CloudFormation Condition Function Example

AWS CloudFormation is a popular tool to automate AWS resource management. There are situations when you need to create a resource or specify a property based on the value of input parameters. You may use Condition Functions to address such types of requirement.

Here is an example of how to use Conditions in a CloudFormation script.

In this example, we want to specify “unlimited”, a CPU credit property only applies to t2/t3 series instance when a user select such instance type. If the user input is not t2 series, for example, m4 series, the CPU credit property cannot be specified as it does not exist.

The code to define a condition that tests if the input parameter is t2.large, t2.xlarge or t2.2xlarge is as follows.

"Conditions": {
     "AviatrixIAMRoleNotExist": {
         "Fn::Equals": [
             {
                 "Ref": "IAMRoleParam"
             },
             "New"
         ]
     },
     "T2SeriesTrue": {
         "Fn::Or": [
              {"Fn::Equals" : [{"Ref" : "InstanceTypeParam"}, "t2.large"]},
              {"Fn::Equals" : [{"Ref" : "InstanceTypeParam"}, "t2.xlarge"]},
              {"Fn::Equals" : [{"Ref" : "InstanceTypeParam"}, "t2.2xlarge"]}

         ]
     }
 },

Where T2SeriesTrue is the condition to test out if input parameter InstanceTypeParam is one of the t2 series. Note “Fn::Or” is used to allow multiple choices, if any of them is True, the condition returns True.

The code to use the condition to specify the CPU Credit type is as follows:

"CreditSpecification": {
    "Fn::If" : [
        "T2SeriesTrue",
        {"CPUCredits": "unlimited"},
        {"Ref": "AWS::NoValue"}
    ]
}

where if condition T2SeriesTrue is True, CPUCredits will be set to “unlimited”, otherwise it is not specified.

To view the entire CloudFormation script that is used to launch the Aviatrix Controller, visit the repo on github.