How connection load balancing works with SCAN. Comparing Oracle's and Progress's JDBC Oracle driver.

From: https://docs.oracle.com/database/121/JJDBC/scan.htm#JJDBC29161 

How Connection Load Balancing Works Using the SCAN 

For clients connecting using Oracle SQL*Net, three IP addresses are received by the client by resolving the SCAN name through DNS. The client then goes through the list that it receives from the DNS and tries connecting through one of the IP addresses in the list. 

If the client receives an error, then it tries connecting to the other addresses before returning an error to the user or application. This is similar to how client connection failover works in earlier Database releases, when an address list is provided in the client connection string. 

When a SCAN Listener receives a connection request, the SCAN Listener checks for the least loaded instance providing the requested service. It then re-directs the connection request to the local listener on the node where the least loaded instance is running. 

Subsequently, the client is given the address of the local listener. The local listener then finally creates the connection to the Database instance. 

The Progress Connect driver for Oracle should be working similarly to this with a minor exception. Similar to the Oracle Thin driver URL, the Progress driver URL should be configured to use the Oracle RAC SCAN name. This will be resolved to 3 IP addresses during the full DNS lookup (InetAddress.getAllByName(host)). 

When this look up occurs in the connection process is controlled by the setting of the LoadBalancing connection option. If load balancing is disabled (the default), then the driver will not do the DNS lookup up right away and will instead resolve the host name in the URL using InetAddress.getByName(host). 

This returns a single IP address. If the socket create to that IP address fails, then the driver will obtain the list of all IP addresses associated with the host name and randomly choose from these values in its attempt to establish a socket. 

If the socket establishment continues to fail, then the driver will continue to try all IP addresses returned by the DNS look up. 

If load balancing is enabled, then the driver will instead do the DNS lookup first and then randomly select one of the three IP addresses to use for establishing the socket. If the connection fails, the driver will try each of the IP addresses in the list until the connection succeeds or it has tried all the addresses. 

In both cases, the IP addresses are not cached. If the address has been tried (with failure or success), then the address will be removed from the list. If connection retry count is set, then each time the driver framework calls into the driver, the driver will do the IP address lookup again.