Question:
I’m trying to deploy stepfunctions with CloudFormation, and I’d like to reference the actual stepfunction definition from an external file in S3.
Here’s how the template looks like:
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StepFunction1: Type: "AWS::StepFunctions::StateMachine" Properties: StateMachineName: !Ref StepFunction1SampleName RoleArn: !GetAtt StepFunctionExecutionRole.Arn DefinitionString: Fn::Transform: Name: AWS::Include Parameters: Location: Fn::Sub: 's3://${ArtifactsBucketName}/StepFunctions/StepFunction1/definition.json' |
However, this doesn’t seem to be supported, as we are getting error
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Property validation failure: [Value of property {/DefinitionString} does not match type {String}] |
I am doing something similar for APIs, referencing the actual API definition from an external swagger file, and that seems to be working fine.
Example:
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SearchAPI: Type: "AWS::Serverless::Api" Properties: Name: myAPI StageName: latest DefinitionBody: Fn::Transform: Name: AWS::Include Parameters: Location: Fn::Sub: 's3://${ArtifactsBucketName}/ApiGateway/myAPI/swagger.yaml' |
How can I make this work?
Answer:
The trick is to escape the StepFunction DefinitionString property, and include the actual property, DefinitionString, in the external CloudFormation referenced file. Escaping only the stepfunction definition string would fail, CloudFormation complaining that the referenced Transform/Include template, is not a valid yaml/json.
Here’s how it looks like:
Template:
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StepFunction1: Type: "AWS::StepFunctions::StateMachine" Properties: StateMachineName: !Ref StepFunction1SampleName RoleArn: !GetAtt StepFunctionExecutionRole.Arn Fn::Transform: Name: AWS::Include Parameters: Location: Fn::Sub: 's3://${ArtifactsBucketName}/StepFunctions/StepFunction1/definition.json' |
External stepfunction definition file:
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{ "DefinitionString" : {"Fn::Sub" : "{\r\n \"Comment\": \"A Retry example of the Amazon States Language using an AWS Lambda Function\",\r\n \"StartAt\": \"HelloWorld\",\r\n \"States\": {\r\n \"HelloWorld\": {\r\n \"Type\": \"Task\",\r\n \"Resource\": \"arn:aws:lambda:${AWS::Region}:${AWS::AccountId}:function:${HelloWorldLambdaFunctionName}\", \r\n \"End\": true\r\n }\r\n }\r\n}"} } |
Now, although this solves the problem, it’s a bit more difficult to maintain the StepFunction definition, in this form, in source control.
So I’ve thought about using a CloudFormation custom resource backed by a lambda function. The lambda function would deal with the actual StepFunction DefinitionString escaping part.
Here’s how it looks like:
Template:
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StepFunctionParser: Type: Custom::AMIInfo Properties: ServiceToken: myLambdaArn DefinitionString: Fn::Transform: Name: AWS::Include Parameters: Location: Fn::Sub: 's3://${ArtifactsBucketName}/StepFunctions/StepFunctionX/definition.json' StepFunctionX: Type: "AWS::StepFunctions::StateMachine" Properties: StateMachineName: StepFunction1SampleNameX RoleArn: !GetAtt StepFunctionExecutionRole.Arn DefinitionString: !GetAtt StepFunctionParser.DefinitionString |
External StepFunction definition file:
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{ "Comment": "A Retry example of the Amazon States Language using an AWS Lambda Function", "StartAt": "HelloWorld", "States": { "HelloWorld": { "Type": "Task", "Resource": {"Fn::Sub" : "arn:aws:lambda:${AWS::Region}:${AWS::AccountId}:function:${HelloWorldLambdaFunctionName}" }, "End": true } } } |
Here’s the documentation for creating AWS Lambda-backed Custom Resources.
There’s still a problem with this.
Transform/Include converts external template boolean properties into string properties.
Therefore, DefinitionString
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"DefinitionString": { "States": { "HelloWorld": { "Type": "Task", "Resource": "arn:aws:lambda:${AWS::Region}:${AWS::AccountId}:function:${HelloWorldLambdaFunctionName}", **"End": true** } }, "Comment": "A Retry example of the Amazon States Language using an AWS Lambda Function", "StartAt": "HelloWorld" } |
becomes
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"DefinitionString": { "States": { "HelloWorld": { "Type": "Task", "Resource": _realLambdaFunctionArn_, **"End": "true"** } }, "Comment": "A Retry example of the Amazon States Language using an AWS Lambda Function", "StartAt": "HelloWorld" } |
CloudFormation then complains about the StepFunction definition not being valid:
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Invalid State Machine Definition: 'SCHEMA_VALIDATION_FAILED: Expected value of type Boolean at /States/HelloWorld/End' |
Is this a CloudFormation Transform/Include issue? Can someone from AWS give a statement on this?