Using a function to provide Fn::Join's delimiter?

[Amos S]

I'm trying to parametise the delimiter used by an Fn::Join, e.g. at first I had:

"Name" : { "Fn::Join" : [ ".", [ 

               { "Ref":"serviceName"}, { "Ref": "environment" } ] ] },

Which works well, but then I changed it to:

"Name" : { "Fn::Join" : [ {"Ref":"HostNameSeparator"}, [

               { "Ref":"serviceName"}, { "Ref": "environment" } ] ] },

I get the following error from the validation phase:

A client error (ValidationError) occurred when calling the

    ValidateTemplate operation: Template error: every Fn::Join object

    requires two parameters, (1) a string delimiter and (2) a list of

    strings to be joined or a function that returns a list of strings

    (such as Fn::GetAZs) to be joined.

Is it possible to do what I want, i.e. pass the Join delimiter as a template parameter?

(I've shortened the examples above for clarity sake, please ignore typos)

As a work-around, I calculate the join in the script which calls the cloudformation creation command

and pass the final value as a parameter, but it's not ideal since it means that I repeat the input to the template

more than once (the "serviceName" and "environment" parameters above, for instance).

[Michael Barrett]

Hey Amos - I'm trying to reproduce this.  Can you tell me what sort of resource you are applying this to?

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[Michael Barrett]

Actually - another question - how are you writing the template?  Are you using troposphere, or are you creating the JSON Template by hand and then trying to do something with it?

[Amos S]

I'm (trying to) use it to set the "Name" property of an AWS::Route53::RecordSet resource.

This is what the resource would look like if the Fn::Join worked:

        "LoadBalancerDNSRecord" : {

            "Type" : "AWS::Route53::RecordSet",

            "Condition" : "UpdateDNS",

            "Properties" : {

                "HostedZoneId" : "/hostedzone/Z25PF77UMQRIWR",

                "Comment" : "CNAME for ELB",

                "Name" : { "Fn::Join" : [ {"Ref":"HostNameSeparator"}, [

                         { "Ref":"serviceName"}, { "Ref": "environment" } ] ] },

                "Type" : "CNAME",

                "TTL" : "900",

                "ResourceRecords" : [

                    {"Fn::GetAtt":["LoadBalancer","DNSName"]}

                ]

            }

        },

[Amos S]

For now I maintain the template JSON file by hand using a text editor.

[Michael Barrett]

In the version that you maintain by hand in JSON, do you have any issues using Join & Ref's for the parts & separator, or is that only an issue when you try to do the same in troposphere?

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[Amos S]

I'm sorry I didn't realise that this group is about Troposphere. I was referred here by someone after I didn't get responses on the AWS support forums.

I don't use troposphere.

Back to your latest question - the issue happens using the static JSON file. I don't use Troposphere and haven't heard about it until now.

[Michael Barrett]

Oh, yeah, then in this case it's likely that it's not possible if you're writing it in plain CloudFormation templates and it's throwing a Validation error.  

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[Amos S]

Thanks for the confirmation.