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Cold start and warm start

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Cold start and warm start refer to the two different states of a Lambda function container. A cold start occurs when your function is invoked for the first time or after a period of inactivity. During the cold start, AWS Lambda provisions a new container, loads runtime environments, and initializes…

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AWS Certified Solutions Architect - Cold start and warm start Example Questions

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Question 1

An online store uses multiple Node.js AWS Lambda functions behind API Gateway. During sudden traffic spikes, users experience higher latency due to cold starts. The team wants to prevent cold starts from impacting end-user latency by keeping execution environments initialized ahead of time. Which action should they take?

Correct Answer: Use provisioned concurrency for the Lambda functions.

Provisioned concurrency keeps a specified number of Lambda execution environments initialized and ready to serve requests, effectively eliminating cold starts and the latency spikes they cause. Reserved concurrency only guarantees capacity and prevents throttling but does not pre-initialize environments, so cold starts still occur. Increasing memory may shorten cold-start duration but does not prevent them and offers no consistency under traffic spikes. Decreasing the function timeout has no effect on initialization latency and can cause premature failures.

Question 2

An AWS Lambda function must run inside an Amazon VPC to connect to an Amazon RDS database. The function is experiencing long cold starts due to VPC initialization. Without changing the database engine or removing the function from the VPC, which action will most directly reduce the VPC-related cold start duration?

Correct Answer: Adjust the Lambda function's subnet configuration to increase available IP addresses.

When a Lambda function is configured for VPC access, cold starts include creating and attaching an elastic network interface (ENI) in the target subnets. If those subnets have limited or exhausted IP address capacity, ENI allocation and attachment can be delayed, increasing cold start duration. Expanding the available IP space (for example, using larger CIDR ranges or dedicated subnets with more free IPs for Lambda) reduces ENI provisioning delays and thus shortens VPC-related cold starts. Migrating to DynamoDB changes the database engine and violates the constraint to keep using RDS and VPC. Enabling AWS X-Ray provides observability but does not reduce cold start time. Increasing memory can affect runtime performance and some initialization overhead, but it does not address the ENI creation bottleneck that dominates VPC cold starts.

Question 3

A mobile app uses an AWS Lambda function to process user requests. The app's performance noticeably decreases when multiple users access it simultaneously due to cold starts. What technique should be applied to minimize the cold start impact?

Correct Answer: Enable provisioned concurrency on the Lambda function.

Provisioned concurrency allows pre-warming of a specified number of function instances, minimizing the impact of cold starts. Timeouts, security groups, multi-AZ, memory allocation, or ALB changes do not directly address the cold start issue.

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