Design Cost-Optimized Architectures Study Guide — SAA-C03

Design Cost-Optimized Architectures

This domain accounts for 20% of the AWS SAA-C03 exam — the largest single domain. It tests your ability to select the lowest-cost architecture that still meets the stated requirements. Questions present a workload description with constraints (SLA, elasticity, storage durability, etc.) and ask you to choose the most cost-effective option. You must know exact service tiers, pricing models, and when to trade one cost lever for another.

S3 Storage Classes, Lifecycle Policies, and Intelligent-Tiering

Storage Class Comparison

Lifecycle Policies

Lifecycle rules transition objects between classes or expire them automatically. Key transition constraints:

• Objects must be in Standard or Standard-IA for at least 30 days before transitioning to Standard-IA or One Zone-IA (via lifecycle — not a manual PUT restriction, but a billing minimum).
• You can transition directly from Standard to Glacier classes with no 30-day wait.
• Expiration actions permanently delete objects or delete expired delete markers.

Intelligent-Tiering

S3 Intelligent-Tiering automatically moves objects between tiers:

• Frequent Access → Infrequent Access (after 30 days of no access)
• Archive Instant Access (after 90 days, optional activation)
• Archive Access (after 90–180 days, optional)
• Deep Archive Access (after 180+ days, optional)

There is a per-object monitoring fee for objects ≥ 128 KB. Objects < 128 KB are always stored in Frequent Access and are not monitored. No retrieval fees between automatic tiers.

Common Traps

• One Zone-IA loses data if the AZ fails — never use it for non-reproducible data.
• Choosing Standard-IA for objects accessed more than once per month — retrieval fees make Standard cheaper.
• Forgetting the 90-day minimum for Glacier Instant Retrieval when calculating cost savings vs. deleting early.
• Intelligent-Tiering is NOT free — the monitoring fee exists; for very small objects it adds cost vs. Standard.
• Lifecycle transitions do not apply retroactively to objects that already exist unless the rule matches their prefix/tag.

Worked Scenario

Requirement: Log files are written daily, accessed heavily for the first 7 days, rarely after 30 days, and must be deleted after 1 year. Cost must be minimized.

Reasoning: Standard for days 0–30 (no minimum duration billing risk for IA). Transition to Standard-IA at day 30. Transition to Glacier Flexible Retrieval at day 90. Expire at day 365. Avoid Intelligent-Tiering here because the access pattern is known.

EC2 Purchasing Options

Decision Tree for EC2 Purchasing

Use this flowchart to select the correct purchasing model for a given workload.

Key Differentiators

• Standard RIs are sold on the Reserved Instance Marketplace; Convertible RIs are not.
• Compute Savings Plans apply to Lambda and Fargate as well as EC2 — the broadest coverage.
• EC2 Instance Savings Plans apply to a single instance family in a region but any size, OS, or tenancy within that family.
• Spot interruptions give a 2-minute warning via instance metadata and EventBridge.
• Spot Blocks (defined duration) are no longer available to new customers.

Common Traps

• Choosing Dedicated Instances when BYOL is the requirement — only Dedicated Hosts expose socket/core counts needed for licensing.
• Assuming Savings Plans replace all RI features — Standard RIs still offer higher discounts for locked workloads and are marketable.
• Spot instances for databases or stateful workloads — exam distractors often suggest Spot for any "cost savings" scenario.
• Convertible RIs require exchange via AWS, not the Marketplace.