A resilient application is an application that can be restarted on a different cluster node without requiring you to reconfigure the clients.
See Making application programs resilient to learn about what characteristics make an application resilient.
A resilient application needs the ability to recognize the temporary loss of the Internet Protocol (IP) connection between the client and the server. The client application must be aware that the IP connection will be temporarily unavailable and must retry access rather than ending or initiating a failover. Similarly, if you are performing a switchover, server applications need to be aware that the IP connection is no longer available. Eventually, an error condition is returned to the server application. Once this error condition is received, it is best if the server application recognizes the condition and ends normally.
IP address takeover is a high availability function that is used to protect clients from application server outages. An application takeover IP address is a floating address that is to be associated with an application. The concept is to use IP address aliasing to define a floating IP address that is associated with multiple application servers or hosts. When one application server in a cluster fails, another cluster node assumes the responsibilities of the application server without requiring you to reconfigure the clients.
Also introduced in support of IP address takeover is the concept of application cluster resource groups (CRGs). Application CRGs are cluster resource groups that contain an application takeover IP address resource and a recovery domain. The recovery domain contains the list of application servers within the cluster that support a particular application. If a single resource fails, cluster resource services initiates a failover on the group to which the failing resource belongs.