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Logging Step Functions to CloudWatch

Many AWS Services log to CloudWatch. Some do it out of the box, others need to be configured to log properly. When Amazon released Step Functions, they didn’t include support for logging to CloudWatch. In February 2020, Amazon announced StepFunctions could now log to CloudWatch. Step Functions still support CloudTrail logs, but CloudWatch logging is more useful for many teams.

Users need to configure Step Functions to log to CloudWatch. This is done on a per State Machine basis. Of course you could click around he console to enable it, but that doesn’t scale. If you use CloudFormation to manage your Step Functions, it is only a few extra lines of configuration to add the logging support.

In my example I will assume you are using YAML for your CloudFormation templates. I’ll save my “if you’re using JSON for CloudFormation you’re doing it wrong” rant for another day. This is a cut down example from one of my services:

---
AWSTemplateFormatVersion: '2010-09-09'
Description: StepFunction with Logging Example.
Parameters:
Resources:
  StepFunctionExecRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: '2012-10-17'
        Statement:
        - Effect: Allow
          Principal:
            Service: !Sub "states.${AWS::Region}.amazonaws.com"
          Action:
          - sts:AssumeRole
      Path: "/"
      Policies:
      - PolicyName: StepFunctionExecRole
        PolicyDocument:
          Version: '2012-10-17'
          Statement:
          - Effect: Allow
            Action:
            - lambda:InvokeFunction
            - lambda:ListFunctions
            Resource: !Sub "arn:aws:lambda:${AWS::Region}:${AWS::AccountId}:function:my-lambdas-namespace-*"
          - Effect: Allow
            Action:
            - logs:CreateLogDelivery
            - logs:GetLogDelivery
            - logs:UpdateLogDelivery
            - logs:DeleteLogDelivery
            - logs:ListLogDeliveries
            - logs:PutResourcePolicy
            - logs:DescribeResourcePolicies
            - logs:DescribeLogGroups
            Resource: "*"
  MyStateMachineLogGroup:
    Type: AWS::Logs::LogGroup
    Properties:
      LogGroupName: /aws/stepfunction/my-step-function
      RetentionInDays: 14
  DashboardImportStateMachine:
    Type: AWS::StepFunctions::StateMachine
    Properties:
      StateMachineName: my-step-function
      StateMachineType: STANDARD
      LoggingConfiguration:
        Destinations:
          - CloudWatchLogsLogGroup:
             LogGroupArn: !GetAtt MyStateMachineLogGroup.Arn
        IncludeExecutionData: True
        Level: ALL
      DefinitionString:
        !Sub |
        {
          ... JSON Step Function definition goes here
        }
      RoleArn: !GetAtt StepFunctionExecRole.Arn

The key pieces in this example are the second statement in the IAM Role with all the logging permissions, the LogGroup defined by MyStateMachineLogGroup and the LoggingConfiguration section of the Step Function definition.

The IAM role permissions are copied from the example policy in the AWS documentation for using CloudWatch Logging with Step Functions. The CloudWatch IAM permissions model is pretty weak, so we need to grant these broad permissions.

The LogGroup definition creates the log group in CloudWatch. You can use what ever value you want for the LogGroupName. I followed the Amazon convention of prefixing everything with /aws/[service-name]/ and then appended the Step Function name. I recommend using the RetentionInDays configuration. It stops old logs sticking around for ever. In my case I send all my logs to ELK, so I don’t need to retain them in CloudWatch long term.

Finally we use the LoggingConfiguration to tell AWS where we want to send out logs. You can only specify a single Destinations. The IncludeExecutionData determines if the inputs and outputs of each function call is logged. You should not enable this if you are passing sensitive information between your steps. The verbosity of logging is controlled by Level. Amazon has a page on Step Function log levels. For dev you probably want to use ALL to help with debugging but in production you probably only need ERROR level logging.

I removed the Parameters and Output from the template. Use them as you need to.