Windows Server Failover clustering is a feature of the server generation of Windows operating systems for improving high availability of applications and services. In a Windows server failover cluster, one of the servers is an active server while another server is a standby server. For the servers to participate in a failover cluster, they must have similar hardware specifications and they must be sharing a storage device.
The two servers communicate through a series of heartbeat messages over a dedicated link. They are called push heartbeats and pull heartbeats. If the signals are initiated by an active server and sent to standby signal at a specified interval, these are called push heartbeats. Pull heartbeats refer to signals which are sent from the standby server to the active server. If the standby server does not receive a push heartbeat within a certain amount of time, it takes over and becomes the active server.