Support (and document) use case for using existing Transit Gateway

[korenyoni]

Is your request related to a new offering from AWS?

No.

Is your request related to a problem? Please describe.

var.create_tgw short-circuits creation of aws_ec2_transit_gateway_route and prevents using this module when a TGW already exists but the TGW routes should be created:

terraform-aws-transit-gateway/main.tf

Lines 2 to 5 in 3a3d9f1

	# List of maps with key and route values
	vpc_attachments_with_routes = chunklist(flatten([
	for k, v in var.vpc_attachments : setproduct([{ key = k }], v.tgw_routes) if var.create_tgw && can(v.tgw_routes)
	]), 2)

This use case is relevant when the Transit Gateway is managed in one configuration / state, but the VPC attachments and their routes are managed elsewhere.

It is not clear if this use case is actually intended to be supported, hence why this issue is a feature request and not a bug report. For example, aws_ec2_transit_gateway_vpc_attachment is created even when var.create_tgw, but as mentioned above, aws_ec2_transit_gateway_route is not. Moreover, this use case is not documented.

terraform-aws-transit-gateway/main.tf

Line 68 in 3a3d9f1

transit_gateway_id = var.create_tgw ? aws_ec2_transit_gateway.this[0].id : each.value.tgw_id

Describe the solution you'd like.

Change this logic such that aws_ec2_transit_gateway_route when var.create_tgw is false

Describe alternatives you've considered.

None

Additional context

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