adstrtal.sh: Database connection could not be established. Either the database is down or the APPS credentials supplied are wrong

Problem:
While trying to start application on middle tier, I was getting below error: $ ./adstrtal.sh apps/<apps password>
adstrtal.sh version 115.19
adstrtal.sh: Database connection could not be established. Either the database is down or the APPS credentials supplied are wrong.
USAGE:  adstrtal.sh <appsusername/appspassword>
adstrtal.sh: exiting with status 1

Cause:  

• The database listener may not be up and running and please check if you can connect to the database remotely as apps/apps.
• Also, check is the apps password is correct 
• Please make sure you source the Correct environment file before using adstrtal.sh
  

Possibilities:
In our case, both the database and listener were running fine. So, it might be something else which is not letting us start the applications.
I invoked APPSORA.env to set the environment, and tried to start application again but all in vain still the same issue.

Now, when I'm trying to connect to database from CM node via SQL*Plus .. I got ORA-12545 error
 $ sqlplus
SQL*Plus: Release 8.0.6.0.0 - Production on Sat Mar 15 08:25:58 2014
(c) Copyright 1999 Oracle Corporation.  All rights reserved.
Enter user-name: apps
ERROR:
ORA-12545: Connect failed because target host or object does not exist

Alright, it seems easier to troubleshoot now as we know 

ORA-12545 is a generic error as a result of TNS configuration issue at client.

TNS configuration issue is unable to resolve host / IP specified in TNS entry

We can analyze what exactly is wrong by performing below steps

1. ping  <DB_HOST/IP>

2. telnet <DB_HOST/IP> <PORT>

3. tnsping <TNS_Alias/SID>

4. sqlplus apps@<TNS_Alias>/<apps_password>

Checks 1 and 2 were  successful when ORA-12545 is faced, but 3 and 4 were not working and making them successful will resolve the error.

Solution:

I would recommend you to follow the below steps to overcome the issue. 

• Bounce or reload listener on DB node
• Check if all services are down on application node, no process should be running. 
• Run cmclean -- on Concurrent Processing node - CMCLEAN.SQL - Non Destructive Script to Clean Concurrent Manager Tables [ID 134007.1] 
• Run autoconfig on db tier -- make sure it completes successfully. 
• Run autoconfig on all apps tier -- make sure it completes successfully. 
• Now, try to start applications on middle tier.

                   ++++++++++++++XXXXX+++++++++++++++
Please see the following docs, if the above fix doesn't work.
R12: "adstrtal.sh: Database connection could not be established" Error When Running Script adstrtal.sh and adstpall.sh [ID 1492611.1]
R12 Adstrtal.sh Fails with "Database Connection Could Not Be Established error starting all services on the target" [ID 1499837.1]

HTH

Thanks for reading.