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

Cause: The listener received a request to establish a connection to a database or other service. The connect descriptor received by the listener specified in addition to the service name an instance name for an instance (usually a database instance) that either has not yet dynamically registered with the listener or has not been statically configured for the listener. This may be a temporary condition such as after the listener has started, but before the database instance has registered with 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

– 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.

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