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Optional Parameter in CloudFormation

Michael Wittig – 23 Mar 2016

Sometimes you want a CloudFormation Parameter to be optional. Unfortunately a blank Parameter contains an empty string. If you pass this empty string to e.g. the KeyName Property of an EC2 Instance or Launch Configuration you end up with a validation error.

Imagine the following CloudFormation template:

{
"AWSTemplateFormatVersion": "2010-09-09",
"Description": "Optional Parameters in CloudFormation: Problem",
"Parameters": {
"KeyName": {
"Description": "Provides the name of the EC2 key pair",
"Type": "String"
}
},
"Resources": {
"LaunchConfiguration": {
"Type": "AWS::AutoScaling::LaunchConfiguration",
"Properties": {
"KeyName": {"Ref": "KeyName"}
[...]
}
}
}
}

If you leave the KeyName blank you will get the following error:


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Parameter validation failed: parameter value for parameter name KeyName does not exist. Rollback requested by user.

The solution is the make use of CloudFormation Conditions, the Condition Function Fn::If and the Pseudo Parameter AWS::NoValue.

{
"AWSTemplateFormatVersion": "2010-09-09",
"Description": "Optional Parameters in CloudFormation: Solution",
"Parameters": {
"KeyName": {
"Description": "Provides the name of the EC2 key pair",
"Type": "String"
}
},
"Conditions": {
"HasKeyName": {"Fn::Not": [{"Fn::Equals": ["", {"Ref": "KeyName"}]}]}
},
"Resources": {
"LaunchConfiguration": {
"Type": "AWS::AutoScaling::LaunchConfiguration",
"Properties": {
"KeyName": {"Fn::If": ["HasKeyName", {"Ref": "KeyName"}, {"Ref": "AWS::NoValue"}]}
[...]
}
}
}
}

Now you can leave the KeyName parameter blank to not add a EC2 KeyPair to the LaunchConfiguration.

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