CloudFormation allowing users to use existing vs create new resources.

Question

Hi,
I have a scenario where I am developing a CloudFormation template for my users. So, I want to give my users the choice of using their existing VPS,Subnet,Security Group or they want to create a new one from the template.
So, Is it possible to do this using the parameters where all the existing VPC ID would be listed along with the "create new" option in the same dropdown?

Thanks

Answer

You can do this, but it can get quickly rather complex. I have done it like this
Have parameters for everything. If you already have the resource, input your resource ID, and if you want to create a new one, leave it empty. For example NAT GW Elastic IP parameter

```
EipA:
    Type: String
    Description: Pre-allocated EIP ID for AZ-A (optional)
    Default: ''
```
Then in Conditions block, check if parameters has a value or if it is left empty
```
CreateEipA: !Equals [!Ref EipA, "" ] 
```
Then use the condition when creating (or not) creating the resource
```
  NatEipA:
    Type: AWS::EC2::EIP
    Condition: CreateEipA
    Properties:
      Domain: vpc
```
And finally when creating the NAT GW select if you want to use parameter value or reference to resource that was created.
```
  NatGwA:
    Type: AWS::EC2::NatGateway
    Properties:
      ConnectivityType: public
      SubnetId: !Ref PubSubnetA
      AllocationId: !If [ CreateEipA, !GetAtt NatEipA.AllocationId, !Ref EipA ]
```

CloudFormation allowing users to use existing vs create new resources.

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