Cloudformation: Reference Created Subnets in ElastiCache SubnetGroup - amazon-web-services

Question

I'm having a challenge creating an ElastiCache SubnetGroup which dynamically references the correct subnets. I'd like to use the same template in both the east and west region so I'm specifying the subnets for the subnet group in mappings. However, when I attempt to run update my stack, I get the following error:
Value of property SubnetIds must be of type List of String
Here's a gist showing roughly what i am trying to do: https://gist.github.com/brockhaywood/b71ed34c6a554a0a0fec
This unanswered question on the AWS forums appears to be a very similar problem: https://forums.aws.amazon.com/message.jspa?messageID=532454

Answer 1

I think SubnetIds should be an array, where you have a single object.
"ElastiCacheSubnetGroup": {
"Type": "AWS::ElastiCache::SubnetGroup",
"Properties": {
"SubnetIds": [
{
"Fn::FindInMap":["RegionMap", { "Ref":"AWS::Region" }, AppSubnets" ]
}
]
}
}

Answer 2

The specific issue is that you can't use Ref within a Mappings value, as noted in the Mappings documentation:
You cannot include parameters, pseudo parameters, or intrinsic functions in the Mappings section.
As an alternative, you can use Conditions to accomplish what your template is attempting. Here's a complete working example:
{
"Description": "Create an ElastiCache SubnetGroup with different subnet depending on the current AWS region."
"Conditions": {
"us-east-1": {"Fn::Equals": [{"Ref":"AWS::Region"}, "us-east-1"]},
"us-west-2": {"Fn::Equals": [{"Ref":"AWS::Region"}, "us-west-2"]}
},
"Resources": {
"VPC": {
"Type": "AWS::EC2::VPC",
"Properties": {
"CidrBlock": "10.0.0.0/16"
}
},
"AppSubnetA": {
"Type": "AWS::EC2::Subnet",
"Properties": {
"VpcId": {"Ref": "VPC"},
"CidrBlock": "10.0.0.0/24",
"AvailabilityZone": {"Fn::Select": [1, {"Fn::GetAZs": ""}]}
}
},
"AppSubnetB": {
"Type": "AWS::EC2::Subnet",
"Properties": {
"VpcId": {"Ref": "VPC"},
"CidrBlock": "10.0.1.0/24",
"AvailabilityZone": {"Fn::Select": [1, {"Fn::GetAZs": ""}]}
}
},
"ElastiCacheSubnetGroup": {
"Type": "AWS::ElastiCache::SubnetGroup",
"Properties": {
"Description": "SubnetGroup",
"SubnetIds": {"Fn::If": ["us-east-1", [
{"Ref": "AppSubnetA"}
],
{"Fn::If": ["us-west-2",
[
{"Ref": "AppSubnetB"}
],
{"Ref":"AWS::NoValue"}
]}
]}
}
}
}
}