Compute Savings Plan and Lambda VPC Configuration for Cost Optimization

SAA-C03 By islever Editorial Last updated

Design cost-optimized compute solutions.

Short answer

C Purchase a Compute Savings Plan and connect Lambda functions to the private subnets containing the EC2 instances.

Question

A company runs its application by using Amazon EC2 instances and AWS Lambda functions. The EC2 instances run in private subnets of a VPC. The Lambda functions need direct network access to the EC2 instances for the application to work. The application will run for 1 year. The number of Lambda functions that the application uses will increase during the 1-year period. The company must minimize costs on all application resources. Which solution will meet these requirements?
  1. Purchase an EC2 Instance Savings Plan. Connect the Lambda functions to the private subnets that contain the EC2 instances.
  2. Purchase an EC2 Instance Savings Plan. Connect the Lambda functions to new public subnets in the same VPC where the EC2 instances run.
  3. Purchase a Compute Savings Plan. Connect the Lambda functions to the private subnets that contain the EC2 instances. Correct Answer
  4. Purchase a Compute Savings Plan. Keep the Lambda functions in the Lambda service VPC.

Key Takeaways

Tests the distinction between EC2 Instance Savings Plans (EC2 only) and Compute Savings Plans (EC2 + Lambda), combined with secure VPC connectivity.

To minimize costs while maintaining network access, use a Compute Savings Plan covering both EC2 and Lambda, and attach Lambda to private subnets.

Common mistake: Choosing A or B due to misunderstanding that EC2 Instance Savings Plans do not cover Lambda functions, leading to higher uncommitted costs.

Expert Analysis

Why the Answer Is Correct

A Compute Savings Plan is the correct choice because it provides significant discounts for both Amazon EC2 and AWS Lambda usage, whereas an EC2 Instance Savings Plan only covers EC2. Since the question requires minimizing costs for all resources including increasing Lambda functions, the Compute Savings Plan is essential. Connecting Lambda to private subnets allows direct, secure communication with EC2 instances within the same VPC without incurring data transfer costs associated with internet gateways.

Why the Other Options Are Wrong

Options A and B suggest using an EC2 Instance Savings Plan, which does not apply to Lambda functions, leaving Lambda charges at on-demand rates. Option B also suggests public subnets, which is unnecessary and potentially less secure than private subnets for internal communication. Option D suggests keeping Lambda in its service VPC, but this would prevent direct network access to the customer's EC2 instances as required by the application architecture.

Community Comment Notes

Community consensus strongly supports C, noting that "Compute savings include lambda and EC2" while instance plans do not. Some users initially considered D due to default networking behavior but were corrected that explicit VPC attachment is needed for private subnet access. One user highlighted that "Lambda functions in private subnets can directly access EC2 instances... reducing security risks," reinforcing the architectural choice.

Official Reference

https://aws.amazon.com/savingsplans/compute-pricing/

This link opens the official vendor documentation in a new tab.

Exam Strategy

Always check if the Savings Plan type covers all compute services mentioned in the scenario. For mixed workloads of EC2 and Lambda, Compute Savings Plans are almost always the cost-effective answer over specific instance plans.

Frequently Asked Questions

Why not use an EC2 Instance Savings Plan?

An EC2 Instance Savings Plan only covers EC2 usage. It does not provide any discount for Lambda functions, which are part of the workload.

Can Lambda access EC2 in private subnets from its service VPC?

No. By default, Lambda runs in a service-managed VPC. To access resources in your VPC, you must explicitly configure the Lambda function's VPC settings.

More SAA-C03 FAQ →

Community Votes

C
84%
A
16%

84% of anonymous learners picked answer C. Votes are pick records left by other test-takers — they are not the verified answer.

Discussion 7 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 7

  1. Guru4Cloud 👍 7 Answered C
    Compute Savings Plan: This plan offers significant discounts on Lambda functions compared to on-demand pricing. Since the application will run for a year, a sustained use discount like Compute Savings Plan is ideal. Private Subnets: Lambda functions in private subnets can directly access EC2 instances within the VPC without needing internet access, reducing security risks and potential egress costs.
  2. sheilawu 👍 2 Answered A
    In this question has point out "access EC2 instances" within VPC,=> Lambda VPC to an ENI (Elastic network interface) in your account VPC.=>No charge. Therefore I stick with A, Not D.
  3. jacinml 👍 2 Answered C
    Compute savings include lamdba and EC2, Ec2 savings only EC2 instances. https://aws.amazon.com/savingsplans/compute-pricing/
  4. mk168898 👍 2
    A and B is not because it is talkign about EC2, but the question is asking for Lambda
  5. LeonSauveterre 👍 1 Answered C
    Why Compute Savings Plan: Covers both EC2 and Lambda, providing flexibility as Lambda usage increases. Why Private Subnets: Ensures network access between Lambda and EC2 instances. !! An EC2 Instance Savings Plan doesn't cover Lambda. Also, Lambda functions in public subnets cannot directly communicate with EC2 instances in private subnets.
Show 2 more comments
  1. Lin878 👍 1
    I confuse this Question. Instance saving plan is cheaper than compute saving plan. https://aws.amazon.com/savingsplans/compute-pricing/
  2. Scheldon 👍 1 Answered D
    AnswerD https://docs.aws.amazon.com/lambda/latest/dg/foundation-networking.html

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