Anyone know what is fail over concept for a oracle server? what is resource group?

Hi -

For your first question: The concept of failover in Oracle is basically 2 things
1) A Real Application Cluster (RAC)
2) A Data Guard Configuration (DG)

RAC is the concept of having 2 or more Oracle instances accessing the same Oracle database and being able to work as one entity. If one of the nodes in the RAC fails, the remaining node(s) keep on working and users keep on working.
DG is the concept of having a "Standby" database, which is typically kept in "managed recovery mode" and just takes the online redo longs and/or archived logs from the Primary instance and applies them to the Standby Database. If the Primary database crashes or is lost, then the Standby database can be activated, rolled forward with any remaining logs available, and opened as a Primary database.

For additional inof, see here:

http://www.oracle.com/pls/db102/portal.p...

Resource Groups, one the other hand, really have nothing to do with failover. A Resource Group is an entity that you create (like a set of users in a particular department for example) and then you can define how much resources in the system that group can consume at any given point in time. For example, during the last 3-5 days of the month, you may assign the Payroll Resource Group a higher consumption rate on the system while reducing the Sales Resource Groups' consumption rate. Because at the end of the month, all the Sales info is in the database and the Payroll people need the computing power to spit out the pay checks.