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Oracle tns_admin tips

Oracle Database Tips by Donald BurlesonJanuary 1, 2015

Question: What is the tns_admin parameter and how do I use tns_admin to define a common location for my TNS files?

Answer:  The tns_admin parameter determines the location of the tns administration files (e.g. tnsnames.ora and listener.ora, etc.) 

The tns_admin parameter is super useful on servers with many databases on different release levels (hence different $ORACLE_HOME directories) because all databases tns_admin values can be set to a single location, allowing a single set of SQL*Net files for all databases on the server. 

Without the tns_admin variable, each instance would have it's own set of TNS files in the $ORACLE_HOME/network/admin directory.

Search path steps with tns_admin

The tns_admin defines a list of directories to be searched to find the tnsnames.ora files.  SQL*Net configuration files are resolved in this precedence:

1 - Search for SQL*Net files in present working directory (PWD/CWD)

2 - Set tns_admin for each session or by a user-defined script

3 - Set tns_admin as a global environment variable (in kshrc, cshrc, bashrc, etc.)

4 - tns_admin is defined in the registry (In Windows only)

5 - Search the default location for SQL*Net files in the  $ORACLE_HOME/network/admin directory location (default).

There should be only one tnsnames.ora per server, no matter how many instances reside on it!

Using tns_admin in Windows

In windows, look for the tns_admin registry entry. If  tns_admin is set then you should be able to use a network alias, not the full "(DESCRIPTION..." connect string. The TNS_ADMIN parm tells Oracle clients where to find the tnsnames.ora file.

Also, make sure that  tns_admin is set in your DOS $PATH variable:

C:> set TNS_ADMIN=$PATH;ORACLE_HOME\network\admin

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