Add support to increase number of sole tenant nodes in node group

[tamilvs]

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Description

Currently increasing number of sole tenant nodes in the node group is not supported. and can see a warning message on the resource page stating that "Due to limitations of the API, Terraform cannot update the number of nodes in a node group and changes to node group size either through Terraform config or through external changes will cause Terraform to delete and recreate the node group" but I could see addNodes api in GCP doc, please find the reference link and enhance the node group rosource

https://cloud.google.com/compute/docs/reference/rest/v1/nodeGroups/addNodes

New or Affected Resource(s)

• google_compute_node_group

Potential Terraform Configuration

# Propose what you think the configuration to take advantage of this feature should look like.
# We may not use it verbatim, but it's helpful in understanding your intent.

References

• b/185382242

[tamilvs]

No one owning this issue :(

[slevenick]

Thanks for the request. We have looked at adding support for this, but it is quite a difficult task in terraform due to the one-way direction of adding a node. We are exploring ways to support this functionality, so I'll leave this issue open.

[rileykarson]

This was brought up to cover autoscaling policies as well internally, which we don't super support without using lifecycle.ignore_changes on the size field. I think we can support the entire scaling use case better in a way that's backwards compatible:

• Introduce an initial_node_count or initial_size field that is mutually exclusive with size, specifically marking them as AtLeastOneOf with each other. Whichever is specified is used on create as the initialNodeCount query parameter
  • We will never detect drift on this field, and changing it locally will recreate the resource.
• Mark the size field as Optional + Computed, use CustomizeDiff to support updates only in the additive case
  • note: if a field is O+C and not present in user config, there will not be a diff on the field and the value in state will be the value stored on Read

Note that users are already able to specify size and autoscaling_policy together, and we have no reason to break them- it'll be a tiny bit of a footgun, but not worth a backwards incompatibility. Users could then use the following configurations, all of which are valid:

# works today
resource "google_compute_node_group" "nodes" {
  name          = "soletenant-group"
  node_template = google_compute_node_template.soletenant-tmpl.id

  size = 1
}

resource "google_compute_node_group" "nodes" {
  name          = "soletenant-group"
  node_template = google_compute_node_template.soletenant-tmpl.id

  initial_size  = 1
}

# works today
resource "google_compute_node_group" "nodes" {
  name          = "soletenant-group"
  node_template = google_compute_node_template.soletenant-tmpl.id

  size = 1
  autoscaling_policy {
    mode      = "ON"
    min_nodes = 1
    max_nodes = 10
  }

  lifecycle {
    ignore_changes = ["size"]
  }
}

resource "google_compute_node_group" "nodes" {
  name          = "soletenant-group"
  node_template = google_compute_node_template.soletenant-tmpl.id

  initial_size = 1
  autoscaling_policy {
    mode      = "ON"
    min_nodes = 1
    max_nodes = 10
  }
}

It's possible that this will be difficult/impossible to implement with our current code generator, in which case we can defer fixing it. I believe it'll be possible (specifically, I think we can d.Get the size value in the initial_size expander if d.Get initial_size is 0)

[github-actions]

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