Serverless developers frequently import libraries and dependencies into their AWS Lambda functions. While you can zip these dependencies as part of the build and deployment process, in many cases it’s easier to use layers instead. In this repo, I explain how layers work, and how you can build and include layers in your own applications.

A Lambda layer is an archive containing additional code, such as libraries, dependencies, or even custom runtimes. When you include a layer in a function, the contents are extracted to the /opt directory in the execution environment. You can include up to five layers per function, which count towards the standard Lambda deployment size limits.

Layers are deployed as immutable versions, and the version number increments each time you publish a new layer. When you include a layer in a function, you specify the layer version you want to use. Layers are automatically set as private, but they can be shared with other AWS accounts, or shared publicly. Permissions only apply to a single version of a layer.

Using layers can make it faster to deploy applications with the AWS Serverless Application Model (AWS SAM) or the Serverless framework. By moving runtime dependencies from your function code to a layer, this can help reduce the overall size of the archive uploaded during a deployment.