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What event sources can trigger a Lambda function to execute, and how does this support building event driven applications? What is a cold start in Lambda, and what strategies help reduce its impact on application performance? How does Lambda concurrency work, and what is the difference between reserved concurrency and the account level concurrency limit? How do Lambda layers help share common code and dependencies across multiple functions without duplicating that code in every deployment package? How should an organization approach designing observability and error handling for a complex, multi function serverless architecture built on Lambda?
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How does Lambda support running functions within a VPC, and what performance considerations does this introduce compared to functions that do not need VPC access?

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Answer

Lambda functions can be configured to run within a specific VPC, which is necessary when the function needs to access resources like an RDS database or ElastiCache cluster that are not publicly accessible, and while modern Lambda VPC networking has significantly improved cold start performance compared to older implementations, functions still need properly configured subnets with sufficient available IP addresses and appropriate route tables if they also need outbound internet access through a NAT Gateway.
aws lambda update-function-configuration --function-name my-function --vpc-config SubnetIds=subnet-12345,SecurityGroupIds=sg-12345
Real-world example A Lambda function needing to query a private RDS database is configured to run within the same VPC as that database, using a properly sized subnet to avoid IP address exhaustion issues during periods of high concurrent execution.

Common follow-ups: How do you provide internet access to a Lambda function running inside a private VPC subnet?;What IP address exhaustion issues can occur with VPC connected Lambda functions at high concurrency?

VPC & Networking;RDS & Databases
All 7 Lambda & Serverless questions