ORA-12519: TNS:no appropriate service handler found (4257715)

When adding Oracle RAC databases in Foglight for Oracle, the following error is generated for one of the RAC nodes:

Listener refused the connection with the following error: ORA-12519, TNS:no appropriate service handler found.

1. Ensure that you can perform a SQLPlus and tnsping against the failing node from a remote client machine.
2. Run the following on the Oracle node host and ensure that the results do not indicate any errors (refer to Oracle documentation for details):
   lsnrctl services
   lsnrctl status
3. Compare the tnsnames.ora and listener.ora between the 2 nodes and identify if there is a significant difference between the two that may have contributed to the issue.
4. When attempting to add the agents on Foglight, try using the IP addresses of the RAC nodes instead of their hostnames.  If that works, then edit the ../etc/hosts file of the FglAM host to include the RAC nodes' IP and hostnames.