How does an EC2 Auto Scaling Group work, and how would you configure it to scale on load?
What they are really testing: Whether you understand the moving parts (launch template, desired/min/max, scaling policy, health checks) and prefer target tracking over hand-tuned thresholds.
A real interview question
How does an EC2 Auto Scaling Group work, and how would you configure it to scale on load?
What most people say
drag me
“You set a minimum and maximum number of instances and it adds more when CPU goes high.”
It captures the gist but skips the launch template, multi-AZ, health-check self-healing, and the choice of scaling policy. CPU is also not always the right signal, and there is no mention of warmup/cooldown.
The follow-ups they ask next
Why might CPU be a poor scaling metric for a web app?
Load can be I/O- or latency-bound while CPU stays low, so you under-scale during a real overload. Requests-per-target or response latency often tracks user-facing load better.
What does a cooldown or warmup period prevent?
Counting instances before they are ready (causing premature scale-downs) and rapid oscillation. It lets metrics stabilize after a scaling action before the next one.
What the interviewer is listening for
- Knows launch template + min/max/desired + multi-AZ
- Prefers target tracking
- Mentions health-check self-healing and warmup/cooldown
What sinks the answer
- Only "min/max and CPU"
- Unaware of health-check replacement
- No notion of the right scaling metric
If you genuinely do not know
Say this instead of freezing. Reasoning out loud from what you do know beats silence every single time, and a good interviewer is listening for exactly that.
“An ASG [keeps a fleet between min/max around desired, from a launch template across AZs]. I scale with [target tracking, hold a metric like CPU 50% or requests-per-target], use [step/scheduled for known patterns], and rely on [health-check replacement for self-healing]. I tune [warmup/cooldown] and pick [a metric that reflects real load, often requests/latency over CPU].”
Keep going with aws
Foundation
What are the EC2 purchasing options (On-Demand, Reserved, Savings Plans, Spot), and how do you mix them?
Foundation
Walk me through the main S3 storage classes and how you would use lifecycle policies to control cost.
Junior
How does AWS decide whether an IAM request is allowed? Explain the evaluation logic.
Mid
How do you choose between SQS, SNS, EventBridge, and Kinesis?
Mid
What are VPC endpoints, and when would you use a gateway endpoint versus an interface endpoint?
Mid
How does CloudFront work, and how would you use it to serve a web app efficiently and securely?
Knowing the answer is not the same as recalling it under pressure
Sign in to send the questions you fumble to spaced recall, so they come back right before you would forget them, and learn the concepts behind them with hands-on labs.
Start free