Use a default value less than MinLength in an AWS CloudFormation parameter

I'm building an AWS CloudFormation template which has the following parameter:

Naming:
  Type: String
  Default: aws-ab-
  MinLength: 12
  Description: 'aws-ab-{customer}-{environment}'

The first characters are fixed, but the rest needs to be configured.

However, this template is giving me an error:

Template contains errors.: Parameter 'Naming' must contain at least 12 characters

Is there a way to have a default/placeholder value that knowingly violates the validation rules, expecting that the user will complete it?

Solution

In this case, you've defined the Naming parameter with a MinLength constraint of 12 characters, and it's causing an error because the default value you provided, 'aws-ab-', is shorter than the minimum length constraint.

To achieve your goal of having a default/placeholder value that knowingly violates the validation rules and expecting users to complete it, you can work around this constraint by using a combination of intrinsic functions like Fn::Sub to construct the default value. Here's how you can do it:

Parameters:
  Customer:
    Type: String
    Default: customer
    AllowedPattern: ^[a-z0-9]+$  # Only lowercase letters and digits are allowed
  Environment:
    Type: String
    Default: environment
    AllowedPattern: ^[a-z0-9]+$  # Only lowercase letters and digits are allowed

Resources:
  MyResource:
    Type: AWS::SomeResourceType
    Properties:
      Naming:
        Fn::Sub: 'aws-ab-${Customer}-${Environment}'

In this example:

We have two additional parameters, Customer and Environment, which have default values.
The Fn::Sub function is used to substitute the default value of Naming with the concatenated string 'aws-ab-${Customer}-${Environment}'.

This way, the default value for Naming is constructed using the Fn::Sub function, and it doesn't violate the MinLength constraint because the placeholders ${Customer} and ${Environment} will be replaced by their default values when the CloudFormation stack is created. Users can later override these values when creating the stack if needed.