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What is AWS Lambda and how does the serverless computing model differ from traditional server based applications? 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 does Lambda support running functions within a VPC, and what performance considerations does this introduce compared to functions that do not need VPC access? How should an organization approach designing observability and error handling for a complex, multi function serverless architecture built on Lambda?
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AWS › Lambda & Serverless

How do Lambda layers help share common code and dependencies across multiple functions without duplicating that code in every deployment package?

Intermediate

Answer

A Lambda layer is a separate archive containing libraries, custom runtimes, or other dependencies that can be attached to multiple Lambda functions, meaning shared code, such as a common logging library or a large third party dependency, only needs to be packaged and updated in one place, keeping individual function deployment packages smaller and making it easier to maintain consistency across many related functions.
aws lambda publish-layer-version --layer-name common-utilities --zip-file fileb://layer.zip
Real-world example A company with dozens of Lambda functions extracts their shared logging and error handling code into a single Lambda layer, attaching it to every function so that a bug fix in that shared logic only needs to be updated and republished once rather than in every individual function's deployment package.

Common follow-ups: How many layers can a single Lambda function use at once?;What is the maximum combined size limit when using layers alongside a function's own code?

AWS Serverless Application Model (SAM);IaC (CloudFormation)
All 7 Lambda & Serverless questions