AWS SAA-C03 Domain 4: Cost-Optimized Architectures Complete Guide

Domain 4 of the AWS Certified Solutions Architect – Associate (SAA-C03), Design Cost-Optimized Architectures, carries 20 percent of the scored content. The theme is cutting waste without sacrificing performance or availability.

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Principles – right-sizing and elasticity

Cost optimization rests on right-sizing (matching resource size to demand) and elasticity (scaling with load). Analyze utilization first, establish the always-needed baseline, then choose purchase options for the stable part and On-Demand or Spot for the variable part.

Compute purchase options

• On-Demand: pay per use, no commitment. Best for unpredictable or short workloads.
• Savings Plans: commit to steady spend ($/hr) for 1 or 3 years; Compute Savings Plans are most flexible.
• Reserved Instances: commit to instance attributes for 1 or 3 years for a discount.
• Spot Instances: up to ~90% off spare capacity, but interruptible. Best for fault-tolerant work.

Decision axes: can you commit, and can you tolerate interruption? Steady long-term load means Savings Plans or RI; interruptible batch means Spot; unpredictable bursts mean On-Demand. A common trap is buying Reserved capacity you cannot fully use – commit only to the analyzed baseline.

Storage – S3 classes and lifecycle

Choose the S3 storage class from the access pattern and automate transitions with lifecycle policies. Frequent access stays in S3 Standard; unpredictable access fits Intelligent-Tiering; archival goes to Glacier or Glacier Deep Archive. Unknown patterns point to Intelligent-Tiering; lowest-cost long-term archive points to Glacier Deep Archive.

Data transfer – the hidden cost

Inbound is generally free, but outbound to the internet and cross-Region transfer are charged. Serving through CloudFront cuts origin egress, and keeping traffic within a Region/AZ avoids unnecessary charges.

Pricing tables make sense once you understand how foundation-model applications actually consume tokens.

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Cost management tools

Cost Explorer visualizes spend, Budgets alerts on thresholds, and Cost and Usage Reports give line-item detail. Alert before exceeding a budget points to Budgets; analyze where spend goes points to Cost Explorer.

Summary

Cost optimization is right-sizing, the correct compute purchase model, storage classes matched to access, and data-transfer control. Commit the baseline, absorb variability with On-Demand/Spot, automate tiering, watch egress.

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