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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