CloudFormation Template - any way to get a Spot-Fleet-Request ID?

Question

I'm attempting to create a single template that creates the following:

AWS::EC2::SpotFleet resource 2 AWS::ApplicationAutoScaling::ScalingPolicy resources (scale up, scale down)

Initially, my template included only the SpotFleet resource, and I confirmed that the stack would create without issue. Once I added the ScalingPolicy resources, the stack would rollback because there was "No scalable target registered for namespace..." So, I added an additional resource.

AWS::ApplicationAutoScaling::ScalableTarget resource. (From http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-applicationautoscaling-scalabletarget.html#cfn-applicationautoscaling-scalabletarget-resourceid )

{
  "Type" : "AWS::ApplicationAutoScaling::ScalableTarget",
  "Properties" : {
    "MaxCapacity" : Integer,
    "MinCapacity" : Integer,
    "ResourceId" : String,
    "RoleARN" : String,
    "ScalableDimension" : String,
    "ServiceNamespace" : String
  }
}

The ResourceID is a required property. I have the data for all the other properties, but when researching what data is needed for the ResourceID property, I have found that the data I need is the spot-fleet-request ID, (something like this: "SpotFleetRequestId": "sfr-73fbd2ce-aa30-494c-8788-1cee4EXAMPLE") .

So here's the problem: Since I am creating the spot fleet request in the same template as the scaling policy, I can't put the SpotFleetRequestId in manually, since to my knowledge this is created when the resource is and there's no way to anticipate what the request ID will be. In other templates, with other kinds of resources, I've simply used "Ref" or "Fn::GetAtt" to pass in the arn of a resource without having to manually input this. However--there seems to be no way to do this with a SpotFleetRequestID. All the research I've done has turned up nothing, not even a single template example that uses a method like I'm describing - the only examples available assume that the scalable target resource already exists and the SpotFleetRequestID is known prior to creating the ScalingPolicy.

Does anyone have any idea if referring to the SpotFleetRequestID of an AWS::EC2::SpotFleet initialized in the same template is even possible? Or am I just missing something REALLY obvious?

-KungFuBilly

Answer 1

Turns out that if you "Ref" the logical name of the AWS::EC2::SpotFleet it will return the request ID. Then, it's a matter of using "Fn::Join" to get the right data for the ResourceID. Should look something like this:

"ResourceId":  {
                "Fn::Join": [
                    "/",
                    [
                        "spot-fleet-request",
                        {
                            "Ref": "SpotFleet"
                        }
                    ]
                ]
            },

That will output: spot-fleet-request/"SpotFleetRequestID"