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Using CloudFormation templates is it possible to ensure a subnet is created if it does not already exist?

The use case is for when running a stack deployment in an existing VPC, that VPC may or may not have other resources using the specified subnet.

It seems like this would be a common request but I can't find any documentation on it.

I guess I could assume the subnets I'm working with always exist but that doesn't seem particularly robust.

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Generally speaking, no. CloudFormation's resources are always created.

The reason for this is to ensure that a CloudFormation stack's resouces are isolated and can be fully controlled by one-and-only-one stack.

If you want to use an existing subnet, you have a few options:

  1. Hardcode the subnet ID into your template and use it,
  2. Pass a subnet ID into your stack as a parameter,
  3. Use the new CloudFormation stack resource references to reference resource in other stacks.

If you really need to use a create-if-not-exists resource, you may be able to do it using custom resources. However, you'll have to worry about what to do on stack cleanup and rollbacks.

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