Building Lambda layers in AWS SAM

You can use AWS SAM to build custom Lambda layers. Lambda layers allow you to extract code from a Lambda function that can then be re-used across several Lambda functions. Building only Lambda layers (instead of building your entire application) can benefit you in a few ways. It can help you reduce the size of your deployment packages, separate core function logic from dependencies, and allow you to share dependencies across multiple functions. For information about layers, see AWS Lambda layers in the AWS Lambda Developer Guide.

How to build a Lambda layer in AWS SAM

Note

Before you can build a Lambda layer, you must first write a Lambda layer in your AWS SAM template. For information and examples on doing this, see Increase efficiency using Lambda layers with AWS SAM.

To build a custom layer, declare it in your AWS Serverless Application Model (AWS SAM) template file and include a Metadata resource attribute section with a BuildMethod entry. Valid values for BuildMethod are identifiers for an AWS Lambda runtime, or makefile. Include a BuildArchitecture entry to specify the instruction set architectures that your layer supports. Valid values for BuildArchitecture are Lambda instruction set architectures.

If you specify makefile, provide the custom makefile, where you declare a build target of the form build-layer-logical-id that contains the build commands for your layer. Your makefile is responsible for compiling the layer if necessary, and copying the build artifacts into the proper location required for subsequent steps in your workflow. The location of the makefile is specified by the ContentUri property of the layer resource, and must be named Makefile.

Note

When you create a custom layer, AWS Lambda depends on environment variables to find your layer code. Lambda runtimes include paths in the /opt directory where your layer code is copied into. Your project's build artifact folder structure must match the runtime's expected folder structure so your custom layer code can be found.

For example, for Python you can place your code in the python/ subdirectory. For NodeJS, you can place your code in the nodejs/node_modules/ subdirectory.

For more information, see Including library dependencies in a layer in the AWS Lambda Developer Guide.

The following is an example Metadata resource attribute section.

    Metadata:
      BuildMethod: python3.8
      BuildArchitecture: arm64

Note

If you don't include the Metadata resource attribute section, AWS SAM doesn't build the layer. Instead, it copies the build artifacts from the location specified in the CodeUri property of the layer resource. For more information, see the ContentUri property of the AWS::Serverless::LayerVersion resource type.

When you include the Metadata resource attribute section, you can use the sam build command to build the layer, both as an independent object, or as a dependency of an AWS Lambda function.

• As an independent object. You might want to build just the layer object, for example when you're locally testing a code change to the layer and don't need to build your entire application. To build the layer independently, specify the layer resource with the sam build layer-logical-id command.

• As a dependency of a Lambda function. When you include a layer's logical ID in the Layers property of a Lambda function in the same AWS SAM template file, the layer is a dependency of that Lambda function. When that layer also includes a Metadata resource attribute section with a BuildMethod entry, you build the layer either by building the entire application with the sam build command or by specifying the function resource with the sam build function-logical-id command.

Examples

Template example 1: Build a layer against the Python 3.9 runtime environment

The following example AWS SAM template builds a layer against the Python 3.9 runtime environment.

Resources:
  MyLayer:
    Type: AWS::Serverless::LayerVersion
    Properties:
      ContentUri: my_layer
      CompatibleRuntimes:
        - python3.9
    Metadata:
      BuildMethod: python3.9   # Required to have AWS SAM build this layer

Template example 2: Build a layer using a custom makefile

The following example AWS SAM template uses a custom makefile to build the layer.

Resources:
  MyLayer:
    Type: AWS::Serverless::LayerVersion
    Properties:
      ContentUri: my_layer
      CompatibleRuntimes:
        - python3.8
    Metadata:
      BuildMethod: makefile

The following makefile contains the build target and commands that will be executed. Note that the ContentUri property is set to my_layer, so the makefile must be located in the root of the my_layer subdirectory, and the filename must be Makefile. Note also that the build artifacts are copied into the python/ subdirectory so that AWS Lambda will be able to find the layer code.

build-MyLayer:
  mkdir -p "$(ARTIFACTS_DIR)/python"
  cp *.py "$(ARTIFACTS_DIR)/python"
  python -m pip install -r requirements.txt -t "$(ARTIFACTS_DIR)/python"

Example sam build commands

The following sam build commands build layers that include the Metadata resource attribute sections.

# Build the 'layer-logical-id' resource independently
$ sam build layer-logical-id
            
# Build the 'function-logical-id' resource and layers that this function depends on
$ sam build function-logical-id

# Build the entire application, including the layers that any function depends on
$ sam build