ORA-12520: TNS:listener could not find available handler for requested type of server

What does it mean?

The listener could not find a suitable server process (handler) for the requested connection type (dedicated or shared). The client can reach the listener, but the connection cannot be handed off to a server process.

Why does it happen?

The instance is not registered with the listener or its registration is broken; the PROCESSES/SESSIONS limit is reached and no new dedicated process can be spawned; no/insufficient dispatchers in a shared server configuration; or the instance has just started and has not yet registered with the listener.

How to fix it

Check the services the listener sees: lsnrctl services (handler count and states). Re-register the instance with the listener: ALTER SYSTEM REGISTER; Check the PROCESSES limit: SELECT resource_name, current_utilization, max_utilization, limit_value FROM v$resource_limit WHERE resource_name IN ('processes','sessions'); If needed, increase it with ALTER SYSTEM SET processes=... SCOPE=SPFILE; and restart the instance.