Error in SNS configuration for pre-existing SNS topic?

Serverless Framework
1

Hi,

Not sure if this is an error … but the behavior is surprising, so I thought I’d report it.

  handleSesSuppressions:
    handler: handleSesSuppressions.handler
    events:
      - sns:
          arn: ${{self:resources.Outputs.BouncesSnsTopic.Value}}
          topicName: ${{file(./commonResources/serverless.yml):provider.environment.BouncesSnsTopic}}
      - sns:
          arn: ${{self:resources.Outputs.ComplaintsSnsTopic.Value}}
          topicName: ${{file(./commonResources/serverless.yml):provider.environment.ComplaintsSnsTopic}}

This deploys. As seen in cloudformation-template-update-stack.json:

    "HandleSesSuppressionsSnsSubscriptionIrhsesbounces": {
      "Type": "AWS::SNS::Subscription",
      "Properties": {
        "TopicArn": {
          "Fn::Sub": "arn:aws:sns:${AWS::Region}:${AWS::AccountId}:irh-ses-bounces"
        },
        "Protocol": "lambda",
        "Endpoint": {
          "Fn::GetAtt": [
            "HandleSesSuppressionsLambdaFunction",
            "Arn"
          ]
        }
      }
    },

However, this does not deploy:

  handleSesSuppressions:
    handler: handleSesSuppressions.handler
    events:
      - sns: ${{self:resources.Outputs.BouncesSnsTopic.Value}}
      - sns:
          arn: ${{self:resources.Outputs.ComplaintsSnsTopic.Value}}
          topicName: ${{file(./commonResources/serverless.yml):provider.environment.ComplaintsSnsTopic}}

Produces this error message:

     Missing or invalid topicName property for sns event
     in function "handleSesSuppressions" The correct syntax
     is: sns: topic-name-or-arn OR an object with  arn and
     topicName OR topicName and displayName. Please check
     the docs for more info.

It seems like I am meeting the requirements for using a pre-existing SNS topic by not including the Topic Name:


Which seems redundant, since it’s included in the ARN:

    BouncesSnsTopic:
      Description: SNS topic to receive SES bounces.
      Value:
        Fn::Sub: arn:aws:sns:${AWS::Region}:${AWS::AccountId}:${{file(./commonResources/serverless.yml):provider.environment.BouncesSnsTopic}}

Am I reading the documentation incorrectly?

Thanks in advance!

2

The docs say that using:

- sns: ${{self:resources.Outputs.BouncesSnsTopic.Value}}

will only work if ${{self:resources.Outputs.BouncesSnsTopic.Value}} resolves to a value starting with arn:

Read the second half of the docs covering the usage with intrinsic CloudFormation functions. The topicName is required and the topciArn must contain the value given in topicName.

3

Ah, gotcha, thanks for explaining. The reason it threw both me and my fellow developers is because similar approaches have worked in other places, I think - in other words, replacing a literal string starting with “arn:” could be replace with a “Fn::Sub: arn:xxx:” etc. string. For example, it works in the “arn:” field but not the “sns:” field. So I think you could imagine our confusion that wasn’t resolved by the documentation of that page.

Thanks!