Exams / SAA-C03 / Database cost optimisation / Question 1
SAA-C03 practice question: Database cost optimisation (1 of 2)
Design Cost-Optimized Architectures A free question from our original SAA-C03 practice exam. Try it, then open the answer to see why each option is right or wrong.
A ticketing start-up is launching a new app backed by an Amazon DynamoDB table. For most of the day the table receives a few requests a second, but when tickets for a popular event go on sale, writes jump within seconds to about 2,000 a second for 2 to 3 minutes. The team cannot predict when sales will open or how large they will be. Requests must not be throttled, and nobody wants to manage capacity. Which configuration is MOST cost-effective?
Choose one answer.
- AA. Use provisioned capacity with DynamoDB auto scaling at a 70% target utilisation, starting from the quiet-period baseline.
- BB. Use provisioned capacity sized for 2,000 writes a second, and buy DynamoDB reserved capacity for that amount.
- CC. Use on-demand capacity mode, so that the table is billed per request and scales with the traffic.
- DD. Use provisioned capacity sized for the quiet periods, and put a DynamoDB Accelerator (DAX) cluster in front of the table.
Show the answer and explanation
Answer: C
- A. Wrong: Auto scaling only changes capacity after usage stays above the target for a couple of minutes, and the update then takes several more minutes. A spike that arrives in seconds and lasts 2 to 3 minutes would be throttled before the extra capacity arrived. Learn more: Managing throughput capacity automatically with DynamoDB auto scaling (opens in a new tab)
- B. Wrong: Reserved capacity discounts provisioned capacity that is used steadily, but the company would pay for 2,000 writes a second all day while using a tiny fraction of it. The peak is also only a guess, so a bigger sale could still be throttled. Learn more: DynamoDB reserved capacity (opens in a new tab)
- C. Right: Correct. On-demand mode charges per read and write request and needs no capacity planning. A new on-demand table can sustain up to 4,000 writes a second, and the mode instantly handles up to double the previous peak, so the sudden 2,000-write spikes are absorbed while quiet hours cost very little. Learn more: DynamoDB on-demand capacity mode (opens in a new tab)
- D. Wrong: DAX caches reads, but it is a write-through cache: every write is passed to DynamoDB, and if DynamoDB throttles the write, the error goes back to the caller. The write spike would still hit the table's small provisioned capacity and be throttled. Learn more: DAX and DynamoDB consistency models (opens in a new tab)
Explanation
Sudden, short, unpredictable spikes on a mostly quiet table are what on-demand capacity mode is for. It is billed per request, needs no capacity planning, and a new table can sustain up to 4,000 writes a second. Auto scaling reacts in minutes, not seconds; provisioning or reserving for the peak pays for idle capacity; and DAX does not reduce write load.
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