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The Listeners main role in Oracle Database is to act as an agent for establishing user process connection to the database.
This is how a client connection happens to a database.
you give a connect string in your application which contains the database server IP address, the listener port (usually its1521) and the username and password.
The connection request lands on port number1521 of the database server machine.
The listener then takes the request to the database engine where the username and password are authenticated. if they are correct, a connection is established to the database. i.e. on a unix system a new process is forked by the operating system and a process id is assigned to it.
At this point the work of the listener stops.
Even if you shutdown the listener when the database is up and running, nothing happens to the already connected sessions. Only new connections will not be established.
For a system with high concurrency and multiple applications, it is better to have many listeners configured on different ports on the database. This helps in connection load balancing.
Also the IANA registered port numbers for oracle listeners are2483 and2484 and not the1521 port.
This means that if you use2483 and2484 ports for the listener, you will never have conflicts with any other process trying to use the same port number on the machine.
The Oracle listener is set up to start automatically whenever your server machine is restarted. However, when your system encounters unforeseen circumstances, or when you have manually stopped the listener, you can restart it at the command line. To do so, use the following:
lsnrctl startYou can use Enterprise Manager to stop the listener. To do so, navigate to the Listener: listener_name page by clicking Listener on the Home page. To shut down the listener, click Stop.
You can also stop it at the command line using the following:
lsnrctl stop
