The answer is Lambda reserved concurrency, which sets a hard limit on concurrent executions to control costs during unpredictable traffic spikes. This approach prevents runaway scaling by capping the number of invocations your function can handle simultaneously, directly avoiding excessive DynamoDB read/write capacity consumption and keeping your bill predictable without pre-warming instances. On the AWS Certified Solutions Architect Professional SAP-C02 exam, this scenario tests your understanding of throttling mechanisms versus provisioned concurrency—a common trap is choosing provisioned concurrency for cost savings, but that incurs charges even when idle, while reserved concurrency only costs for actual invocations. Remember the key distinction: reserved concurrency is a cost-control ceiling, not a performance accelerator. Memory tip: “Reserved for cost, provisioned for speed.”
SAP-C02 Design for New Solutions Practice Question
This SAP-C02 practice question tests your understanding of design for new solutions. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 solutions architect is designing a new serverless application using AWS Lambda for business logic, Amazon API Gateway for RESTful APIs, and Amazon DynamoDB for data storage. The application will experience unpredictable traffic spikes. What is the MOST cost-effective way to handle concurrency and scaling?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Use Lambda reserved concurrency to set a limit on concurrent executions.
Option B is correct because Lambda reserved concurrency sets a hard limit on the number of concurrent executions for a function, preventing runaway scaling and controlling costs during unpredictable traffic spikes. It ensures that the function does not consume more concurrency than allocated, which avoids excessive DynamoDB read/write capacity usage and keeps costs predictable without needing to pre-warm instances.
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 Lambda provisioned concurrency to pre-warm instances.
Why it's wrong here
Provisioned concurrency adds cost and is not needed for unpredictable spikes; it's for latency-sensitive applications.
✓
Use Lambda reserved concurrency to set a limit on concurrent executions.
Why this is correct
Reserved concurrency controls the maximum number of concurrent Lambda invocations, preventing excessive scaling and cost.
Related concept
Read the scenario before looking for a memorised answer.
✗
Configure DynamoDB auto scaling to handle traffic spikes.
Why it's wrong here
DynamoDB auto scaling manages read/write capacity, not Lambda concurrency.
✗
Set a usage plan in API Gateway with a throttling limit.
Why it's wrong here
API Gateway throttling limits API requests but does not directly control Lambda concurrency.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates confuse provisioned concurrency (which reduces latency but adds cost) with reserved concurrency (which controls scaling and cost), or they mistakenly think DynamoDB auto scaling or API Gateway throttling directly manages Lambda concurrency.
Detailed technical explanation
How to think about this question
Lambda reserved concurrency allocates a specific number of concurrent executions from the account-level concurrency pool (default 1000 per region). When set, it prevents the function from exceeding that limit, and any additional invocations are throttled (returning a 429 error) or queued for up to 6 hours. This is critical for unpredictable spikes because it caps cost exposure and avoids overwhelming downstream resources like DynamoDB, which has its own provisioned capacity limits.
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.
Design for New Solutions — This question tests Design for New Solutions — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Use Lambda reserved concurrency to set a limit on concurrent executions. — Option B is correct because Lambda reserved concurrency sets a hard limit on the number of concurrent executions for a function, preventing runaway scaling and controlling costs during unpredictable traffic spikes. It ensures that the function does not consume more concurrency than allocated, which avoids excessive DynamoDB read/write capacity usage and keeps costs predictable without needing to pre-warm instances.
What should I do if I get this SAP-C02 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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