Two legacy application VMs must survive planned maintenance and a single host failure. The vendor requires both VMs to stay in the same region, and a datacenter outage is not part of the requirement. What should the administrator use?
Availability sets are designed for host-level resilience inside one datacenter. They spread VMs across fault domains and update domains, which helps reduce impact from hardware failures and planned maintenance. Because the requirement does not include surviving a full datacenter outage, an availability set is the right level of protection without the added complexity of zones.
Why this answer
An availability set protects against planned maintenance and single host failures by placing VMs in separate fault domains (different physical hardware) and update domains (different maintenance windows). This ensures that during planned Azure maintenance, only one VM is rebooted at a time, and if a host fails, only VMs in that fault domain are affected. Since the requirement specifies a single host failure (not a datacenter outage) and both VMs must stay in the same region, an availability set is the correct choice.
Exam trap
The trap here is that candidates confuse availability zones (which protect against datacenter outages) with availability sets (which protect against host and rack failures), leading them to choose zones even though the requirement explicitly excludes a datacenter outage.
Why the other options are wrong
The requirement is to survive a single host failure, not a datacenter outage. Availability zones protect against datacenter failures, but placing one VM in each zone would not protect against a single host failure because each zone contains multiple hosts, and the VMs could still be on the same host within a zone.
A virtual machine scale set in a single zone does not protect against a single host failure because all VMs could be on the same host; it also doesn't meet the requirement of exactly two legacy VMs with separate fault domains.
A proximity placement group ensures low network latency between VMs but does not protect against planned maintenance or single host failure, as VMs can still be placed on the same host or fault domain.
When would these options actually be correct?
If the requirement were to survive a full datacenter outage (e.g., due to a regional disaster) while keeping VMs in the same region, then availability zones with one VM in each zone would be correct. For example: 'Two VMs must remain available if an entire datacenter fails, but must stay within the same region.'
An administrator needs to deploy multiple identical VMs that automatically scale based on load, and high availability across zones is not required; a scale set in a single zone provides load balancing and scaling.
An administrator needs to minimize network latency between two VMs for a high-performance computing workload, and availability requirements are met by other means (e.g., application-level replication).
Why candidates pick the wrong answer
Candidates may confuse 'host failure' with 'datacenter failure' and think zones provide high availability for any failure, or they may assume that distributing VMs across zones automatically protects against host failures, not realizing that zones are for datacenter-level redundancy.
Candidates may think a scale set inherently provides high availability, but without multiple zones or fault domains, it doesn't protect against host failures.
Candidates may confuse proximity placement groups with high availability solutions, thinking that placing VMs close together somehow provides redundancy.