The most likely cause is that the Lambda function's security group does not allow outbound traffic to the RDS security group. When a Lambda function is deployed inside a VPC, it loses default internet access and relies entirely on VPC routing and security group rules to reach resources like an RDS instance in a private subnet. Even if the function and database are in the same VPC, the Lambda security group must have an outbound rule permitting traffic to the RDS security group on the database port, and the RDS security group must have a corresponding inbound rule; a missing or misconfigured rule will cause the connection to hang until timeout. On the AWS Certified Developer Associate DVA-C02 exam, this scenario tests your understanding of VPC networking for serverless workloads—a common trap is assuming that being in the same VPC is sufficient without checking security group bidirectional rules. Remember the memory tip: "Lambda in a VPC needs a two-way handshake—outbound from Lambda, inbound on RDS."
DVA-C02 Deployment Practice Question
This DVA-C02 practice question tests your understanding of deployment. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. 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.
An application deployed on AWS Lambda needs access to an Amazon RDS database. The Lambda function is in a VPC with a public subnet. The RDS instance is in a private subnet. The function can connect to the database but experiences frequent timeouts. What is the MOST likely cause?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Lambda function's security group does not allow outbound traffic to the RDS security group.
Option C is correct: Lambda functions in a VPC need a NATgateway to access the internet, but for RDS in private subnet, they need a VPC endpoint or the Lambda function must be in the same VPC with proper routing. Timeouts often occur if the Lambda function does not have a route to the RDS subnet (e.g., missing route table entry or security group issue). Option A (memory) would not cause connection timeouts. Option B (timeout) is set but not the cause. Option D (IAM) would cause access denied, not timeout.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 Lambda function timeout is set too low for the database queries.
Why it's wrong here
While possible, the question states frequent timeouts, implying a systemic issue, not just query duration.
✗
The IAM role for the Lambda function does not have the 'rds:Connect' permission.
Why it's wrong here
RDS connections are managed by database credentials, not IAM; IAM is used for authentication only with IAM database auth.
✗
The Lambda function does not have enough memory allocated.
Why it's wrong here
Insufficient memory causes out-of-memory errors, not connection timeouts.
✓
The Lambda function's security group does not allow outbound traffic to the RDS security group.
Why this is correct
If the security group blocks outbound traffic to the RDS port, connections will timeout.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
Real-world example
How this comes up in practice
A company's IT admin needs to give a contractor read-only access to production logs without sharing account credentials. Using role-based access control (RBAC) and temporary scoped permissions — not a permanent shared password — is the correct pattern. Questions like this test whether you can apply least-privilege access across cloud identity services.
What to study next
Got this wrong? Here's your next step.
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DVA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Deployment — This question tests Deployment — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The Lambda function's security group does not allow outbound traffic to the RDS security group. — Option C is correct: Lambda functions in a VPC need a NAT gateway to access the internet, but for RDS in private subnet, they need a VPC endpoint or the Lambda function must be in the same VPC with proper routing. Timeouts often occur if the Lambda function does not have a route to the RDS subnet (e.g., missing route table entry or security group issue). Option A (memory) would not cause connection timeouts. Option B (timeout) is set but not the cause. Option D (IAM) would cause access denied, not timeout.
What should I do if I get this DVA-C02 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related DVA-C02 subnetting questions on CIDR, address ranges, and subnet selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
CIDR notation defines the prefix length.
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