feature/mandatory-UI-arn

[grumbling-tom]

I would like to make the Metaflow UI a mandatory component for a Metaflow deployment.

The problem I am having is with the UI count:

• this count value depends on the string value of the SSL certificate ARN
• I am typically creating those SSL certificates in another module
• the string value for the ARNs is not known at plan time if they do not yet exist (i.e. when the module has not been created before)
• as such, the value of the count for the Metaflow UI cannot be determined, meaning the terraform plan fails with this error:

╷
│ Error: Invalid count argument
│ 
│   on .terraform/modules/ncr_metaflow.metaflow/main.tf line 44, in module "metaflow-ui":
│   44:   count  = var.ui_certificate_arn == "" ? 0 : 1
│ 
│ The "count" value depends on resource attributes that cannot be determined until apply, so Terraform cannot predict how many instances will be created. To work around this, use the -target
│ argument to first apply only the resources that the count depends on.
╵
╷

The workaround suggested requires that we run the terraform apply in two stages. This is not desirable because:

• it is not compatible with our CI workflow
• we want to be able to create both this module and the SSL modules at the same time, to speed up deployments of new infrastructure

Other things that I have tried:

• Setting the default value to be null, rather than an empty string, with the hope that terraform would check the type for the input ui_certificate_arn variable. It did not.
• Adding a lifecycle precondition that checks the ui_certificate_arn value, but these are not supported for modules

We could also add a second variable, create_ui, which we instead use as the conditional to count, rather than the string value of the ARN. This would be similar to how the terraform-eks module handles conditional resource creation.

The trade-off of this change is that we end up with a change in our terraform-aws fork that will almost certainly not be accepted into the upstream Outerbounds repo, which might make proposing changes later on harder.