CloudFormation Conditions Query

Question

I have a CloudFormation template with me and I want to have a choice of whether to use Existing VPC,Subnet for creating an EC2instance using Input parameter.
What I am looking for is something like this:-
The template will have a parameter **"UseExistingResources"**, if user selects **'true**' and provide existing resources ID in other inputs like **"ExistingVpcId"** , **"ExistingSubnetId"** then it should use the existing ones.
And if the user selects **"false"** and then if user left **"ExistingVpcId"** , **"ExistingSubnetId"** either empty or provide something then it should be ignored and the template should create new resources.

Can anyone please provide a condition statement to deal with this.

Thanks

Answer

Hi.

First you need to create a Condition in the template:

```
AWSTemplateFormatVersion: "2010-09-09"
Parameters:
   UseExistingResources:
   Type: String
   Default: false
    AllowedValues:
      - true
      - false

Conditions:
  DoUseExistingResources: !Equals [!Ref UseExistingResources, true]

....
```

Then you need to create the Instance using the condition specify either the existing resources or the new, something like this.

```
EC2Instanace:
   Type: AWS::EC2::Instance
   Properties: 
      ......
      SubnetId:
         Fn::If:
            [DoUseExistingResources, !Ref ExistingSubnetID, !Ref NewCreatedSubnetId]
```

Hope that helps you.

CloudFormation Conditions Query

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