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: ${opt:stage, self:provider.stage, 'dev'}-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.
    #   - '*'
    # note: uncommenting this will give all AWS users access to this layer unconditionally.
    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

Keep in mind that all include and exclude patterns (even when inherited from the service config) are resolved against the layer's path and not the service path.

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 }