TNS-12154 on Database Link When Connected Through Listener

I ran into a problem when creating a database link that uses a new TNS entry. The database link seemed to work fine when I created it, but the developers complained about getting TNS-12154 errors when they tried to use it. Checked it again on my side, and the database link worked just fine.

 

After some discussion with the development team, we noticed that the database did work when I connected locally from the database server, but it threw the TNS-12154 when connecting through the listener. So I focussed on why the listener prevented the database link from recognizing my newly created TNS entry. Turns out it has a lot to do with setting a TNS_ADMIN environment variable, the dynamic registration to the listener and the way the listener uses environment variables.

 

So, to paint a picture of the situation:

·        We have a database on Linux (probably any Linux or UNIX variant will have this problem)

·        We have set the TNS_ADMIN environment variable

·        The listener uses both static and dynamic service registration (because we are using OEM)

 

Solutions proposed so far

 

This problem is described in a lot of places, but I could not find a satisfactory solution until recently. The answers I found so far are not satisfactory, some more so than others. They include:

·        Restart the database. When creating a new TNS entry, this will not been seen by the dynamic service registration to the listener. Restarting the database somehow resets this and then the database link also “magically” works.

o   My problem with this: Restarting a (Production) database is not something I want to do when I just want to add a new TNS entry for a database link.

·        Use a different naming method when creating the database link. Instead of the USING clause with a TNS entry, you can specify an EZConnect string or the full description of the TNS entry. Both imply that server and database specific information is to be used in the database link itself.

o   But: I use the tnsnames.ora to have a central location for all descriptions and use that to point to any database/service I need. I don’t want to repeat sevrer/instance information in every database link I create.

·        Set the ENVS clause of the static registration (SID_LIST_LISTENER) to include TNS_ADMIN.

o   This has several drawbacks: You need to use static registration and static registration only. Dynamic takes precedence over Static, and Dynamic cannot be used to set TNS_ADMIN. Even when you decide to use only Static, then you have to maintain the TNS_ADMIN in the environment variable itself and in the listener.ora which means extra maintenance.

·        Don’t use TNS_ADMIN. When not using TNS_ADMIN, all works fine. As the database and the listener will then both use the default location ($ORACLE_HOME/network/admin), they can always find the tnsnames.ora and will pick up new entries right away.

o   The problem is obvious: we use TNS_ADMIN to create a centralized location for our tnsnames.ora file. Not using the TNS_ADMIN would defeat this initial setup.

 

My solution

 

The last “solution”, however, opens up a final solution. When the listener (the dynamic service registration) fails to pick up (or refresh, or whatever you like to call it) changes to tnsnames.ora in the TNS_ADMIN location, it will look at the default location. Creating an OS link there will allow the listener to find the file anyway and use the new TNS entry without having to restart the database. And yes, there is a drawback to this solution as well: you need to create this link in every RDBMS home you created and point it to the TNS_ADMIN location. I think this is still slightly better than not using TNS_ADMIN at all, because we will still have a centralized tnsnames.ora file, and only use links to that file in the RDBMS home directory.

 

Technical “proof”

 

The example sketched below is from an AIX machine, using an 11gR2 database. I also tested this on a 12c database on Linux. Both have the exact same results when it comes to recognizing the new TNS entry.

 

Using the original setup, I modified the tnsnames.ora file in the TNS_ADMIN location:

 

dbserver::../home/oracle:> cd $TNS_ADMIN

dbserver::../oracle/tns:> ls -l

total 16

-rw-r--r--    1 oracle   dba             483 Dec 16 16:37 listener.ora

-rw-r--r--    1 oracle   dba            1581 Dec 16 16:14 tnsnames.ora

dbserver::../oracle/tns:> vi tnsnames.ora

 

I added an “orcldb_self_ref.world” entry, pointing toward my original database. After this, without restarting the database or reloading the listener, I created the database link.

 

dbserver::../oracle/tns:> sqlplus system@orcldb

 

SQL*Plus: Release 11.2.0.2.0 Production on Thu Dec 17 09:41:34 2015

Copyright (c) 1982, 2010, Oracle.  All rights reserved.

Enter password:

Connected to:

Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production

With the Partitioning, OLAP, Data Mining and Real Application Testing options

 

SQL> create database link dblink_test_erik using 'orcldb_self_ref.world';

 

Database link created.

 

SQL> select * from dual@dblink_test_erik;

select * from dual@dblink_test_erik

                   *

ERROR at line 1:

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

 

And there you have it, the TNS-12154. Normally, you would think that we did something wrong defining the TNS entry or something like that. However, when not connecting through the listener, the same database link does work:

 

dbserver::../oracle/tns:> sqlplus system

 

SQL*Plus: Release 11.2.0.2.0 Production on Thu Dec 17 09:41:34 2015

Copyright (c) 1982, 2010, Oracle.  All rights reserved.

Enter password:

Connected to:

Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production

With the Partitioning, OLAP, Data Mining and Real Application Testing options

 

SQL> select * from dual@dblink_test_erik;

 

D

-

X

 

So, on to the next part, creating an OS link:

 

dbserver::../home/oracle:> cd $ORACLE_HOME/network/admin

dbserver::../network/admin:> ln -s $TNS_ADMIN/tnsnames.ora ./tnsnames.ora

dbserver::../network/admin:> ls -l

total 8

drwxr-xr-x  2 oracle dba 256 Oct 23 09:49 samples

-rw-r--r--  1 oracle dba 187 May 07 2007  shrept.lst

lrwxrwxrwx  1 oracle dba  28 Dec 17 09:45 tnsnames.ora -> /u01/oracle/tns/tnsnames.ora

 

Directly after creating the link, our database link works as it should. Just to prove it works right away, I dropped and re-created it.

 

dbserver::../oracle/tns:> sqlplus system@orcldb

 

SQL*Plus: Release 11.2.0.2.0 Production on Thu Dec 17 09:49:10 2015

Copyright (c) 1982, 2010, Oracle.  All rights reserved.

Enter password:

Connected to:

Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production

With the Partitioning, OLAP, Data Mining and Real Application Testing options

 

SQL> drop database link dblink_test_erik;

 

Database link dropped.

 

SQL> create database link dblink_test_erik using 'orcldb_self_ref.world';

 

Database link created.

 

SQL> select * from dual@dblink_test_erik;

 

D

-

X

 

Note that when you are already connected, it might be necessary to re-connect to pick up the changes. But that beats restarting the entire database...

 

Final remarks

 

Though this is my preferred solution, it doesn’t mean that it is the only solution. If any of the other options suits your needs better, then by all means: use them ;-)

 

This solution uses an existing setup with TNS_ADMIN set and dynamic service registrations. Both good things in my opinion. Then, by creating a simple link, you can make sure that any changes to your TNS entries are useable right away, instead of having to configure other files or restarting the database.