serverless/docs/providers/aws/guide/layers.md

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AWS - Layers

If you are using AWS as a provider, all layers inside the service are AWS Lambda layers.

Configuration

All of the Lambda layers in your serverless service can be found in serverless.yml under the layers property.

# serverless.yml
service: myService

provider:
  name: aws

layers:
  hello:
    path: layer-dir # required, path to layer contents on disk
    name: ${self:provider.stage}-layerName # optional, Deployed Lambda layer name
    description: Description of what the lambda layer does # optional, Description to publish to AWS
    compatibleRuntimes: # optional, a list of runtimes this layer is compatible with
      - python3.8
    licenseInfo: GPLv3 # optional, a string specifying license information
    allowedAccounts: # optional, a list of AWS account IDs allowed to access this layer.
      - '*'
    retain: false # optional, false by default. If true, layer versions are not deleted as new ones are created

You can add up to 5 layers as you want within this property.

# serverless.yml

service: myService

provider:
  name: aws

layers:
  layerOne:
    path: layerOne
    description: optional description for your layer
  layerTwo:
    path: layerTwo
  layerThree:
    path: layerThree

Your layers can either inherit their packaging settings from the global package property.

# serverless.yml
service: myService

provider:
  name: aws

package:
  exclude:
    - layerSourceTarball.tar.gz

layers:
  layerOne:
    path: layerOne

Or you can specify them at the layer level.

# serverless.yml
service: myService

provider:
  name: aws

layers:
  layerOne:
    path: layerOne
    package:
      exclude:
        - layerSourceTarball.tar.gz

You can also specify a prebuilt archive to create your layer. When you do this, you do not need to specify the path element of your layer.

# serverless.yml
service: myService

provider:
  name: aws

layers:
  layerOne:
    package:
      artifact: layerSource.zip

Permissions

You can make your layers usable by other accounts by setting the allowedAccounts property:

# serverless.yml
service: myService

provider:
  name: aws

layers:
  layerOne:
    path: layerOne
    allowedAccounts:
      - 111111111111 # a specific account ID
      - 222222222222 # a different specific account ID

Another example, making the layer publicly accessible:

# serverless.yml
service: myService

provider:
  name: aws

layers:
  layerOne:
    path: layerOne
    allowedAccounts:
      - '*' # ALL accounts!

Using your layers

Using the layers configuration key in a function makes it possible for your layer with a function

functions:
  hello:
    handler: handler.hello
    layers:
      - arn:aws:lambda:region:XXXXXX:layer:LayerName:Y

To use a layer with a function in the same service, use a CloudFormation Ref. The name of your layer in the CloudFormation template will be your layer name TitleCased (without spaces) and have LambdaLayer appended to the end. EG:

layers:
  test:
    path: layer
functions:
  hello:
    handler: handler.hello
    layers:
      - { Ref: TestLambdaLayer }