9.1 Overview of Oracle Net Listener
A listener is configured with one or more listening protocol addresses, information about supported services, and parameters that control its runtime behavior. The listener configuration is stored in a configuration file named listener.ora.
Note:
The release of the listener must be the same as or later than the latest release of all Oracle databases being serviced through the listener.
Because the configuration parameters have default values, it is possible to start and use a listener with no configuration. This default listener has a name of LISTENER, supports no services on startup, and listens on the following TCP/IP protocol address:
(ADDRESS=(PROTOCOL=tcp)(HOST=host_name)(PORT=1521))The listener forwards client requests to supported services. These services are
dynamically registered with the listener. This dynamic registration feature is called
service registration. The registration is performed by the Listener Registration (LREG) process. Dynamic service
registration does not require any manual configuration in the
listener.ora file.
Service registration offers the following benefits:
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Because the listener always monitors the state of the instances, service registration facilitates automatic failover of a client connect request to a different instance if one instance is down.
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Service registration enables the listener to forward client connect requests to the least-loaded instance and dispatcher or dedicated server. Service registration balances the load across the service handlers and nodes.
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High-availability for Oracle Real Application Clusters and Oracle Data Guard