PROBLEM:

In one of our 12.1.0.2 database, below error was logged in alert log.

2017-11-06 02:28:10.545000 +03:00
Error ORA-235 occurred during an un-locked control file transaction. This

CAUSE AND SOLUTION:

As per Oracle, this is an expected behavior and can be ignored safely.

Starting from Oracle 12c, controlfile transactions are enhanced. We are no longer request a controlfile enqueue before scanning a particular controlfile section. This is done to minimize locking issues and improve performance and scalability. Due to this change, the process reading a controlfile section may sometimes encounter ORA-235 if the same section is being modified by another process. When this occurs, the reading process will simply re-read the section again. You can safely ignore this message, it’s not an issue.

The errors cannot be suppressed at database level by setting any event or initialization parameter. The reason is that although the errors are harmless, they do give an idea of the concurrency on the controlfile and can help debug issues relating to slow controlfile i/o etc so, this enhancement has been added specifically by development. It’s best to modify any monitoring scripts to ignore/exclude this error. Since this is working as designed and these are simply informational, the messages can simply be ignored.