SAA-C03 Practice Question 20: DynamoDB Capacity Mode Cost
AWS Certified Solutions Architect – Associate (SAA-C03) · Test 2 · question 10 of 10
An Amazon DynamoDB table in on-demand capacity mode backs a telemetry ingestion service. For the past year its traffic has been nearly flat around the clock, at about 4,000 reads and 1,500 writes per second, varying by less than 10% and growing slowly and predictably. The table holds only about 200 GB, so throughput makes up almost the entire DynamoDB bill. Reads are spread evenly across millions of keys, with very little repetition.
Finance will accept a one-year commitment. Which change lowers the table's cost the most?
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Answer:
On-demand mode is the right default for unknown or spiky traffic. This workload is the opposite: it is steady, predictable, and easy to forecast, which is the case AWS describes for provisioned capacity. Provisioned mode bills for the capacity you set, and auto scaling keeps that setting close to the real ±10% variation, so little capacity sits idle. With a one-year commitment you can then buy reserved capacity for the flat baseline. That is a partial upfront payment for a discounted hourly rate on committed provisioned throughput, saving up to 54% off standard rates for a one-year term. Any capacity above the reservation is billed at normal provisioned rates.
Option A is not possible. DynamoDB reserved capacity applies only to provisioned-mode tables on the Standard table class. It is not available for on-demand tables, so there is nothing for the commitment to discount.
Option B spends money on the wrong tool. DAX pays off when the same keys are read repeatedly and the cache hit rate is high. With reads spread across millions of keys with little repetition, most requests would miss the cache and still reach the table, and DAX does nothing for the 1,500 writes per second. The cluster would be added cost.
Option D is the trap. Standard-IA lowers storage prices but raises read and write prices, so it only pays off when storage dominates the bill, and here throughput does. It also cannot be combined with reserved capacity, which is not offered for Standard-IA tables.
References: DynamoDB reserved capacity; DynamoDB throughput capacity