When you, or more accurately an application program, attempts to establish a connection to an Oracle database, it must look up the connection data for the database service that you are trying to connect to. Where it looks for this connection data depends on how your client is configured; there are several ways you can configure this:
- Local naming through a tnsnames.ora file.
- Oracle Names service.
- LDAP.
The application will pass a connect identifier which the Oracle client will then try and resolve into a connect descriptor (a complete database connection string including the destination Oracle database service and network route information) using one of the above naming methods.
If the connect descriptor lookup fails for some reason, for example if you are using tnsnames.ora and it doesn’t find a matching service configured in that file, then the Oracle client will raise an “ORA-12154: TNS Could Not Resolve Service Name” error.
The most common occurrence of the ORA-12154 error is when an application is attempting to connect to an Oracle database service, but it is not the only way this error can occur. Itcan also happen when one database instance tries to connect to another database service through a database link.
It is important to understand that the ORA-12154 error is raised by the Oracle client code where the application is running and indicates a misconfiguration on the client side. It is not raised on the remote server where the database service you are trying to connect to is running.
If you get an ORA-12154 error it basically means that your application program has found an installed Oracle client, but the Oracle service it is looking up is not listed in the tnsnames.ora file (or is not found in Oracle Names or LDAP, depending on your naming configuration).
Common Causes Of The “ORA-12154: TNS Could Not Resolve Service” Error
Most sites use the tnsnames.ora file for their naming resolution and the most usual causes of the ORA-12154 error are:
- The corresponding entry is missing from the tnsnames.ora file.
- The corresponding entry in the tnsnames.ora file has a typo in it or has some other error in it.
- The application is using the tnsnames.ora file from a different ORACLE_HOME.
- The application is not using a fully-qualified service name, but no default domain is enabled in sqlnet.ora
How To Resolve The ORA-12154 Error
The first step you should take to resolve the ORA-12154 error is to check which naming method you are using.
The Oracle client decides which method to use based on the data in the sqlnet.ora file. Both this file and the tnsnames.ora file are by default located in ORACLE_HOME/network/admin.
However, the location can also be set via the TNS_ADMIN environment variable so you should check to see if you have a TNS_ADMIN set in order to be sure which location your configuration files are stored in. If you don’t have a sqlnet.ora file, or if it does not have a naming resolution method configured in it, then Oracle defaults to using the tnsnames.ora file.
Assuming that you are using a tnsnames.ora file then once you have found the correct configuration file, check that you have an entry in it that matches the database service your application is configured to connect to. If it doesn’t have one, you should add one, making sure the syntax you use is correct.
If your tnsnames.ora file does have an entry in it, then check it for syntax errors and typos. Check for any unmatched parentheses or stray characters.
Finally, you should also check the security privileges of the tnsnames.ora file to make sure it is accessible.