AWS IoT JITPovisioning template with Fn::Join - aws-iot

Question

I am trying to add registrationConfig for my CA certificate in AWS IoT. I would like to do some manipulation of data for Thing attributes. But I can't seem to get that JITP to work if the template body has Fn::Join in it.
Following are extract of the template body (string unescaped for reading purpose)
NOT working:
"Resources": {
"thing": {
"Type": "AWS::IoT::Thing",
"Properties": {
"ThingName": {
"Ref": "AWS::IoT::Certificate::CommonName"
},
"ThingTypeName" : "w2-device",
"ThingGroups" : ["w2-devices"],
"AttributePayload": {
"location": {
"Fn::Join":["",["ThingPrefix_",{"Ref":"SerialNumber"}]]
},
"organization": {
"Ref": "AWS::IoT::Certificate::Organization"
},
"version": "w2",
"country": {
"Ref": "AWS::IoT::Certificate::Country"
}
}
}
},
In the above when I have Fn::Join in AttributePayload/location it fails to create the Thing during JITP. I don't see any errors in CloudWatch either.
Working:
"Resources": {
"thing": {
"Type": "AWS::IoT::Thing",
"Properties": {
"ThingName": {
"Ref": "AWS::IoT::Certificate::CommonName"
},
"ThingTypeName" : "w2-device",
"ThingGroups" : ["w2-devices"],
"AttributePayload": {
"location": {
"Ref": "AWS::IoT::Certificate::StateName"
},
"organization": {
"Ref": "AWS::IoT::Certificate::Organization"
},
"version": "w2",
"country": {
"Ref": "AWS::IoT::Certificate::Country"
}
}
}
},
Note: I have also asked this in aws forum but without any answer there yet.

Answer 1

Provisioning templates for JITP define a set of parameters beginning with AWS::IoT::Certificate.
The AWS::IoT::Certificate::SerialNumber parameter should be used instead of just SerialNumber in the attribute payload. e.g.
"AttributePayload": {
"location": {
"Fn::Join":["",["ThingPrefix_",{"Ref":"AWS::IoT::Certificate::SerialNumber"}]]
},
https://docs.aws.amazon.com/iot/latest/developerguide/jit-provisioning.html lists the defined parameters for JITP as:
AWS::IoT::Certificate::Country
AWS::IoT::Certificate::Organization
AWS::IoT::Certificate::OrganizationalUnit
AWS::IoT::Certificate::DistinguishedNameQualifier
AWS::IoT::Certificate::StateName
AWS::IoT::Certificate::CommonName
AWS::IoT::Certificate::SerialNumber
AWS::IoT::Certificate::Id
The SerialNumber examples in the AWS documentation (without the AWS::IoT::Certificate prefix are used for the Bulk Registration process.