Jeff’s Note #
Unlike generic exam dumps, ADH analyzes this scenario through the lens of a Real-World Lead Developer.
For DVA-C02 candidates, the confusion often lies in understanding how to connect Lambda functions to resources inside a VPC. In production, this is about knowing exactly how Lambda networking interacts with private subnets and IAM permissions to reach RDS securely. Let’s drill down.
The Certification Drill (Simulated Question) #
Scenario #
FinTech startup MercuryPay is building a serverless microservice for payment processing. One Lambda function must query a critical Amazon RDS PostgreSQL database instance that is deployed inside a private subnet within a VPC for data security reasons. The team created an IAM role granting the Lambda function permissions to access the RDS instance. However, the developer notices the Lambda still cannot connect to the database.
The Requirement: #
What additional step must the developer take to allow the Lambda function to connect successfully to the RDS instance inside the private subnet?
The Options #
- A) Assign a public IP address to the RDS instance. Modify its security group to allow inbound traffic from the IP address of the Lambda function.
- B) Set up an AWS Direct Connect connection between the Lambda function and the RDS instance.
- C) Configure an Amazon CloudFront distribution to create a secure connection between the Lambda function and the RDS instance.
- D) Configure the Lambda function to connect to the private subnets in the VPC. Add security group rules to allow traffic to the RDS instance from the Lambda function.
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Correct Answer #
D
Quick Insight: The Developer Imperative #
Lambda functions do not have VPC network access by default. To reach any private subnet resource such as RDS inside a VPC, you must explicitly configure the Lambda function to run inside those VPC subnets and adjust security groups accordingly. Simply granting IAM permissions is not enough.
Content Locked: The Expert Analysis #
You’ve identified the answer. But do you know the implementation details that separate a Junior from a Senior?
The Expert’s Analysis #
Correct Answer #
Option D
The Winning Logic #
When a Lambda function needs to access a resource inside a private subnet, such as an RDS instance, it must be configured to run within that VPC. This means:
- Specifying the VPC ID, subnet IDs (private subnets), and associated security groups in the Lambda function’s network configuration.
- Ensuring the Lambda function’s security group allows outbound traffic to the RDS instance.
- Configuring the RDS security group to allow inbound traffic on the database port (e.g., 5432 for PostgreSQL) only from the Lambda’s security group.
IAM roles grant Lambda the permission to interact with AWS resources, but do not provide network connectivity. Without VPC configuration, Lambda executes in a default isolated environment without access to private subnets.
The Trap (Distractor Analysis): #
- Why not A? Assigning a public IP to RDS violates security best practices and still doesn’t grant the Lambda function network access without configuring Lambda’s VPC. Also, Lambda functions in the default environment cannot communicate directly with specific public IPs without VPC config.
- Why not B? AWS Direct Connect is for dedicated network connections from on-premises data centers; irrelevant inside a Lambda-to-RDS communication scenario.
- Why not C? CloudFront is a CDN for HTTP content, not applicable for database connections from Lambda to RDS.
The Technical Blueprint #
# Example CLI to update a Lambda function's VPC config:
aws lambda update-function-configuration \
--function-name ProcessPaymentsFunction \
--vpc-config SubnetIds=subnet-0abc12345de678fgh,subnet-0def23456ghijklm7,SecurityGroupIds=sg-0123456789abcdef0
Security Group rules for RDS might look like:
aws ec2 authorize-security-group-ingress \
--group-id sg-rds123456 \
--protocol tcp \
--port 5432 \
--source-group sg-lambda345678
The Comparative Analysis #
| Option | API Complexity | Performance | Use Case |
|---|---|---|---|
| A | Low | Risk of latency, public exposure | Not recommended for private DB access |
| B | High | Not applicable | Dedicated network for on-premises |
| C | Medium | Not applicable | CDN for HTTP/S static content |
| D | Moderate (VPC config) | Optimal and secure | Correct Lambda private subnet access |
Real-World Application (Practitioner Insight) #
Exam Rule #
“For the exam, always configure Lambda’s VPC settings when accessing resources in private subnets.”
Real World #
“In production, setting Lambda in the VPC adds cold start latency due to ENI creation, so it’s worth weighing trade-offs with networking needs.”
(CTA) Stop Guessing, Start Mastering #
Disclaimer
This is a study note based on simulated scenarios for the DVA-C02 exam.