Amazon web services - Is there anyway that I can check if the List<AWS::EC2::Subnet::Id> is empty in cloud formation

rosepalette
August 7, 2023
263 views
0 votes
2 Answers

I am trying to let the user choose the subnet from the cloud formation params and I keep getting errors when the params of the subnet are not selected with any choices.

When I do not select any subnet, it will throw an error Parameter validation failed: parameter value for parameter name SubnetIds does not exist. Rollback requested by user.

AWSTemplateFormatVersion: "2010-09-09"

Parameters:
  SubnetIds:
    Description: "Select multiple subnets from the list"
    Type: List<AWS::EC2::Subnet::Id>

Conditions:
  SubnetListNotEmpty:
    Fn::Not:
      - Fn::Equals:
          - Fn::Join: ["", !Ref SubnetIds]
          - ""

Resources:
  NullResource:
    Type: AWS::CloudFormation::WaitConditionHandle

Outputs:
  ConditionValid:
    Description: "valid condition"
    Value: !If [SubnetListNotEmpty, "valid", "invalid"]

It would be a good help if anyone could drop some hints.

I understood the error, the AWS:NoValue and "" does not exist in the AWS::EC2::Subnet::Id. How do I handle if there is a chance that the subnet is not needed?

Updated stack

AWSTemplateFormatVersion: "2010-09-09"

Parameters:
  SubnetId:
    Description: "Select a subnet ID or leave blank"
    Type: AWS::EC2::Subnet::Id
    Default: ""

Conditions:
  SubnetSelected: !Not [!Equals [!Ref SubnetId, ""]]

Resources:
  NullResource:
    Type:
      AWS::CloudFormation::WaitConditionHandle
      # Other properties

Outputs:
  SubnetIdOutput:
    Description: "Selected subnet ID or None"
    Value: !If [SubnetSelected, !Ref SubnetId, "None selected"]

Answers

- DennisTraub
- August 7, 2023 at 5:27 pm
- 0 votes
0
You can add the AllowedPattern: ".+" attribute to make sure it’s not empty:
```
Parameters:
  SubnetIds:
    Description: "Select multiple subnets from the list"
    Type: List<AWS::EC2::Subnet::Id>
    AllowedPattern: ".+"
```
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- Paolo
- August 7, 2023 at 8:37 pm
- 0 votes
0
It’s not possible, because by definition the list cannot be empty. As you found out, AWS performs validation for AWS-specific parameter types, which means that null values are not accepted for such parameter types.

This is confirmed by the documentation (emphasis mine)

AWS-specific parameter types are helpful in catching invalid values at the start of creating or updating a stack. To specify parameters with AWS-specific types, a template user must enter existing AWS values that are in their AWS account. AWS CloudFormation validates these input values against existing values in the account.

As a workaround, your best bet is to use a CommaDelimitedList, which alas implies not having the nice functionality offered by the AWS-specific parameter types.

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