When attempting to connect to an instance that is down, which Oracle Net feature or component facilitates automatic connect-time fail-over to another instance for the user?
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Heterogeneous Services
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Oracle Connection Manager
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Dynamic Service Registration
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Oracle Names Server
When an instance is down, Oracle Net's Dynamic Service Registration enables automatic failover. With service registration, listeners track instance availability and can redirect connection requests to surviving instances without manual configuration. Heterogeneous Services (A) is for non-Oracle data sources. Oracle Connection Manager (B) provides connection pooling and firewall features, not failover. Oracle Names Server (D) is a deprecated naming/directory service.
Dynamic Service Registration is the Oracle Net/PMON feature by which each database instance automatically registers its available services with the listener. When multiple instances offer the same service (as in RAC) and a client tries to connect while one instance is down, the listener — aware through dynamic registration of which instances are currently up for that service — can route or fail the connection over to a surviving instance at connect time. Oracle Connection Manager is a proxy/multiplexing gateway, not a failover mechanism. Heterogeneous Services connects Oracle to non-Oracle databases. Oracle Names Server was a deprecated centralized name-resolution service, unrelated to instance failover.