What is a listener in Oracle Database?

Oracle Listeners: Your Database's Watchman

In the world of Oracle databases, a listener acts like a doorman, managing incoming connection requests from clients. It's a separate process that resides on the database server, patiently waiting for connections. Without a listener, clients can't communicate with the database.

ORA-12170 TNS-12535 TNS-12560 TNS-00505 - Oracle Listener errors & Solution

Configuring Listener can be a tough job on a rare day & frustrating enough to pull your hair out. Especially when you are facing the below errors.

ORA-12170: TNS:Connect timeout occurred

OR

Connecting to (DESCRIPTION=(ADDRESS=(PROTOCOL=TCP)(HOST=oravm19c.practice.com)(PORT=1521)))
TNS-12535: TNS:operation timed out
 TNS-12560: TNS:protocol adapter error
  TNS-00505: Operation timed out

Instance Status RESTRICTED in Oracle Listener

When you don't want any user to connect to database, you can simply restrict your instance from any incoming connection. But if you don't know this concept and you have come across the below error, it means your database is in sort of maintenance mode and you need to disable it to enable users to connect to it via listener.

Note: Maintenance mode still allows users to connect to database without listener, of course you can only do it when you are on DB server and you are not using listener for DB connection i.e. no use of @SID.

ORA-12154: TNS: could not resolve the connect identifier specified

When listener is not aware of your database, you might come across the below error when trying to connect to a database.

PS C:\Users\parvi> sqlplus sys@practice as sysdba

SQL*Plus: Release 19.0.0.0.0 - Production on Tue Jan 7 21:16:26 2025
Version 19.3.0.0.0

Copyright (c) 1982, 2019, Oracle.  All rights reserved.

Enter password:
ERROR:
ORA-12154: TNS:could not resolve the connect identifier specified

You'll need to make your instance known to listener in this case, there are multiple ways to do this, but I'll specify the 2 easiest ones which will help you fix this error.