An Auto Scaling group launches instances registered with an Application Load Balancer target group. One instance passes its EC2 status checks (the underlying hardware, network, and OS are healthy), but its application process has crashed and no longer responds to any requests. The Auto Scaling group's health check type is left at its default value. What happens to this instance?
- The Auto Scaling group marks it unhealthy and replaces it, because it also evaluates target group health by default
- The Auto Scaling group leaves it running, because by default it only evaluates EC2 status checks
- The Application Load Balancer automatically restarts the crashed application process on the instance
- The Auto Scaling group launches an additional instance instead of taking any action on the unhealthy one
Correct answer: B. The Auto Scaling group leaves it running, because by default it only evaluates EC2 status checks
An Auto Scaling group's health_check_type defaults to EC2, meaning it only reacts to EC2 status check failures at the hardware, network, and OS level; an instance that is otherwise healthy but whose application process has crashed will keep running and keep failing to serve traffic until an operator explicitly changes the health check type to ELB, which makes the Auto Scaling group also honour the target group's health checks and replace instances that fail them. The first option describes ELB-type behaviour, which is not the default the question specifies. The third option is false because an Application Load Balancer only routes traffic and reports target health; it never restarts processes running inside an instance's operating system. The fourth option misunderstands how health checks interact with scaling: a failing health check does not, by itself, trigger a scale-out event, since scale-out is driven by scaling policies, not health check type.
Source: AWS documentation: Amazon EC2 Auto Scaling — health checks for Auto Scaling instances