The cause and solution of Oracle's TNS-12502 error

A few days ago, I received a letter from a reader friend asking the following questions:

There was an error in my listening log Tns-12502:tns:listener received no connect_data from client

After looking for information to understand that this error should be the client Tnsnames.ora did not write connect_data, I checked the client machine did not find the problem.

The current phenomenon:

1. This error occurs every few minutes (see attachment), even in the early hours of the morning, where we have no developers to use Oracle in the wee hours.

2, so far also did not find the client machine can not connect the database properly.

Today is the time to study, for TNS-12502 errors, Oracle's explanation is as follows:

Error: ORA-12502 / TNS-12502
Text: TNS:listener received no CONNECT_DATA from client
Cause: No CONNECT_DATA was passed to the listener.
Action: Check that the service name resolved from TNSNAMES.ORA has the
CONNECT_DATA component of the connect descriptor.

That is, this problem occurs only if Connect_data is not included in the Tnsnames.ora file.

The log may be logged as an error if the listener port is detected through some network tools or the HA tool. We can simply simulate the connectivity of the client through the Telnet database server's 1521 port test:

C:\>telnet 172.16.30.11 1521

The following information is recorded in the log at this time:

TNS-12502:
TNS:listener received no CONNECT_DATA from client
09-AUG-2006 16:21:03 * 12502
TNS-12502: TNS:listener received no CONNECT_DATA from client
09-AUG-2006 16:21:13 * 12502
TNS-12502: TNS:listener received no CONNECT_DATA from client
09-AUG-2006 16:21:22 * 12502
TNS-12502: TNS:listener received no CONNECT_DATA from client

If the client is normal, such errors do not affect the application, and of course can be thoroughly examined to find the root cause.