Most Cost-Effective DynamoDB Capacity Mode for Predictable Workloads

SAA-C03 By islever Editorial Last updated

Design cost-optimized storage solutions.

Short answer

B Use provisioned mode and specify the RCUs and WCUs to match the constant, predictable DynamoDB workload at the lowest cost.

Question

A company collects data from a large number of participants who use wearable devices. The company stores the data in an Amazon DynamoDB table and uses applications to analyze the data. The data workload is constant and predictable. The company wants to stay at or below its forecasted budget for DynamoDB.
Which solution will meet these requirements MOST cost-effectively?
  1. Use provisioned mode and DynamoDB Standard-Infrequent Access (DynamoDB Standard-IA). Reserve capacity for the forecasted workload.
  2. Use provisioned mode. Specify the read capacity units (RCUs) and write capacity units (WCUs). Correct Answer
  3. Use on-demand mode. Set the read capacity units (RCUs) and write capacity units (WCUs) high enough to accommodate changes in the workload.
  4. Use on-demand mode. Specify the read capacity units (RCUs) and write capacity units (WCUs) with reserved capacity.

Key Takeaways

The question tests whether you recognize that constant, predictable workloads call for provisioned mode with defined RCUs and WCUs, and that on-demand mode does not allow you to specify capacity units at all.

For a constant, predictable DynamoDB workload, provisioned mode with specified read and write capacity units is the most cost-effective option. This question tests the choice between provisioned and on-demand capacity modes and the proper use of table classes like Standard-IA.

Common mistake: Option A is the most tempting wrong answer because it adds reserved capacity, but Standard-IA table classes are intended for infrequently accessed data, and the company actively analyzes this data. Some learners also pick C, not realizing that on-demand mode does not let you set RCUs or WCUs.

Expert Analysis

Why the Answer Is Correct

Option B directly matches AWS guidance: provisioned mode is designed for workloads with steady, predictable traffic, and you simply specify the RCUs and WCUs your application needs. Since the question explicitly states the workload is "constant and predictable," provisioned mode delivers the lowest cost because you pay only for the capacity you provision rather than a higher per-request on-demand rate. Because the demand is already known, there is no need for auto-scaling or the flexibility that on-demand mode provides.

Why the Other Options Are Wrong

Option A pairs provisioned mode with Standard-IA, which is designed to reduce storage costs for data accessed far less than once per day — yet the company actively analyzes this wearable data, so Standard-IA is inappropriate. Additionally, reserved capacity historically applies only to Standard table classes, not Standard-IA. Option C is internally inconsistent: in on-demand mode, you cannot specify RCUs and WCUs at all; DynamoDB handles capacity automatically, and you pay per request at a premium rate. Option D is also contradictory because reserved capacity cannot be combined with on-demand mode — reserved capacity is exclusive to provisioned mode.

Community Comment Notes

As hovnival noted, "it is not possible to set Read Capacity Units(RCU)/Write Capacity Units(WCU) in on-demand mode," which immediately rules out C and D. Pentium75 echoed this, adding that Standard-IA doesn't fit because the workload is constant with no indication of infrequent access. MrAWSAssociate pointed out that "Reserved Capacity can only be used with DynamoDB Standard table class," reinforcing why A fails. Lonojack also highlighted that the company needs to analyze the data, making Standard-IA unsuitable. The community strongly converged on B with substantive technical reasoning.

Official Reference

These links open the official vendor documentation in a new tab.

Exam Strategy

When a question describes workloads as constant, predictable, or steady-state, immediately look for provisioned capacity as the most cost-effective answer. Be wary of options that combine contradictory features — on-demand mode with specified RCUs/WCUs or reserved capacity is a red flag that the option is designed to be eliminated.

Frequently Asked Questions

Why is Standard-IA not suitable when adding reserved capacity seems cost-effective?

Standard-IA table classes are designed for data accessed infrequently (roughly once per day or less). Since the company actively analyzes this data, Standard-IA would increase read costs and defeat the purpose of cost optimization.

Can you set RCUs and WCUs in DynamoDB on-demand mode?

No. In on-demand mode, DynamoDB automatically handles capacity based on traffic, and you pay per request. You cannot specify RCUs or WCUs, and reserved capacity is not available with on-demand mode.

More SAA-C03 FAQ →

Discussion 21 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 21

  1. everfly 👍 11 Answered B
    The data workload is constant and predictable.
  2. hovnival 👍 6 Answered B
    I think it is not possible to set Read Capacity Units(RCU)/Write Capacity Units(WCU) in on-demand mode.
  3. pentium75 👍 4 Answered B
    C and D are impossible because you don't set or specify RCUs and WCUs in on-demand mode. A is wrong because there is no indication of "infrequent access", and "the data workload is constant", there is no different between the current and the "forecasted" workload.
  4. Lonojack 👍 4 Answered B
    The problem with (A) is: “Standard-Infrequent Access“. In the question, they say the company has to analyze the Data. That’s why the Correct answer is (B)
  5. MrAWSAssociate 👍 4
    Sorry, A will not work, since Reserved Capacity can only be used with DynamoDB Standard table class. So, B is right for this case.
Show 16 more comments
  1. Samuel03 👍 4 Answered B
    As the numbers are already known
  2. kayodea25 👍 3
    Option C is the most cost-effective solution for this scenario. In on-demand mode, DynamoDB automatically scales up or down based on the current workload, so the company only pays for the capacity it uses. By setting the RCUs and WCUs high enough to accommodate changes in the workload, the company can ensure that it always has the necessary capacity without overprovisioning and incurring unnecessary costs. Since the workload is constant and predictable, using provisioned mode with reserved capacity (Options A and D) may result in paying for unused capacity during periods of low demand. Option B, using provisioned mode without reserved capacity, may result in throttling during periods of high demand if the provisioned capacity is not sufficient to handle the workload.
  3. wsdasdasdqwdaw 👍 3
    predictable/constant => provisioned mode. On-demand mode is more suitable for workloads that are unpredictable and can vary widely from minute to minute. The use case is not for Standard-IA which is described here: https://aws.amazon.com/dynamodb/standard-ia/ => Option B
  4. Lonojack 👍 3
    The problem with (A) is: “Standard-Infrequent Access“. In the question, they say the company has to analyze the Data. Correct answer is (B)
  5. boxu03 👍 3
    you forgot "The data workload is constant and predictable", should be B
  6. Steve_4542636 👍 2
    "The data workload is constant and predictable." https://docs.aws.amazon.com/wellarchitected/latest/serverless-applications-lens/capacity.html "With provisioned capacity you pay for the provision of read and write capacity units for your DynamoDB tables. Whereas with DynamoDB on-demand you pay per request for the data reads and writes that your application performs on your tables."
  7. TariqKipkemei 👍 2 Answered B
    I rule out A because of this 'Standard-Infrequent Access ', clearly the company uses applications to analyze the data. The data workload is constant and predictable making provisioned mode the best option.
  8. Charly0710 👍 2 Answered B
    The data workload is constant and predictable, then, isn't on-demand mode. DynamoDB Standard-IA is not necessary in this context
  9. pentium75 👍 2
    You can't 'set RCUs and WCUs' in on-demand mode.
  10. bdp123 👍 2 Answered A
    workload is constant
  11. Sushmita42 👍 1 Answered A
    The important clues are: Workload is constant and predictable - Provisioned mode Company wants to control/stay within a forecasted budget - long term commitment - reserved capacity Data is collected continuously and stored in DynamoDB They want the solution most cost-effectively - Dynamo DB Standard IA - is designed for data that is accessed less frequently and can provide lower storage costs when the access pattern fits.
  12. Clouddon 👍 1 Answered A
    Option A is preferred because it combines the benefits of provisioned mode, DynamoDB Standard-IA for lower storage costs, and reserved capacity for significant savings on provisioned throughput. This combination is ideal for your scenario with a constant and predictable workload, ensuring you stay within your forecasted budget.
  13. Guru4Cloud 👍 1 Answered A
    Option B lacks the cost benefits of Standard-IA. Option C uses more expensive on-demand pricing. Option D does not actually allow reserving capacity with on-demand mode. So option A leverages provisioned mode, Standard-IA, and reserved capacity to meet the requirements in a cost-optimal way.
  14. ThangAWS2022 👍 1 Answered A
    Provisioned Mode and reserved capacity
  15. Bofi 👍 1
    Kayode olode..lol
  16. MrAWSAssociate 👍 1 Answered A
    A is correct!

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