SAA-C03 Design Cost-Optimized Architectures Practice Question
A data engineering team runs a nightly ETL job on EC2. The job can be checkpointed every 5 minutes and can be retried from the last checkpoint if the instance terminates. The job runtime varies from 2 to 4 hours, and the team has no need for a specific instance type, as long as it completes before 7:00 AM local time. They currently run the job on On-Demand EC2, leading to high monthly compute cost.
Which change best reduces cost while maintaining the business deadline?
⚠ Common exam trap
The trap here is that candidates may overlook the checkpointing requirement and choose Reserved Instances (B) thinking they always reduce costs, or disable checkpointing (D) assuming simplicity is better, without realizing that Spot Instances require fault tolerance to be cost-effective.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use Spot Instances for the ETL workload, and configure the job to checkpoint frequently and restart on interruption.
Spot Instances offer significant cost savings (up to 90%) compared to On-Demand, and the ETL job's ability to checkpoint every 5 minutes and restart from the last checkpoint makes it resilient to Spot interruptions. This allows the team to meet the 7:00 AM deadline while drastically reducing compute costs, as the job can be retried on new Spot capacity if interrupted.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Use Spot Instances for the ETL workload, and configure the job to checkpoint frequently and restart on interruption.
Why this is correct
Spot can significantly reduce costs, and checkpointing plus retries mitigate interruption risk.
- ✗
Use Reserved Instances with a 1-year term to lower costs, since reservations provide discounts for any usage.
Why it's wrong here
Reserved Instances discount On-Demand-like capacity but do not exploit the largest discount available for interruptible work.
When this WOULD be correct
A company runs a 24/7 web server fleet with consistent baseline usage. They need to reduce costs for the always-on instances and can commit to a 1-year or 3-year term. Reserved Instances would provide a significant discount over On-Demand for this steady-state workload.
- ✗
Switch to On-Demand but enable Auto Scaling so the job finishes faster during peak hours.
Why it's wrong here
Auto Scaling can improve speed, but it does not address the primary cost driver of always-on On-Demand usage.
When this WOULD be correct
A question where the ETL job is at risk of missing a tight deadline due to variable runtime, and cost is not the primary concern. For example: 'A batch job must complete within 1 hour, but often takes 90 minutes on a single instance. Which change ensures it finishes on time?'
- ✗
Use Spot Instances but disable checkpointing to simplify the application.
Why it's wrong here
Without checkpointing, interruptions can cause substantial recomputation and may miss the 7:00 AM deadline.
When this WOULD be correct
If the ETL job were idempotent and very short (e.g., under 5 minutes), or if checkpointing introduced unacceptable overhead, then disabling it might be acceptable. For example, a simple data transformation that runs in under 5 minutes and can be safely restarted without data loss.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The SAA-C03 exam frequently reuses these exact scenarios with slightly different constraints.
✓Use Spot Instances for the ETL workload, and configure the job to checkpoint frequently and restart on interruption.Correct answer▾
Why this is correct
Spot can significantly reduce costs, and checkpointing plus retries mitigate interruption risk.
✗Use Reserved Instances with a 1-year term to lower costs, since reservations provide discounts for any usage.Wrong answer — click to see why▾
Why this is wrong here
Reserved Instances require a 1-year commitment and are cost-effective only for steady-state, predictable workloads. The nightly ETL job runs only 2-4 hours per day, so the discount does not offset the cost of paying for 24/7 reserved capacity, making it more expensive than Spot Instances.
★ When this WOULD be the correct answer
A company runs a 24/7 web server fleet with consistent baseline usage. They need to reduce costs for the always-on instances and can commit to a 1-year or 3-year term. Reserved Instances would provide a significant discount over On-Demand for this steady-state workload.
Why candidates choose this
Candidates know Reserved Instances offer discounts and may assume any usage benefits, overlooking that the discount applies only to the reserved capacity, which is wasted when the instance is idle for most of the day.
✗Switch to On-Demand but enable Auto Scaling so the job finishes faster during peak hours.Wrong answer — click to see why▾
Why this is wrong here
Auto Scaling does not reduce cost; it adds more instances, increasing cost. The job already runs within the deadline, so scaling out is unnecessary and more expensive.
★ When this WOULD be the correct answer
A question where the ETL job is at risk of missing a tight deadline due to variable runtime, and cost is not the primary concern. For example: 'A batch job must complete within 1 hour, but often takes 90 minutes on a single instance. Which change ensures it finishes on time?'
Why candidates choose this
Candidates may think Auto Scaling always reduces cost by optimizing resource usage, but here it would increase cost by adding instances without need.
✗Use Spot Instances but disable checkpointing to simplify the application.Wrong answer — click to see why▾
Why this is wrong here
Disabling checkpointing removes the ability to resume from the last checkpoint on interruption, which is critical for Spot Instances that can be terminated at any time. Without checkpointing, the job would have to restart from scratch, likely missing the 7:00 AM deadline.
★ When this WOULD be the correct answer
If the ETL job were idempotent and very short (e.g., under 5 minutes), or if checkpointing introduced unacceptable overhead, then disabling it might be acceptable. For example, a simple data transformation that runs in under 5 minutes and can be safely restarted without data loss.
Why candidates choose this
Candidates may think simplifying the application by removing checkpointing reduces complexity and overhead, not realizing that checkpointing is essential for fault tolerance with Spot Instances to meet deadlines.
Analysis generated from the official SAA-C03blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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