JDBC Connections Using SCAN Fail With ORA-12516

Symptoms

On : 19.7.0.0.0 version, RDBMS
listener_scan.log:

2024-07-31T02:00:00.006290+08:00
31-JUL-2024 02:00:00 * (CONNECT_DATA=(CID=(PROGRAM=JDBC Thin Client)(HOST=jdbc)(USER=ptdb))(SERVICE_NAME=ptdb)) * (ADDRESS=(PROTOCOL=tcp)(HOST=192.168.12.27)(PORT=47898)) * establish * ptdb * 12516
TNS-12516: TNS:listener could not find available handler with matching protocol stack
31-JUL-2024 02:00:00 * (CONNECT_DATA=(CID=(PROGRAM=JDBC Thin Client)(HOST=jdbc)(USER=ptdb))(SERVICE_NAME=ptdb)) * (ADDRESS=(PROTOCOL=tcp)(HOST=192.168.12.27)(PORT=47900)) * establish * ptdb * 12516
TNS-12516: TNS:listener could not find available handler with matching protocol stack
2024-07-31T02:00:01.010650+08:00
31-JUL-2024 02:00:01 * (CONNECT_DATA=(CID=(PROGRAM=JDBC Thin Client)(HOST=jdbc)(USER=ptdb))(SERVICE_NAME=ptdb)) * (ADDRESS=(PROTOCOL=tcp)(HOST=192.168.12.27)(PORT=47910)) * establish * ptdb * 12516
TNS-12516: TNS:listener could not find available handler with matching protocol stack
31-JUL-2024 02:00:01 * (CONNECT_DATA=(CID=(PROGRAM=JDBC Thin Client)(HOST=jdbc)(USER=ptdb))(SERVICE_NAME=ptdb)) * (ADDRESS=(PROTOCOL=tcp)(HOST=192.168.12.27)(PORT=47912)) * establish * ptdb * 12516
TNS-12516: TNS:listener could not find available handler with matching protocol stack
....................

06-AUG-2024 04:06:37 * (CONNECT_DATA=(CID=(PROGRAM=JDBC Thin Client)(HOST=jdbc)(USER=ptdb))(SERVICE_NAME=ptdb)) * (ADDRESS=(PROTOCOL=tcp)(HOST=192.168.12.31)(PORT=52314)) * establish * ptdb * 12516
TNS-12516: TNS:listener could not find available handler with matching protocol stack
06-AUG-2024 04:06:37 * (CONNECT_DATA=(CID=(PROGRAM=JDBC Thin Client)(HOST=jdbc)(USER=ptdb))(SERVICE_NAME=ptdb)) * (ADDRESS=(PROTOCOL=tcp)(HOST=192.168.12.31)(PORT=52316)) * establish * ptdb * 12516
TNS-12516: TNS:listener could not find available handler with matching protocol stack
06-AUG-2024 04:06:37 * (CONNECT_DATA=(CID=(PROGRAM=JDBC Thin Client)(HOST=jdbc)(USER=ptdb))(SERVICE_NAME=ptdb)) * (ADDRESS=(PROTOCOL=tcp)(HOST=192.168.12.31)(PORT=52320)) * establish * ptdb * 12516
TNS-12516: TNS:listener could not find available handler with matching protocol stack
2024-08-06T04:06:38.265467+08:00

Cause

Based on the scan listener logs and VIP listener logs, it is observed that the TNS-12516 errors started appearing on July 31st.
The customer stated they did not make any changes.
The TNS-12516 errors occur in the scan listener logs, while the VIP listener logs show no issues.
The errors are concentrated on the PDB: ptdb.
The scan listener logs indicate that connections to other PDBs are normal.
Occasionally, successful connections to PDB: ptdb are still observed in the scan listener logs.
The primary application server IPs are: 192.168.12.27/31.
Neither the database PROCESS nor SESSION parameters have reached their maximum limits.
The customer uses a load balancer device between the application servers and database servers.

The JDBC driver version is: 11.2.0.4.
This version is relatively old and is known to cause issues such as JDBC Connections Using SCAN Fail With ORA-12516 Or ORA-12520 (Doc ID 1555793.1).

Additionally, the alert.log in your environment also contains errors like:
ORA-03137: malformed TTC packet from client rejected: [3146] [94] [] [] [] [] [] [].
This error is related to ORA-03137: malformed TTC packet from client rejected: [3146] [94] [] [] [] [] [] [] While Using JDBC 12.2.0.1 (Doc ID 2519886.1).
Upgrading the JDBC version is also required to resolve this issue.

Solution

Upgrade the JDBC version to 19c or a later version.

Ref:
https://www.oracle.com/database/technologies/appdev/jdbc-downloads.html
Oracle Database 19c (19.24.0.0) JDBC Driver & UCP Downloads - Long Term Release ««««««<

References

ORA-03137: malformed TTC packet from client rejected: [3146] [94] [] [] [] [] [] [] While Using JDBC 12.2.0.1 (Doc ID 2519886.1)
JDBC Connections Using SCAN Fail With ORA-12516 Or ORA-12520 (Doc ID 1555793.1)