SAA-C03 Design Cost-Optimized Architectures Practice Question
A dev sandbox has unpredictable DynamoDB traffic with long idle periods and occasional spikes. Which capacity mode should minimize operational overhead and avoid paying for idle provisioned capacity?
⚠ Common exam trap
Test-takers frequently confuse 'reserved capacity' (an EC2/RDS concept) with DynamoDB capacity modes, or think that provisioned capacity with auto-scaling is always cheaper, ignoring the cost of idle capacity during long idle periods.
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
✓
DynamoDB on-demand capacity mode
DynamoDB on-demand capacity mode (Option C) is ideal for unpredictable workloads with long idle periods and occasional spikes because it automatically scales to handle traffic without requiring any capacity planning. You pay only for the reads and writes you actually perform, eliminating the cost of idle provisioned capacity and the operational overhead of managing scaling.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Reserved capacity for maximum daily traffic
Why it's wrong here
Reserved capacity (formerly reserved capacity) locks you into a minimum level of provisioned throughput for a 1-year commitment, with a reduced hourly rate in exchange for that fixed spend. In a dev sandbox with long idle periods, that commitment is wasted every minute the table is unused, because you pay for capacity regardless of actual traffic. It also cannot scale down to zero to match the unpredictable, idle-heavy workload, making it a poor fit for this scenario.
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Provisioned capacity set for peak traffic
Why it's wrong here
Provisioned capacity set for peak traffic forces you to pay a fixed per-hour rate for the peak read/write capacity needed only during the busiest moments, while the table sits idle for the rest of the time. This creates the classic cost-inefficiency problem: you are paying for capacity you do not actually consume. Furthermore, if traffic unpredictably exceeds that peak value (even briefly), DynamoDB will throttle requests and return errors unless you also enable auto scaling, which still has a minimum threshold and cannot eliminate the idle waste.
- ✓
DynamoDB on-demand capacity mode
Why this is correct
DynamoDB on-demand capacity mode charges you only for the actual read and write requests your application makes, with no minimum capacity and no need to forecast traffic patterns. It instantly accommodates spikes from unpredictable developer workloads, and when the table is idle, you pay nothing beyond storage costs. This directly solves the problem of paying for unused capacity during long idle periods, which is exactly why on-demand is the correct choice for this dev sandbox.
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Global tables in every Region
Why it's wrong here
Global tables in every Region replicate the DynamoDB table across multiple AWS Regions to provide low-latency reads and disaster recovery, but they also add per-Region storage and cross-Region replication write costs for every item change. Selecting this option would not address the idle-capacity waste at all; instead, it would multiply the cost by the number of Regions and introduce additional operational complexity. Since the workload is a dev sandbox that does not require multi-Region access or high availability, this option is both irrelevant and counterproductive.
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