Get Availability Zone from Subnet Parameter in CloudFormation - amazon-web-services

Question

I have
Parameters
Zookeeper1SubnetParam:
Description: Subnet where Zookeeper 1 should run
Type: AWS::EC2::Subnet::Id
Zookeeper1AZ:
Description: Availability Zone of the Subnet
Type: AWS::EC2::AvailabilityZone::Name
From this I'm creating an ENI (which requires a subnet) and an EBS Volume (which requires an availability zone).
Here's the ENI:
Zookeeper1IPResource:
Properties:
Description: Zookeeper1-IP
GroupSet:
- Fn::GetAtt:
- ZookeeperSecurityGroup
- GroupId
PrivateIpAddress:
Ref: Zookeeper1IPParam
SubnetId:
Ref: Zookeeper1SubnetParam
Type: AWS::EC2::NetworkInterface
And here's the EBS:
Zookeeper1EBSVolume:
Properties:
AvailabilityZone:
Ref: Zookeeper1AZ
Size: 8
VolumeType: gp2
Type: AWS::EC2::Volume
I find it really bad for user experience, to also ask as a parameter for an availability zone, because it can be deducted from the selected subnet
Now, the million dollar question, how do I get the Availability Zone from the Subnet in CloudFormation? As far as I can tell, I can't do a GetAtt for AZ on my ENI.
Any solution welcome!

Answer 1

To answer your question, you can't retrieve the Availability Zone from the Subnet.
But if you have total control of the template or resources that supplies the parameter to your template there are workarounds
If have control over the source the provides you the Subnet parameter, you can return also the Availability Zone from that source as an Outputs and supply it in your template as a parameter where you create ENI and EBS.
In addition, you could also create the Subnet in the same template you will create the ENI and EBS and use the { "Fn::GetAtt" : [ "mySubnet", "AvailabilityZone" ] }
Question(sorry, my rep can't allow me to comment yet)
Do you happen to have dynamic values or resources to be created that depends on availability zones? If yes, you can create Mappings and if that is not enough, you could add Conditions in your template.

Answer 2

I don't know if it is something new, but according to the documentation you can get the AZ of a subnet with GetAttr.
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-ec2-subnet.html#aws-resource-ec2-subnet-returnvalues
Quoting the documentation:
{ "Fn::GetAtt" : [ "mySubnet", "AvailabilityZone" ] }
UPDATE:
This suggestion is wrong, let me quote another documentation from AWS:
Supported Functions
For the Fn::GetAtt logical resource name, you cannot use functions. You must specify a string that is a resource's logical ID.
For the Fn::GetAtt attribute name, you can use the Ref function.
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-getatt.html#getatt-supported-functions