Can I use "Fn::Join" in "Parameters" of AWS Cloudformation json template

Question

I want to use in Parameters of Cloudformation json template shortcut of some Policy/Loadbalancers tags name, like that:

"SomeScalingGroupName": {
            "Type": "String",
            "Default": {"Fn::Join": ["", ["Process-", {"Ref": "Env"}, "-Some-Worker-Name"]]}
        },

And I get error:

Template validation error: Template format error: Every Default member must be a string.

So my question if that proper way to use function join in Parameters? Or I they have any other way to do that? Or you have any better suggestions to using that?

Thanks!

Solution

You cannot use intrinsic functions within the parameters section of your template.

You can use intrinsic functions only in specific parts of a template. Currently, you can use intrinsic functions in resource properties, metadata attributes, and update policy attributes.

http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference.html

You will need use this function within your resource properties. For example:

"Parameters" : {
  "Env" : {
    "Type" : "String",
    "Default" : "test"
  },
  "WorkerName" : {
    "Type" : "String",
    "Default" : "my-worker"
  }
}

"Resources" : {
  "LoadBalancer" : {
    "Type" : "AWS::ElasticLoadBalancing::LoadBalancer",
    ...
    "Properties" : {
      "Tags" : [ 
        { "Key" : "Name", "Value": { "Fn::Join" : [ "-", [ "process", { "Ref" : "Env" }, { "Ref" : "SomeWorkerName" }]]}},
      ]
    }
  }
}

This will apply a 'Name' tag to your Load Balancer with a value of 'process-test-my-worker'. You can also use this same function anywhere else within the properties of your resources.

Comments

[Mircea]

the error tells you what is wrong. You cannot use a function (fn:join) for default values.

[muzafarow]

@thext I don't see (fn:join) in error. Can you suggest how can I transform this peace of code for not getting error? Thanks!