A company runs a critical application on Compute Engine instances in a managed instance group (MIG) with autoscaling. During a traffic spike, some instances become unhealthy but are not automatically replaced. What is the most likely cause?
Trap 1: The MIG is regional and one zone failed.
Regional MIGs automatically redistribute instances across zones; a single zone failure would cause instance recreation in other zones.
Trap 2: The instance template has a startup script error.
A startup script error would cause instances to never become healthy but would not prevent replacement of already unhealthy instances.
Trap 3: The HTTP load balancer's health check is failing.
The load balancer health check determines traffic routing, not instance replacement by the MIG.
- A
The MIG is regional and one zone failed.
Why wrong: Regional MIGs automatically redistribute instances across zones; a single zone failure would cause instance recreation in other zones.
- B
The autohealing health check is misconfigured.
MIG autohealing relies on a health check to detect unhealthy instances and replace them; a misconfiguration prevents detection.
- C
The instance template has a startup script error.
Why wrong: A startup script error would cause instances to never become healthy but would not prevent replacement of already unhealthy instances.
- D
The HTTP load balancer's health check is failing.
Why wrong: The load balancer health check determines traffic routing, not instance replacement by the MIG.