The answer is Amazon DynamoDB with on-demand capacity. This is the most cost-effective choice because DynamoDB provides single-digit millisecond latency for key-value lookups by user ID, and its on-demand mode automatically scales to handle read-heavy workloads with occasional writes while charging only for the reads and writes consumed, eliminating the need to provision for peak capacity. On the AWS Certified Database Specialty DBS-C01 exam, this scenario tests your understanding of when to choose on-demand versus provisioned capacity—a common trap is selecting provisioned capacity for a startup with unpredictable traffic, which would lead to over-provisioning costs. Remember that on-demand is ideal for variable or unknown workloads, while provisioned suits steady, predictable traffic. Memory tip: think "startup surprise" for on-demand—unpredictable traffic means you pay only for what you use.
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.
A startup is building a mobile application that requires a database to store user preferences and session data. The data is accessed by user ID and requires single-digit millisecond latency. The workload is read-heavy with occasional writes. Which database service is MOST cost-effective?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Amazon DynamoDB with on-demand capacity
Amazon DynamoDB with on-demand capacity is the most cost-effective choice because it provides single-digit millisecond latency for key-value lookups by user ID, scales automatically to handle read-heavy workloads with occasional writes, and charges only for the reads and writes consumed, avoiding the cost of provisioning for peak capacity. The on-demand mode eliminates the need for capacity planning, making it ideal for unpredictable or variable traffic patterns typical of a startup's mobile application.
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.
✗
Amazon Aurora Serverless
Why it's wrong here
Aurora Serverless is relational and more expensive for simple key-value access.
✗
Amazon ElastiCache for Memcached
Why it's wrong here
ElastiCache is a cache, not a durable database; data loss risk.
✓
Amazon DynamoDB with on-demand capacity
Why this is correct
DynamoDB provides single-digit millisecond latency and is cost-effective for variable read-heavy workloads.
Related concept
Read the scenario before looking for a memorised answer.
✗
Amazon RDS for MySQL with provisioned IOPS
Why it's wrong here
RDS is relational and may be more expensive for simple key-value access.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often choose Amazon ElastiCache for Memcached (Option B) because of its low latency, but they overlook the requirement for a durable database that persists session data, whereas Memcached is a volatile cache with no built-in persistence or replication for data durability.
Detailed technical explanation
How to think about this question
DynamoDB achieves single-digit millisecond latency by using SSD-backed storage and a distributed hash table architecture that partitions data across multiple nodes based on the partition key (user ID). The on-demand capacity mode uses a pay-per-request pricing model where you pay for read request units (RRUs) and write request units (WRUs) consumed, with automatic scaling up to the account-level throughput limits, which is ideal for workloads with unpredictable traffic spikes. In contrast, provisioned capacity requires estimating read/write capacity units upfront, leading to over-provisioning costs or throttling under unexpected load.
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: Amazon DynamoDB with on-demand capacity — Amazon DynamoDB with on-demand capacity is the most cost-effective choice because it provides single-digit millisecond latency for key-value lookups by user ID, scales automatically to handle read-heavy workloads with occasional writes, and charges only for the reads and writes consumed, avoiding the cost of provisioning for peak capacity. The on-demand mode eliminates the need for capacity planning, making it ideal for unpredictable or variable traffic patterns typical of a startup's mobile application.
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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Question Discussion
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