Leadership says the AWS bill is too high. How do you systematically bring it down?
What they are really testing: Senior signal: measure-then-optimize with attribution, not random instance-shrinking. They want a process, the big cost levers, and the data-transfer trap juniors miss.
A real interview question
Leadership says the AWS bill is too high. How do you systematically bring it down?
What most people say
drag me
“I would look for large instances and switch them to smaller ones to reduce the bill.”
It jumps straight to shrinking instances with no measurement or attribution, and ignores the biggest levers: commitments, Spot, storage tiering, and especially data-transfer costs that often dominate and that rightsizing never touches.
The follow-ups they ask next
What cost driver do engineers most often overlook?
Data transfer: cross-AZ traffic, NAT gateway processing, and internet egress. It does not show up as an instance you can shrink, so it hides, VPC endpoints, same-AZ placement, and CloudFront address it.
Why insist on tagging before optimizing?
Without attribution you cannot tell which team/service owns the spend or prioritize the biggest levers, and you cannot hold teams accountable. Tagging turns one big bill into actionable, owned line items.
What the interviewer is listening for
- Measure + tag/attribute before cutting
- Names commitments, Spot, rightsizing, lifecycle
- Calls out data-transfer/cross-AZ costs
- Sets up ongoing budgets/anomaly alerts
What sinks the answer
- Jumps to shrinking instances
- No measurement or attribution
- Ignores data transfer and storage tiering
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.
“Measure first: [Cost Explorer + CUR + tagging to attribute spend and find the big line items]. Kill waste: [rightsize via Compute Optimizer, delete idle EBS/snapshots/EIPs/dev envs]. Structural levers: [Savings Plans/RIs for baseline, Spot for batch, Graviton]. Then [S3 lifecycle and the data-transfer trap, cross-AZ/egress, fixed with VPC endpoints/CloudFront]. Make it ongoing with [budgets + anomaly alerts].”
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