Some developers report ORA-12850 error in their JDBC log.
ORA-12850: Could not allocate slaves on all specified instances: 2 needed, 0 allocated
There could be many ways to solve this, but I run the following statement to remove ORA-12850.
SQL> alter system flush shared_pool;
For a RAC system, you should flush shared pool on other nodes, too.
In this way, we don't have to restart the database.
Met an error when I tried to monitor the database by querying sessions in Toad for Oracle.
To work around the error, we should prevent Toad to allocate slave processes on other node in this moment, you can uncheck "Use RAC Views" like this.