This issue is faced in some versions of oracle as ORA-12154: TNS: Could not resolve service name. Mostly this issue comes in Oracle 10g and Oracle 11g Express edition which i have faced.
When you or any application try to connect to an Oracle Service via Oracle Networking SQL*Net, it looks up the connect data for the service. When this lookup fails, Oracle client show an error message ORA-12154.
Sometimes this error comes when one database instance tries to connect to another database instance via database link.
You try to connect to Oracle but the service name is either missing from the TNSNAMES.ORA file or is incorrectly defined.
You have to make sure that the TNSNAMES.ORA file exists in the correct directory and accessible.
Ex: In Oracle 11g express, it is located at below path.
You have to make sure that the service name that you are trying to connect is included in the TNSNAMES.ORA file and that it is correctly defined.
Make sure that TNSNAMES is listed as one of the values of the names.directory_path parameter in the sqlnet.ora Oracle Net profile.
You have to make sure that the syntax of TNSNAMES.ORA file is correct.
For example, if there are unmatched brackets in the file (ie: open bracket without the corresponding close bracket), the file will be rendered unusable.
(ADDRESS = (PROTOCOL = TCP)(HOST = TESTING.COM)(PORT = 1521))
(SERVER = DEDICATED)
(SERVICE_NAME = XE)
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