Reference AWS Lambda API Endpoints in serverless.yml

Generating an AWS Lambda API endpoint

A handy trick to reference an AWS Lambda API endpoint is to use serverless-pseudo-parameters and Serverless variables to create the endpoint URL that can be passed to lambda function through an environment variable.

Consider the two lambda functions defined in this serverless.yml:

• get-index: Returns an HTML page
• get-stores: Return a list of stores

provider:
  name: aws
  runtime: nodejs12.x

functions:
  get-index:
    handler: functions/get-index.handler
    events:
      - http:
          path: /
          method: get
    environment:
      stores_api: https://#{ApiGatewayRestApi}.execute-api.#{AWS::Region}.amazonaws.com/${self:provider.stage}/stores
  get-stores:
    handler: functions/get-stores.handler
    events:
      - http:
          path: /stores
          method: get

plugins:
  - serverless-pseudo-parameters

The get-index function needs to make a call to the get-stores function to do some logic, like querying a database.

Use pseudo-parameters and Serverless variables to generate the API endpoint URL of the get-stores API endpoint.

Store this URL as an environment variable for the get-index function:

https://#{ApiGatewayRestApi}.execute-api.#{AWS::Region}.amazonaws.com/${self:provider.stage}/stores

CloudFormation Pseudo Parameters and Serverless Variables

Pseudo parameters are parameters that are predefined by AWS CloudFormation that return AWS resource IDs.

The available CloudFormation Pseudo Parameters are:

• AWS::AccountId
• AWS::NotificationARNs
• AWS::NoValue
• AWS::Partition
• AWS::Region
• AWS::StackId
• AWS::StackName
• AWS::URLSuffix

#{ApiGatewayRestApi} Resource

Serverless generates a cloudformation-template-update-stack.json CloudFormation template that from our serverless.yml.

"Resources": {
  "ApiGatewayRestApi": {      "Type": "AWS::ApiGateway::RestApi",      "Properties": {
        "Name": "dev-api-service",
        "EndpointConfiguration": {
          "Types": [
            "EDGE"
          ]
        },
        "Policy": ""
      }
  }

  ## Other resources...
}

This CloudFormation template specifies ApiGatewayRestApi as an API Gateway REST API resource that contains a collection API Gateway resources.

We can use #{ApiGatewayRestApi} to reference the ApiGatewayRestApi resource from our CloudFormation template.

#{AWS::Region}

#{AWS::Region} references the CloudFormation pseudo parameter to retrieve the AWS Region which our get-stores AWS Lambda resource is being created in.

${self:provider.stage}

${self:provider.stage} self-reference the provider stage variable property in our serverless.yml.