Q

Which instance is a user connected through?

You said in one of your answers that "In normal configurations (without RAC), if the instance goes down, you no longer have access to your data. In RAC environments, if an instance goes down other instances can still access the data." How will it be known that a user is connected through which instance? Is there automatic switchover to another instance?

You said in one of your answers that "In normal configurations (without RAC), if the instance goes down, you no longer have access to your data. In RAC environments, if an instance goes down other instances can still access the data."
  1. How will it be known that a user is connected through which instance?
  2. Is there automatic switchover to another instance?

You can easily determine if you are connected to a RAC instance or a standalone instance by running the following query:

SELECT COUNT(*) FROM GV$INSTANCE;
If the value returned is '1', then you are connected to a standalone instance. If the value is greater than one, then you are connected to a RAC instance and the value tells you how many nodes are running in the cluster.

In Oracle 9i and higher, you can failover to another RAC instance when your current instance goes down. You must configure Transparent Application Failover (TAF) in order for this to work. If you have not configured TAF, then you will not failover. Please read the Oracle docs on TAF as there are a few things one should know about its operations before configuring it.

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