Cost Blind Spot — AWS Developer (DVA-C02)
The architecturally correct answer was also the most expensive. The exam wanted the cost-optimized option that still meets requirements.
DynamoDB On-Demand Costs More at Steady Traffic
A low-traffic application that scales automatically without capacity planning looks like a clean fit for DynamoDB on-demand. On-demand appears simpler and safer. Once traffic becomes predictable and sustained, the per-request price of on-demand exceeds what correctly tuned provisioned auto-scaling would charge for the same workload. On-demand wins for genuinely unpredictable spikes; provisioned with auto-scaling wins when a baseline is stable and measurable. The billing model should follow the actual traffic shape, not the convenience of skipping capacity planning. DynamoDB auto-scaling adjusts provisioned throughput between a defined minimum and maximum based on CloudWatch utilization targets.
The Scenario
The question describes a video transcoding pipeline processing uploaded files — fault-tolerant, no user-facing latency requirements, files reprocessable on failure. You choose a Multi-AZ ECS cluster with On-Demand Fargate tasks and auto-scaling. The correct answer uses EC2 Spot Instances in an Auto Scaling group with a Spot Fleet diversified across instance types and AZs. Same throughput, 60-90% less cost. The workload is explicitly fault-tolerant (files can be reprocessed), which is the textbook Spot qualification. The exam said "most cost-effective" and you optimized for availability that the scenario never required.
How to Spot It
- •When the question says "cost-effective" or "minimize cost," check whether the workload is fault-tolerant. Batch processing, media transcoding, CI/CD builds, data analysis, and any workload with "reprocessable on failure" are Spot Instance candidates. Spot saves 60-90% over On-Demand.
- •Multi-AZ deployments, provisioned IOPS, and dedicated hosts all add cost. If the scenario does not mention an SLA, uptime requirement, or "highly available," these features are cost traps the exam uses to test whether you add unnecessary resilience.
- •S3 Intelligent-Tiering adds a $0.0025/1000 objects monitoring fee. For billions of small objects, that monitoring fee exceeds the storage savings. The exam tests whether you know when Intelligent-Tiering costs more than just picking the right tier manually.
Decision Rules
Determine that inspecting the S3 storage class of objects under the affected prefix is the one step that causally explains both the cost anomaly (Glacier per-GB retrieval fees) and the latency symptom (Glacier restore delay of minutes to hours), given that inter-region egress has been ruled out.
When S3 cost rises and retrieval latency increases from milliseconds to minutes on the same prefix while egress is ruled out, inspect the storage class of the affected objects — Glacier-tier retrieval fees and restore latency are the only S3 cost category that causally explains both symptoms at once.
Which CodeDeploy EC2 deployment strategy satisfies both a 75% minimum-capacity availability floor and automated rollback on health-check failure without doubling fleet cost via a second parallel environment.
Domain Coverage
Difficulty Breakdown
Related Patterns