Microsoft Cloud Witness is a high availability (HA) feature for failover clusters that uses storage in the Microsoft Azure cloud platform to ensure clusters continue to function if there is a site outage.
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The cloud witness feature may benefit organizations that have clusters across multiple sites or are running small Hyper-V clusters in a remote office/branch office environment that need a backup for those scenarios.
Server clusters typically function through a quorum system in which each node in the cluster contributes a single vote to cluster operations. However, most businesses spread the cluster evenly across two sites. If one site fails, the cluster nodes at that site become unavailable, leaving the cluster without a quorum and the cluster may not continue to function properly. By adding another node in Azure storage, the cluster is guaranteed to have a quorum of nodes to keep the cluster functioning.
An organization must have an Azure storage account to use a cloud witness. An administrator adds the Azure account information when configuring the cloud witness portion of a failover cluster quorum. The cluster nodes must have access to the Internet and open HTTPS ports to use the Azure account.