In an IT system, any critical component such as a server, networking, or storage component that must be operational at all times to provide a service or application to users can be protected against failure by providing a backup or redundant system. If
the primary component providing a service fails for any reason, then the provision of the service is switched, or fails over, to the backup component. It is increasingly rare for server applications to be running directly on server hardware. Most applications
are running in virtual servers with the underlying virtualization software and storage systems spread over multiple hosts. In this case, any virtual servers that were running on a failed hardware host are automatically restarted on other virtual server
hosts that are still operational. The penetration of virtualization into networking is not as complete in the field of servers and storage systems. However, it is increasing via Software Defined Networking (SDN) and Network Function Virtualization (NFV).
Failover for SDN and NFV is usually provided in software in the same way as virtual servers are protected. For hardware networking equipment, separate dedicated physical
backup failover systems are still needed.
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