What are the EC2 purchasing options (On-Demand, Reserved, Savings Plans, Spot), and how do you mix them?
What they are really testing: Whether you understand the commitment-versus-flexibility trade-off and can build a cost strategy, not just name the options. This is bread-and-butter AWS cost work.
A real interview question
What are the EC2 purchasing options (On-Demand, Reserved, Savings Plans, Spot), and how do you mix them?
What most people say
drag me
“On-Demand is the normal way to pay, and Reserved Instances are cheaper if you commit.”
It only names two and treats On-Demand as the default to use everywhere. It misses Spot for batch and the actual strategy: baseline on commitments, burst on demand, batch on Spot.
The follow-ups they ask next
Why might you choose a Savings Plan over a Reserved Instance?
Flexibility: a Compute Savings Plan applies across instance families, regions, OS, and even Fargate/Lambda, so it keeps saving even as your architecture changes. Standard RIs are more rigid (though they can be sold/modified).
What kind of workload should never run solely on Spot?
Anything that cannot tolerate sudden termination: a stateful primary database, a long single-node job with no checkpointing. Use Spot for stateless/replicated/batch work that can be reclaimed safely.
What the interviewer is listening for
- Names all four and the commitment trade-off
- Knows Savings Plans flexibility vs RIs
- Describes the baseline/burst/Spot mix
What sinks the answer
- Only On-Demand and RIs
- Suggests Spot for stateful primaries
- No cost strategy, just definitions
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.
“Four options: [On-Demand, no commitment, highest price, for spiky work]; [Reserved/Savings Plans, commit 1-3 yrs for up to ~72% off, Savings Plans more flexible]; [Spot, up to 90% off but reclaimed on 2-min notice, for batch/stateless]. The strategy: [baseline on Savings Plans, bursts on On-Demand, interruptible work on Spot].”
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