Jeff’s Note #
Unlike generic exam dumps, ADH analyzes this scenario through the lens of a Real-World Lead Developer.
For AWS DVA-C02 candidates, the confusion often lies in how Lambda functions access private network resources inside a VPC. In production, this is about knowing exactly how to configure Lambda networking and subnets so it can make HTTP calls to an internal EC2-hosted application without compromising isolation or security. Let’s drill down.
The Certification Drill (Simulated Question) #
Scenario #
Your team at Nextech Innovations is developing a serverless workflow where an AWS Lambda function must send HTTP POST requests to an internal CRM application hosted on EC2 instances. The EC2 instances run inside a private subnet of a VPC, inaccessible from the public internet. Both the Lambda function and the EC2 instances are managed within the same AWS account and region.
The Requirement: #
Design a solution that allows the Lambda function to securely invoke the internal CRM service over HTTP, respecting the private subnet restrictions.
The Options #
- A) Configure a VPC endpoint to connect to the private subnet. Attach the endpoint to the Lambda function.
- B) Attach the Lambda function to the VPC and the private subnet so it can communicate internally.
- C) Configure a VPN connection between the Lambda function and the private subnet. Attach the VPN to the Lambda function.
- D) Modify the VPC route table to include the Lambda function’s IP address.
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Correct Answer #
B
Quick Insight: The Developer Imperative #
To allow a Lambda function to communicate with resources in a private subnet, the function must be configured to run within the VPC itself, attached to the appropriate subnets and security groups. This enables direct network connectivity inside the VPC.
Alternatives like VPC endpoints work for specific AWS services but do not provide generic private network access to EC2 instances. VPNs are unnecessary within the same account’s VPC and add complexity. Route table changes alone cannot grant Lambda network access unless it’s attached to the VPC.
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 B
The Winning Logic #
To enable a Lambda function to reach an internal EC2-hosted application in a private subnet, the Lambda must be configured to run within the same VPC and subnet(s) as the EC2 instances (or at least in subnets with routing that can reach those EC2s). This setup allows the Lambda function to inherit VPC networking, using interfaces connected to the specified subnets, and lets it communicate over private IP addresses securely.
Lambda functions are isolated by default from VPC networks and only have internet access via default NAT configurations. Attaching a Lambda to a VPC explicitly configures elastic network interfaces (ENIs) in the specified subnets for internal communication.
The Trap (Distractor Analysis): #
-
Why not A?
VPC endpoints are specialized gateways for supported AWS services (like S3, DynamoDB, SNS), not for generic EC2 internal apps. There’s no VPC endpoint type that connects specifically to a private subnet for HTTP calls to EC2. -
Why not C?
VPN connectivity between Lambda and private subnet is unnecessary because Lambda in the same VPC can access internal resources directly without establishing encrypted tunnels. -
Why not D?
Adding a Lambda function’s IP address to a route table is invalid because Lambda functions don’t have fixed IP addresses—they require VPC attachment for network presence. Route tables do not directly control Lambda function networking.
The Technical Blueprint #
B) For Developer (Code/CLI Snippet) #
Attach Lambda to VPC and subnets via CLI:
aws lambda update-function-configuration \
--function-name InternalAppCaller \
--vpc-config SubnetIds=subnet-0123456789abcdef0,SecurityGroupIds=sg-0abcdef1234567890
This command configures the Lambda function to use specified VPC subnets and security groups, enabling it to reach EC2 instances in the private subnet.
The Comparative Analysis #
| Option | API/Config Complexity | Performance | Use Case |
|---|---|---|---|
| A | Moderate | High latency | Supports AWS services, not EC2 internal |
| B | Low | Low latency | Direct VPC access, best for internal calls |
| C | High | Complex | Overkill for intra-VPC network communication |
| D | Invalid | N/A | Ip-based routing for Lambda is unsupported |
Real-World Application (Practitioner Insight) #
Exam Rule #
“For the exam, always pick Lambda VPC Attachment when you see the keyword private subnet communication.”
Real World #
“In practice, attaching Lambda to a VPC requires careful subnet and security group planning to avoid cold start latency, but it’s the only way to reach private EC2 services.”
(CTA) Stop Guessing, Start Mastering #
Disclaimer
This is a study note based on simulated scenarios for the AWS DVA-C02 exam.