The correct answer is to use sparse global secondary indexes to reduce write capacity costs, alongside atomic counters for inventory management. Atomic counters in DynamoDB allow you to safely increment or decrement a numeric attribute using the UpdateItem operation with an expression like 'SET inventory = inventory - :qty', which eliminates race conditions in high-traffic e-commerce applications without consuming extra read capacity. Sparse indexes, on the other hand, only index items that contain a specific attribute, dramatically lowering write costs when you need to query a subset of data, such as only active products. On the AWS Certified Database Specialty DBS-C01 exam, this question tests your ability to distinguish between cost-optimization patterns and concurrency control mechanisms—a common trap is confusing sparse indexes with global secondary indexes that index every item. Remember the mnemonic: “Sparse saves spend, atomic avoids angst” to recall that sparse indexes cut write costs while atomic counters prevent concurrency headaches.
DBS-C01 Workload-Specific Database Design Practice Question
This DBS-C01 practice question tests your understanding of workload-specific database design. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
Which TWO of the following are valid design considerations when using Amazon DynamoDB for a high-traffic e-commerce application? (Select TWO.)
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use atomic counters to maintain inventory counts
Option C is correct because atomic counters in DynamoDB allow you to increment or decrement a numeric attribute atomically using the UpdateItem operation with an UpdateExpression like 'SET inventory = inventory - :qty'. This is ideal for maintaining inventory counts in a high-traffic e-commerce application, as it avoids the need for read-modify-write cycles and reduces the risk of race conditions without consuming additional read capacity.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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 scan operations for frequent queries to reduce provisioned capacity
Why it's wrong here
Scan is expensive and inefficient.
✗
Use DynamoDB transactions to reduce read capacity consumption
Why it's wrong here
Transactions consume more capacity.
✓
Use atomic counters to maintain inventory counts
Why this is correct
Atomic counters allow incrementing without read-modify-write.
Related concept
Read the scenario before looking for a memorised answer.
✗
Always use strongly consistent reads for all queries
Why it's wrong here
Strongly consistent reads use more RCU and may not be necessary.
✓
Use sparse global secondary indexes to reduce write capacity costs
Why this is correct
Sparse indexes only index items that have the attribute, saving WCU.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
AWS often tests the misconception that DynamoDB transactions reduce capacity consumption, but in reality, transactions increase capacity costs (e.g., TransactGetItems consumes 2x the read capacity units) and are designed for atomicity, not efficiency.
Detailed technical explanation
How to think about this question
Atomic counters in DynamoDB leverage the UpdateItem API with an atomic increment operation that is idempotent and does not require a conditional check, making them highly efficient for high-traffic scenarios. Under the hood, DynamoDB serializes updates to the same item, ensuring consistency without the overhead of transactions. In a real-world e-commerce application, atomic counters are often used for real-time inventory tracking, where even a brief inconsistency could lead to overselling.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A startup's cloud architect reviews their monthly bill and notices costs are higher than expected for a long-running batch job. Switching from on-demand instances to Reserved Instances — or using Spot/Preemptible VMs — can reduce compute costs by up to 72 %. Questions like this test whether you understand the tradeoffs between commitment, flexibility, and cost across cloud pricing models.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Workload-Specific Database Design — This question tests Workload-Specific Database Design — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use atomic counters to maintain inventory counts — Option C is correct because atomic counters in DynamoDB allow you to increment or decrement a numeric attribute atomically using the UpdateItem operation with an UpdateExpression like 'SET inventory = inventory - :qty'. This is ideal for maintaining inventory counts in a high-traffic e-commerce application, as it avoids the need for read-modify-write cycles and reduces the risk of race conditions without consuming additional read capacity.
What should I do if I get this DBS-C01 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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This DBS-C01 practice question is part of Courseiva's free Amazon Web Services certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the DBS-C01 exam.
Question Discussion
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