Fn :: GetAtt中的AWS Cloudformation Fn :: ImportValue

Question

Is it possible to use Fn::ImportValue inside Fn::GetAtt. Currently, I'm trying to do the following

    "ParentId": {
       "Fn::GetAtt": [
          {
            "Fn::ImportValue": {
               "Fn::Sub": "${ParentStack}:RestApi"
             }
          },
          "RootResourceId"
       ]
    }

But I'm facing an error. "Template error: every Fn::GetAtt object requires two non-empty parameters, the resource name and the resource attribute".

Answer 1

As it currently stands, it's not possible to have "Fn::ImportValue" inside of "Fn::GetAtt" block. The best explanation I have for this is that resource for which you want to get attribute value is not in a scope of your current template.

What you can try to do is to export all attribute values you are interested in from 'parent' template.

So your parent template would look like this:

"Outputs" : {
  "RestApi": {
    "Value" : { "Ref": "RestApi" },
    "Export" : { "Name": "RestApi" }
  },
  "RestApiRootResourceId": {
    "Value" : { "Fn::GetAtt": ["RestApi", "RootResourceId"] },
    "Export" : { "Name" : "RestApiRootResourceId" }
  }
}

And now in your child template you can reference your API root resource id from parent template:

"Resources" : {
  "XApiResource": {
    "Type": "AWS::ApiGateway::Resource",
    "Properties": {
      "RestApiId": {"Fn::ImportValue" : "RestApi"},
      "ParentId": {"Fn::ImportValue" : "RestApiRootResourceId"},
      "PathPart": "apiPath"
    }
  }
}

Answer 2

Though it's a bit complex, the syntax looks correct, so the problem is probably that the first parameter resolves to an empty value (since the second one is clearly a string that you stated).

You could try figuring out if that is the case by removing any parts of the stack that fail, creating an output to the stack, and seeing if it has a value.

Something like:

"Outputs" : {
    "ParentStack" : {
        "Value" : "Fn::Ref": "ParentStack"
    },
    "ParentStackRestAPI" : {
        "Value" : "Fn::Sub": "${ParentStack}:RestApi"
    },
    "ImportedValue" : {
        "Value" : "Fn::ImportValue": {
            "Fn::Sub": "${ParentStack}:RestApi"
        }
    }
}