What ensures high availability for a Domain Controller?

High availability for a Domain Controller is primarily ensured by having redundant power sources. This approach is crucial because a Domain Controller must remain operational to manage authentication requests and other critical network services. If a Domain Controller loses power, it becomes unavailable, potentially disrupting access to resources across the network. By implementing redundant power sources, such as uninterruptible power supplies (UPS) and backup generators, organizations can ensure that the Domain Controller remains functional even during power failures, thus maintaining continuous availability for users and systems relying on it.

While regular backups are essential for data integrity and recovery purposes, they do not inherently provide immediate high availability. Similarly, multiple servers in the same location can improve availability but are at risk if that location experiences an outage. Geographical distribution of servers can enhance disaster recovery and availability in the event of a localized failure, and while this is beneficial, it doesn't directly address the critical issue of power reliability which is immediately necessary for maintaining domain services operational. Thus, the implementation of redundant power sources stands out as the most direct means to secure high availability for a Domain Controller.