN listeners, allowing clients to start using the SCAN to connect to that database.
If the SCAN is resolved by DNS, then DNS returns all three SCAN VIP addresses to the client. If the SCAN is resolved by GNS, then DNS zone delegation sends the lookup request to GNS, which then returns all three SCAN VIP addresses to the client.
Oracle Database 19c database clients use SCAN to connect to the database. Oracle recommends against using the easy connect method with SCAN because the easy connect method does not have the ability to specify timeouts and retries for connection establishment. Instead, applications must use an Oracle Net connect descriptor with the following format:
(DESCRIPTION =
(CONNECT_TIMEOUT=90) (RETRY_COUNT=20)(RETRY_DELAY=3) (TRANSPORT_CONNECT_TIMEOUT=3)
( ADDRESS = (PROTOCOL = TCP)(HOST=scan)(PORT=1521))
(CONNECT_DATA=(SERVICE_NAME=service_name)))scan represents the SCAN for your cluster. If you do not specify a port number, then the default value of 1521 is used for the TCP port identifier. The service_name is the name of a dynamic database service.
The client then uses one of the returned SCAN VIP addresses to contact a SCAN listener. When a SCAN listener receives a connection request from a client, the SCAN listener identifies the least loaded instance in the cluster that provides the requested service. It then redirects the connection request to the local listener on the node where the least loaded instance is running, and the client is given the local listener address. The local listener then creates the connection to the database instance.
Example 6-1 Connecting to Oracle RAC Using an Oracle Net Connect Descriptor
If the Oracle RAC database runs on a cluster for which the SCAN is sales1-scan.mycluster.example.com, then you can submit a connection request for the database service oltp.example.com by using a connect descriptor similar to the following:
(DESCRIPTION =
(CONNECT_TIMEOUT=90) (RETRY_COUNT=20)(RETRY_DELAY=3) (TRANSPORT_CONNECT_TIMEOUT=3)
( ADDRESS = (PROTOCOL = TCP)(HOST=sales1-scan.mycluster.example.com)(PORT=1521))
(CONNECT_DATA=(SERVICE_NAME=oltp.example.com)))If the SCAN is resolved by DNS, then DNS returns all three SCAN VIP addresses to the client. If the SCAN is resolved by GNS, then DNS zone delegation sends the lookup request to GNS, which then returns all three SCAN VIP addresses to the client. The client then uses one of the returned SCAN VIP addresses to contact a SCAN listener.
When a SCAN listener receives a connection request from a client, the SCAN listener identifies the least loaded instance in the cluster that provides the requested service. It then redirects the connection request to the local listener on the node where the least loaded instance is running, and the client is given the local listener address. The local listener then creates the connection to the database instance.