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Cost Aware Architecture and Design Questions

Focuses on how architectural decisions and design patterns affect operating cost and total cost of ownership. Interviewees should be able to reason about trade offs such as managed services versus self managed components, always on virtual machines versus event driven or serverless approaches, reserved versus on demand capacity, use of spot or preemptible instances, and multi region or edge placement. Candidates should demonstrate techniques for reducing cost through storage class selection and lifecycle policies, caching and batching, query and workload optimization, data transfer minimization, and workload isolation. The topic also covers modeling and communicating cost trade offs, estimating ongoing operating expense for alternative designs, and choosing architecture that balances budget constraints with reliability, performance, and engineering effort.

EasyTechnical
0 practiced
You maintain a service running on 3 on-demand EC2 instances (m5.large) 24/7, storing 5 TB in object storage, and transferring 2 TB egress per month. Use these rates: EC2 m5.large $0.096/hr, object storage $0.023/GB-month, data egress $0.09/GB. Calculate the approximate monthly bill and identify the largest cost contributor. Suggest two low-effort optimizations that would reduce the monthly cost and explain why.
EasyTechnical
0 practiced
Explain how serverless compute (for example AWS Lambda) changes cost characteristics compared to always-on VMs: what are the main billing axes to monitor, how do function memory and execution time affect cost, and what operational patterns can inadvertently increase cost (for example: excessive cold starts, high concurrency).
EasyTechnical
0 practiced
You run a stateful service on an autoscaling group with reserved instance commitments covering 60% of baseline capacity. Explain simple heuristics to decide how much additional capacity to keep as buffer (headroom) for traffic spikes versus minimizing cost. What metrics and historical analyses would you use to set that buffer?
EasyTechnical
0 practiced
How does storage class selection impact durability, availability, access cost, and object retrieval latency? Give two examples where object storage is preferable to block storage and two examples where block storage is preferable, and explain cost implications for each choice.
MediumTechnical
0 practiced
An analyst ran an ad-hoc query that scanned 2 TB on your analytics cluster and caused a surprise bill spike. Outline a practical action plan to immediately limit further cost impact and medium-term changes to prevent recurrence. Include schema and query-level tactics such as partitioning, clustering, materialized views, query guards, and cost quota controls.

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