Custom resource Outputs not found

Question

I've written a custom resource in Go using `cloudformation-cli-go-plugin`, it's failing when I try and use it in a stack with
```
Unable to retrieve Guid attribute for MyCo::CloudFormation::Workloads, with error message NotFound guid not found.
```

The stack:
```
AWSTemplateFormatVersion: 2010-09-09
Description: Sample MyCo Workloads Template
Resources:
  Resource1:
    Type: 'MyCo::CloudFormation::Workloads'
    Properties:
      APIKey: ""
      AccountID: ""
      Workload: >-
        workload: {entityGuids: "", name: "CloudFormationTest-Create"}
Outputs:
  CustomResourceAttribute1:
    Value: !GetAtt  Resource1.Guid
```

If I remove the `Outputs` stanza the stack runs successfully and I can see the created resource.

Running with SAM locally I've verified that `Guid` is in fact always returned. FWIW the resource passes all of the contract tests, `Guid` is the `primaryIdentifier`, and is a `readOnlyProperties`.

I've tried several tests playing with the `!GetAtt` definition, all of which fail with schema errors so it appears the CF is aware of the format of the resource's properties.

Suggestions and/or pointers would be appreciated.

Accepted Answer

The issue here was Read failing due to CloudFormation behaving differently than the Contract Tests. The Contract Tests do not follow the CloudFormation model rules, they are more permissive.

Answer

You can pass the output values from the custom resource in the response data being returned as a parameter to the custom resource response objects

Replicating the scenario at my end, I tried to create a sample lambda backed custom resource responsible for creation of a new EC2 instance and then returned the value of the ‘InstanceId' as a return value.

```
Resources:
  LambdaFunction:
    Type: 'AWS::Lambda::Function'
    Properties:
      FunctionName: executeFn
      Timeout: 900
      Code:
        ZipFile: |
          import boto3, json
          import cfnresponse
          def handler(event, context):
              ec2 = boto3.client('ec2')
              print ("Createing new instance:")
              instance = ec2.run_instances(
                ImageId='ami-061e2417489356de3',
                InstanceType='t2.micro',
                MaxCount=1,
                MinCount=1
              )
              instance_id = instance['Instances'][0]['InstanceId']
              responseValue = instance_id
              responseData = {}
              responseData['Data'] = responseValue
              cfnresponse.send(event, context, cfnresponse.SUCCESS, responseData)
      Handler: index.handler
      Role: 'arn:aws:iam::xxxxxxxxxxxx:role/testLambda'
      Runtime: python3.7
  CustomResource:
    Type: 'AWS::CloudFormation::CustomResource'
    DependsOn: LambdaFunction
    Version: '1.0'
    Properties:
      ServiceToken: !GetAtt LambdaFunction.Arn
      Description: Custom resource to invoke lambda
Outputs:
  InstanceId:
    Value: !GetAtt CustomResource.Data
```

Now, here in this example, you can refer that lambda function associated with the custom resource returns the instance id as a value in the Data which is then displayed as an output to the stack created using the GetAtt intrinsic function.

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/crpg-ref-responses.html

Custom resource Outputs not found

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