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 post, I explain how layers work, and how you can build and include layers in your own applications.

This blog post references the Happy Path application, which shows how to build a flexible backend to a photo-processing web application. To learn more, refer to Using serverless backends to iterate quickly on web apps – part 1. This code in this post is available at this GitHub repo.

Overview of Lambda layers

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.

Creating a layer containing the AWS SDK

The AWS SDK allows you to interact programmatically with AWS services using one of the supported runtimes. The Lambda service includes the AWS SDK so you can use it without explicitly importing in your deployment package.

However, there is no guarantee of the version provided in the execution environment. The SDK is upgraded frequently to support new AWS services and features. As a result, the version may change at any time. You can see the current version used by Lambda by declaring an instance of the SDK and logging out the version method:

Logging out the version method

For production workloads, it’s best practice to lock the version of the AWS SDK used in your functions. You can achieve this by including the SDK with your code package. Once you include this library, your code always uses the version in the deployment package and not the version included in the Lambda service.

A serverless application may consist of many functions, which all use a common SDK version. Instead of bundling the SDK with each function deployment, you can create a layer containing the SDK. The effect of this is to reduce the size of the uploaded archive, which makes your deployments faster.

To create an AWS SDK layer:

  1. First, clone this blog post’s GitHub repo. From a terminal window, execute:
    git clone https://github.com/aws-samples/aws-lambda-layers-aws-sam-examples
    cd ./aws-sdk-layer
  2. This directory contains an AWS SAM template and Node.js package.json file. Install the package.json contents:
    npm install
  3. Create the layer directory defined in the AWS SAM template and the nodejs directory required by Lambda. Next, move the node_modules directory:
    mkdir -p ./layer/nodejs
    mv ./node_modules ./layer/nodejs
  4. Next, deploy the AWS SAM template to create the layer:
    sam deploy --guided
  5. For the Stack name, enter “aws-sdk-layer”. Enter your preferred AWS Region and accept the other defaults.
  6. After the deployment completes, the new Lambda layer is available to use. Run this command to see the available layers:
    aws lambda list-layers
    aws lambda list-layers output

After adding a layer to a function, you can use console.log to log out the AWS SDK version. This shows that the function is now using the SDK version in the layer instead of the version provided by the Lambda service:

Use the SDK layer instead of the bundled layer

Creating layers with OS-specific binaries

Many code libraries include binaries that are operating-system specific. When you build packages on your local development machine, by default the binaries for that operating system are used. These may not be the right binaries for Lambda, which runs on Amazon Linux. If you are not using a compatible operating system, you must ensure you include Linux binaries in the layer.

The simplest way to package these libraries correctly is to use AWS Cloud9. This is an IDE in the AWS Cloud, which runs on Amazon EC2. After creating an environment, you can clone a git repository directly to the local storage of the instance, and run the necessary build scripts.

The Happy Path application resizes images using the Sharp npm library. This library uses libvips, which is written in C, so the compilation is operating system-specific. By creating a layer containing this library, it simplifies the packaging and deployment of the consuming Lambda function.

To create a Sharp layer using AWS Cloud9:

  1. Navigate to the AWS Cloud9 console.
  2. Choose Create environment.
  3. Enter the name “My IDE” and choose Next step.
  4. Accept all the default and choose Next step.
  5. Review the settings and choose Create environment.
  6. In the terminal panel, enter:
    git clone https://github.com/aws-samples/aws-lambda-layers-aws-sam-examples
    cd ./aws-lambda-layers-aws-sam-examples/sharp-layer
    npm installCreating a layer in Cloud9
  7. From a terminal window, ensure you are in the directory where you cloned this post’s GitHub repo. Execute the following commands:
    cd ./sharp-layer
    npm install
    mkdir -p ./layer/nodejs
    mv ./node_modules ./layer/nodejsCreating the layer in Cloud9
  8. Next, deploy the AWS SAM template to create the layer:
    sam deploy --guided
  9. For the Stack name, enter “sharp-layer”. Enter your preferred AWS Region and accept the other defaults. After the deployment completes, the new Lambda layer is available to use.

In some runtimes, you can specify a local set of packages for development, and another set for production. For example, in Node.js, the package.json file allows you to specify two sections for dependencies. If your development machine uses a different operating system to Lambda, and therefore uses different binaries, you can use package.json to resolve this. In the Happy Path Resizer function, which uses the Sharp layer, the package.json refers to a local binary for development.

Adding development dependencies to package.json

AWS SAM defines Lambda functions with the AWS::Serverless::Function resource. Layers are defined as a property of functions, as a list of layer ARNs including the version:

 MyLambdaFunction: Type: AWS::Serverless::Function Properties: CodeUri: myFunction/ Handler: app.handler MemorySize: 128 Layers: - !Ref SharpLayerARN

Sharing a layer

Layers are private to your account by default but you can optionally share with other AWS accounts or make a layer public. You cannot share layers via the AWS Management Console but instead use the AWS CLI.

To share a layer, use add-layer-version-permission, specifying the layer name, version, AWS Region, and principal:

aws lambda add-layer-version-permission \ --layer-name node-sharp \ --principal '*' \ --action lambda:GetLayerVersion \ --version-number 3 --statement-id public --region us-east-1

In the principal parameter, specify an individual account ID or use an asterisk to make the layer public. The CLI responds with a RevisionId containing the current revision of the policy:

add-layer-version output

You can check the permissions associated with a layer version by calling get-layer-version-policy with the layer name and version:

aws lambda get-layer-version-policy \ --layer-name node-sharp \ --version-number 3 \ --region us-east-1

get-layer-version-policy output

Similarly, you can delete permissions associated with a layer version by calling remove-layer-vesion-permission with the layer name, statement ID, and version:

aws lambda remove-layer-version-permission \ -- layer-name node-sharp \ -- statement-id public \ -- version-number 3

Once the permissions are removed, calling get-layer-version-policy results in an error:

Error invoking after removal

Conclusion

Lambda layers provide a convenient and effective way to package code libraries for sharing with Lambda functions in your account. Using layers can help reduce the size of uploaded archives and make it faster to deploy your code.

Layers can contain packages using OS-specific binaries, providing a convenient way to distribute these to developers. While layers are private by default, you can share with other accounts or make a layer public. Layers are published as immutable versions, and deleting a layer has no effect on deployed Lambda functions already using that layer.

To learn more about using Lambda layers, visit the documentation, or see how layers are used in the Happy Path web application.