Referencing Resources between CloudFormation stacks

Question

If I have two cloudformation stacks, how do I references a resource in one stack from the other stack?

In the example below I have a stack that creates an EBS volume and want to reference that via the Ref: key in the second stack for my EC2 instance but I keep getting a rollback since it can't see that resource from the first stack:

"Template format error: Unresolved resource dependencies"

I already tried the DependsOn clause but it didn't work. Do I need to pass information via Parameters?

{
  "AWSTemplateFormatVersion": "2010-09-09",
  "Resources": {
    "CubesNetworking": {
      "Type": "AWS::CloudFormation::Stack",
      "Properties": {
        "TemplateURL":     "https://s3.amazonaws.com/mybucket/cf_network.json"
      }
   },
   "CubesInstances": {
     "DependsOn": ["CubesNetworking"],
      "Type": "AWS::CloudFormation::Stack",
      "Properties": {
        "TemplateURL": "https://s3.amazonaws.com/mybucket/cf_instances.json"
      }
    }
  }
}

Answer 1

In each of your nested stacks, you should have an output section. Then you can get those values in your calling stack (the one you have listed above) with syntax like:

      { "Fn::GetAtt" : [ "CubesNetworking", "Outputs.VolumeID" ] }

You then pass the values into your other nested stacks via Parameters:

  "Parameters" : {
      "VolumeId" : { "Fn::GetAtt" : [ "CubesNetworking", "Outputs.VolumeID" ] }

You still want the DependsOn since you need the volume created before the instance.

Edit, Mid-2017:

CloudFormation has introduced the ability to export values from one stack, and reference them in other stacks that do not have to be nested.

So your output can specify an export:

Outputs:
  Desc:
    Value: !Ref CubesNetworking.VolumeID
    Export:
      Name: some-unique-name

Then in another stack:

Fn::ImportValue: some-unique-name