Pre-warm Lambda functions

prewarm.yml

Parameters:
  PreWarmEnvironmentParameter:
    Type: String # enabled or disabled, only enable for low-volume environments
  PreWarmFunctionNamesParameter:
    Type: String # JSON string like ["MyFunctionToKeepWarm1","MyFunctionToKeepWarm2"]
Conditions:
  IsPreWarmEnvironmentEnabled:
    !Equals [!Ref PreWarmEnvironmentParameter, enabled]
Resources:
  PreWarmEnvironmentWorkflow:
    Type: AWS::StepFunctions::StateMachine
    Condition: IsPreWarmEnvironmentEnabled
    Properties:
      DefinitionString: !Sub |
        {
          "Comment": "Pre-warm environment",
          "StartAt": "PassFunctionsToPreWarm",
          "States": {
            "PassFunctionsToPreWarm": {
              "Type": "Pass",
              "Next": "PreWarmFunctionsMap",
              "Result": {
                "preWarmFunctionNames": ${PreWarmFunctionNamesParameter}
              }
            },
            "PreWarmFunctionsMap": {
              "Type": "Map",
              "ItemsPath": "$.preWarmFunctionNames",
              "Iterator": {
                "StartAt": "InvokeLambdaFunction",
                "States": {
                  "InvokeLambdaFunction": {
                    "Type": "Task",
                    "Resource": "arn:aws:states:::lambda:invoke",
                    "Parameters": {
                      "FunctionName.$": "$",
                      "Payload": {
                        "preWarm": true
                      }
                    },
                    "End": true
                  }
                }
              },
              "End": true
            }
          }
        }
      RoleArn: !GetAtt PreWarmEnvironmentRole.Arn
      StateMachineType: EXPRESS
  PreWarmEnvironmentRole:
    Type: AWS::IAM::Role
    Condition: IsPreWarmEnvironmentEnabled
    Properties:
      AssumeRolePolicyDocument:
        Version: 2012-10-17
        Statement:
          - Effect: Allow
            Principal:
              Service:
                - !Sub states.${AWS::URLSuffix}
            Action: sts:AssumeRole
      Policies:
        - PolicyName: InvokeFunctions
          PolicyDocument:
            Version: 2012-10-17
            Statement:
              - Effect: Allow
                Action:
                  - lambda:InvokeFunction
                Resource:
                  - "*" # consider using a tag condition or list out PreWarmFunctionNamesParameter if you use this!
  PreWarmEnvironmentSchedule:
    Type: AWS::Scheduler::Schedule
    Condition: IsPreWarmEnvironmentEnabled
    Properties:
      Description: Pre-warm environment
      FlexibleTimeWindow:
        Mode: "OFF"
      ScheduleExpression: rate(1 minute)
      State: ENABLED
      Target:
        Arn: !GetAtt PreWarmEnvironmentWorkflow.Arn
        RoleArn: !GetAtt PreWarmEnvironmentScheduleRole.Arn
  PreWarmEnvironmentScheduleRole:
    Type: AWS::IAM::Role
    Condition: IsPreWarmEnvironmentEnabled
    Properties:
      AssumeRolePolicyDocument:
        Version: 2012-10-17
        Statement:
          - Effect: Allow
            Principal:
              Service:
                - !Sub scheduler.${AWS::URLSuffix}
            Action: sts:AssumeRole
      Policies:
        - PolicyName: StartExecution
          PolicyDocument:
            Version: 2012-10-17
            Statement:
              - Effect: Allow
                Action:
                  - states:StartExecution
                Resource: !GetAtt PreWarmEnvironmentWorkflow.Arn