What are instance connection and service connection in Oracle?

Question

I am configuring a WebLogic app server. I am unclear about what is an instance connection and what is a service connection.

Also, in my SQL Developer, when I make a new connection, it asks for either an SID or a Service Name. What are they?

Answer 1

An instance connection is a TNS connection to a specific instance on a host.

A service connection is to a particular service that a listener on a host knows about.

The two are often used interchangeably, but the SERVICE connection is more flexible. I can configure multiple instances to register as the same service name to the listener, allowing load balancing and behind the scenes replacement of databases transparently to users. If an instance connection is specified, that particular instance must be up and available or the connection will be refused.