Cost Blind Spot — AWS Solutions Architect (SAA-C03)
The architecturally correct answer was also the most expensive. The exam wanted the cost-optimized option that still meets requirements.
High capability does not mean cost-appropriate
The candidate sees an option that maximizes resilience, throughput, or coverage and marks it correct because it solves the technical problem completely. The scenario signals cost sensitivity through phrases like "startup," "minimize spend," or "cost-effective." Multi-AZ RDS, cross-region replication, and provisioned throughput at full headroom are all technically correct but cost-wrong. The exam tests whether cost constraints change your architecture decision, not just whether you can name the service.
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
Whether cold historical records accessed less than once per quarter should remain in RDS at gp3 per-GB-month pricing—or be exported and archived to an S3 storage class tuned for infrequent access—given that long-lived in-database cold data volume, not engine or instance tier, is the dominant cost driver.
Whether to convert data to a columnar format (Parquet) and add date-based S3 prefix partitioning catalogued in AWS Glue to enable partition pruning and columnar predicate pushdown—versus applying query-level or engine-level changes that leave the full-table scan intact.
Whether to switch DynamoDB from on-demand to provisioned capacity with Auto Scaling when the workload's RCU/WCU volume is stable enough that provisioned pricing plus reserved capacity is materially cheaper than on-demand per-request pricing.
Whether to use ALB (Layer 7 HTTP/S with native path-based routing) or NLB (Layer 4 high-throughput TCP) for an HTTPS microservices workload where URL path-based target group routing is a hard requirement.
Whether NLB's native static IP per AZ capability satisfies both the TCP protocol and partner firewall whitelisting constraints without layering additional services or provisioning virtual appliance infrastructure, making NLB the lowest-cost correct fit versus GWLB (appliance-chaining overhead with no inspection need) or ALB (Layer 7 HTTP-only, no native static IP).
Whether to continue routing S3 and DynamoDB traffic through NAT Gateway — which incurs per-GB data-processing fees that compound to thousands of dollars monthly at 10 TB/day — or replace that path with Gateway VPC Endpoints, which route the same traffic privately at zero incremental cost.
Domain Coverage
Difficulty Breakdown
Related Patterns