ORA-12521: TNS:listener does not currently know of instance requested

admin

ORA-12521:TNS:listener does not currently know of instance requested in connect
descriptor
Cause: The listener received a request to establish aconnection to a database or
other service. The connect descriptor received by the listener specified inaddition
to the service name an instance name for an instance (usually a databaseinstance)
that either has not yet dynamically registered with the listener or has notbeen
statically configured for the listener. This may be a temporary condition suchas
after the listener has started, but before the database instance has registeredwith
the listener.
Action:
- Wait a moment and try to connect a second time.
- Check which instances are currently known by the listener by executing:lsnrctl
services <listener name>
- Check that the INSTANCE_NAME parameter in the connect descriptor specifies
an instance name known by the listener.
- Check for an event in the listener.log file.