AWS cloudformation optional line - amazon-web-services

Question

I'm trying to setup a cloudformation template that will either launch a clean instance or one from snapshots. I'd like to be able to use an if / else type statement so that would look something like
pseudo code:
if InputSnapshotId:
"SnapshotId" : {"Ref" : "InputSnapshotId"},
else:
"Size" : 20,
In cloudformation I have tried a number of things like:
"WebserverInstanceDataVolume" : {
"Type" : "AWS::EC2::Volume",
"Properties" : {
"Fn::If" : [
{"Ref" : "FromSnapshot"},
{"SnapshotId" : { "Ref" : "InputSnapshotId" }},
{"Size" : "20"}
],
"VolumeType" : "standard",
"AvailabilityZone" : { "Fn::GetAtt" : [ "WebserverInstance", "AvailabilityZone" ]},
"Tags" : [
{"Key" : "Role", "Value": "data" },
]
},
"DeletionPolicy" : "Delete"
},
Or wrapping the in Fn::If in {}:
{"Fn::If" : [
{"Ref" : "FromSnapshot"},
{"SnapshotId" : { "Ref" : "InputSnapshotId" }},
{"Size" : "20"}
]}
All of which kicks different types or errors. The first one gives a "Encountered unsupported property Fn::If" in cloudformation, the second, just isn't valid JSON. I could snapshot an empty volume and define a size parameter then always pass a SnapshotId and size but I feel like there must be a way to have an optional line in cloudformation.
Any ideas?

Answer 1

You can do like this:
"Conditions" : {
"NotUseSnapshot" : {"Fn::Equals" : [{"Ref" : "InputSnapshotId"}, ""]}
},
"Resources" : {
"WebserverInstanceDataVolume" : {
"Type" : "AWS::EC2::Volume",
"Properties" : {
"Size" : {
"Fn::If" : [
"NotUseSnapshot",
"20",
{"Ref" : "AWS::NoValue"}
]
},
"SnapshotId" : {
"Fn::If" : [
"NotUseSnapshot",
{"Ref" : "AWS::NoValue"},
{"Ref" : "InputSnapshotId"}
]
},
"VolumeType" : "standard",
"AvailabilityZone" : { "Fn::GetAtt" : [ "WebserverInstance", "AvailabilityZone" ]},
"Tags" : [
{"Key" : "Role", "Value": "data" }
]
},
"DeletionPolicy" : "Delete"
}
}
Here is a link to a functional template: https://github.com/caussourd/public-cloudformation-templates/blob/master/conditional_volume_creation.template