AWS Cloudformation: Conditionally create properties of resources - amazon-web-services

Question

I know that it is possible via the use of Conditions to conditionally (what else?) create resources.
I am trying to find a way though to conditionally create properties of resources;
in my case I am creating several EC2 instances in a subnet with default public ip assignment = false.
Sometimes though for debugging purposes I want my instances to get public IPs.
Now I have to comment in/out the SG/Subnet and the NetworkInterfaces properties below (those do not go together)
myEC2:
Type: AWS::EC2::Instance
Metadata:
Comment: My EC2 Instance
AWS::CloudFormation::Init:
config:
commands:
01_provision:
command:
!Sub |
sed -i "s/somestring/${somevar}/" /some/path/
CreationPolicy:
ResourceSignal:
Timeout: PT4M
Properties:
ImageId: !FindInMap [ MyAamiMap, 'myami', amiid ]
InstanceType: "t2.2xlarge"
# SubnetId: !Ref SBNDemo1
# SecurityGroupIds: [!Ref SGInternalDemo]
NetworkInterfaces:
- AssociatePublicIpAddress: "true"
DeviceIndex: "0"
GroupSet:
- Ref: "SGInternalDemo"
SubnetId:
Ref: "SBNDemo1"
UserData:
"Fn::Base64":
!Sub |
#!/bin/bash -xe
# Start cfn-init
/usr/local/bin/cfn-init -s ${AWS::StackId} -r myEC2 --region ${AWS::Region} || echo 'Failed to run cfn-init'
# All done so signal success
/usr/local/bin/cfn-signal -e $? --stack ${AWS::StackId} --resource myEC2 --region ${AWS::Region}
Any suggestions?

Answer 1

This might be a little late, but I recently had a same question.
From AWS docs, you can use Fn::If to set properties accordingly.
The template will look like:
Properties:
ImageId: !FindInMap [ MyAamiMap, 'myami', amiid ]
InstanceType: "t2.2xlarge"
# SubnetId: !Ref SBNDemo1
# SecurityGroupIds: [!Ref SGInternalDemo]
NetworkInterfaces:
!If
- YourCondition
-
AssociatePublicIpAddress: "true"
DeviceIndex: "0"
GroupSet:
- Ref: "SGInternalDemo"
SubnetId:
Ref: "SBNDemo1"
- !Ref "AWS::NoValue"
AWS::NoValue means there will be no NetworkInterfaces properties set.

Answer 2

Perhaps I am misunderstanding but this sounds like a parameter use case rather than a condition use case. I say that because you do not say under what conditions you would like a public ip. Just "sometimes for debugging purposes" How would the template know that you are debugging? You have to tell it with a parameter.
check out the docs https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/parameters-section-structure.html
So you could have a public ip parameter and a subnet id parameter and pass in what you like at stack creation.
One way that conditions could be useful is to create a debug parameter that would toggle public/private ip and subnet. Is this what you were thinking of?
To use conditions on properties use the IF function
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/intrinsic-function-reference-conditions.html
I suggest setting your public subnet to provide a public ip on launch, and of course ensuring your private subnet does not do that. Then just pass the subnet in as a parameter.
https://docs.aws.amazon.com/vpc/latest/userguide/vpc-ip-addressing.html#subnet-public-ip