DVA-C02 DynamoDB Read Capacity Units (RCUs) Practice Question
A developer monitors an AWS Lambda function that processes messages from an Amazon SQS queue. CloudWatch logs show that the function's execution time has increased significantly over the past week, and it now frequently times out at the 5-minute timeout. The function's code has not been changed recently. The function makes calls to an Amazon DynamoDB table. What is the most likely cause of the increased execution time?
⚠ Common exam trap
Watch out — candidates often confuse DynamoDB throttling with Lambda timeout configuration, overlooking that gradual performance degradation often points to downstream resource contention rather than function configuration.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The DynamoDB table's read capacity units are underprovisioned, causing throttling.
The most likely cause is that the DynamoDB table's read capacity units are underprovisioned, leading to throttling (ProvisionedThroughputExceededException). When DynamoDB throttles requests, the Lambda function must retry them, which adds latency and can cause the function to exceed its 5-minute timeout. Since the code hasn't changed, this points to a scaling or capacity issue on the DynamoDB side.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The DynamoDB table's read capacity units are underprovisioned, causing throttling.
Why this is correct
When a Lambda function attempts to read from a DynamoDB table with insufficient Read Capacity Units (RCUs), DynamoDB will throttle the requests. This throttling results in ProvisionedThroughputExceededException errors, forcing the Lambda function to implement retry logic, which significantly prolongs its execution time. Repeated retries against a persistently throttled table can cause the function to approach or exceed its configured timeout, indicating a clear resource bottleneck.
- ✗
The SQS queue's visibility timeout is too short, causing duplicate processing.
Why it's wrong here
A short SQS visibility timeout primarily leads to duplicate message processing because other consumers might pick up the message before the original consumer finishes and deletes it. While this increases the total number of executions and potential cost, it does not inherently increase the execution time of a single Lambda invocation. In fact, if a function fails quickly, a shorter timeout might allow faster reprocessing, not a longer individual execution.
- ✗
The Lambda function's memory is too low, causing CPU throttling.
Why it's wrong here
AWS Lambda allocates CPU proportional to the configured memory, so if memory is too low, the function would experience performance degradation due to insufficient CPU, leading to longer execution times. However, a severe memory shortage would more typically result in OutOfMemoryError exceptions, causing the function to fail outright rather than consistently running for an extended period up to its timeout limit. The primary symptom of low memory is often failure, not just slow execution.
- ✗
The DynamoDB table's indexes are missing, causing full table scans.
Why it's wrong here
Missing or improperly designed indexes in DynamoDB would indeed increase the latency of Query operations, potentially leading to longer Lambda execution times if the function relies on complex filtered queries. However, a "full table scan" specifically refers to a Scan operation, which is distinct from GetItem or Query and typically used for analytical purposes. Furthermore, the absence of indexes would not suddenly manifest as a new problem unless there were recent code changes to use unindexed attributes for queries, or a change in the data access pattern.
Quick reference
Cloud Service Model Comparison
| Model | You Manage | Provider Manages | Examples |
|---|---|---|---|
| IaaS | OS, runtime, apps, data | Hardware, hypervisor, networking | EC2, Azure VMs, GCP Compute Engine |
| PaaS | Apps and data | OS, runtime, middleware, hardware | Elastic Beanstalk, Azure App Service |
| SaaS | Data and settings only | Everything else | Microsoft 365, Salesforce, Workday |
| FaaS / Serverless | Function code only | Infra, scaling, runtime | Lambda, Azure Functions, Cloud Run |
| CaaS | Containers and apps | Kubernetes, OS, hardware | EKS, AKS, GKE |
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Same concept, more angles
1 more way this is tested on DVA-C02
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A developer monitors an AWS Lambda function that processes messages from an Amazon SQS queue. CloudWatch logs show that the function's execution time has increased significantly over the past week. The function's code has not been changed recently. The function makes calls to an Amazon DynamoDB table. CloudWatch metrics show a high rate of DynamoDBProvisionedThroughputExceededException errors. The DynamoDB table has 5 read and 5 write capacity units (RCU/WCU). What is the most effective action to reduce the function's execution time?
medium- A.Increase the Lambda function's memory allocation.
- B.Increase the Lambda function's reserved concurrency.
- ✓ C.Increase the DynamoDB table's read and write capacity units.
- D.Increase the Lambda function's timeout.
Why C: The high rate of DynamoDBProvisionedThroughputExceededException errors indicates that the Lambda function is being throttled by DynamoDB due to insufficient read and write capacity units. This throttling causes the function to retry operations, significantly increasing execution time. Increasing the RCU/WCU from 5 to a higher value directly addresses the bottleneck, allowing operations to complete without retries and reducing overall execution time.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This DVA-C02 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 DVA-C02 exam.