Lesson 5 of 6 · 9 min
Domain 4 — Design cost-optimized architectures
The 20% domain: purchasing models, storage lifecycle, and the data transfer charges the exam loves.
Purchasing models by load shape
| Load | Model | Saving |
|---|---|---|
| Steady 24/7 baseline | Compute Savings Plan, 1 or 3 years | Up to ~72% |
| Predictable but instance-specific | Reserved Instances | Up to ~72% |
| Spiky or unpredictable | On-Demand | Baseline rate |
| Interruption-tolerant batch | Spot | Up to ~90% |
| Licensing tied to physical hardware | Dedicated Hosts | Compliance, not saving |
Data transfer, the invisible line item
Inbound data is free. Outbound to the internet is charged per gigabyte. Traffic between AZs is charged in both directions. Traffic between Regions is charged. Traffic to S3 or DynamoDB through a gateway endpoint is free, while the same traffic through a NAT gateway incurs both NAT processing and per-gigabyte charges.
That is why 'reduce the cost of a workload pulling large volumes from S3' is answered with a gateway endpoint, and why 'reduce egress cost for a popular download' is answered with CloudFront.
Storage cost levers
| Situation | Answer |
|---|---|
| Access pattern unknown or changing | S3 Intelligent-Tiering |
| Known cold after 30 days | Lifecycle rule to Standard-IA, then Glacier |
| Old versions accumulating | Lifecycle expiry for noncurrent versions |
| Incomplete multipart uploads | Lifecycle rule to abort after 7 days |
| Unattached EBS volumes and old snapshots | Delete them — Trusted Advisor lists them |
| Over-provisioned instances | Right-size using Compute Optimizer |
Governance
Budgets alert or act when spend crosses a threshold. Cost Explorer analyses where it went. Cost and Usage Reports give the raw detail for chargeback. Cost allocation tags are what make any of it attributable to a team — which is why 'we cannot tell which team is spending this' is answered with tags plus a tag policy, not with a report.
Worth remembering
- Match the purchasing model to the load shape: baseline committed, spiky on-demand, tolerant spot.
- Data transfer out and cross-AZ traffic are the hidden costs questions target.
- The cheapest option that still meets the stated requirement is the answer — not the cheapest overall.
Test it now
Cost trade-off scenarios cluster in the hard bank.