aws_lb_listener_rule - Ignore weights for ECS blue/green deployments

[muru]

Description

With the new ECS Blue/Green deployment support (support added in #43434), the ALB listener rule weights will be changed by ECS when deployments occur, so these weights can no longer be managed in Terraform.

Ideally, this could have been resolved by using lifecycle { ignore_changes }, but it is not possible to ignore only the weight, because the target_groups are a set of objects, and there's no way to refer to a key in them:

action[0].forward[0].target_group.weight

╷
│ Error: Cannot index a set value
│
│   on webapps_blue_green.tf line 45, in resource "aws_lb_listener_rule" "blue_green":
│   45:       action[0].forward[0].target_group.weight,
│
│ Block type "target_group" is represented by a set of objects, and set elements do not have addressable keys. To find elements matching specific criteria, use a "for" expression with an "if"
│ clause.
╵

action[0].forward[0].target_group.*.weight (similar error for action[0].forward[0].target_group[*].weight)

╷
│ Error: Invalid expression
│
│   on webapps_blue_green.tf line 43, in resource "aws_lb_listener_rule" "blue_green":
│   43:       action[0].forward[0].target_group.*.weight,
│
│ A single static variable reference is required: only attribute access and indexing with constant keys. No calculations, function calls, template expressions, etc are allowed here.
╵

action[0].forward[0].target_group[0].weight

│ Error: Cannot index a set value
│
│   on webapps_blue_green.tf line 45, in resource "aws_lb_listener_rule" "blue_green":
│   45:       action[0].forward[0].target_group[0].weight,
│
│ Block type "target_group" is represented by a set of objects, and set elements do not have addressable keys. To find elements matching specific criteria, use a "for" expression with an "if"
│ clause.

Omitting the weight simply leads to them being set to 1. The only thing that works is ignoring target_group altogether, but if we need to update the target groups, then we will have to recreate the rule altogether (probably with a replace_triggerd_by).

It would be nice if we could indicate to the resource that weights will be set elsewhere. The weights are already optional, so maybe we could have a new attribute that will cause the weights to be treated as computed values?

Affected Resource(s) or Data Source(s)

• aws_lb_listener_rule

Potential Terraform Configuration

resource "aws_lb_listener_rule" "blue_green" {
  action {
    type = "forward"
    forward {
      externally_managed_weights = true
      target_group {
        arn = aws_lb_target_group.blue.arn
      }
      target_group {
        arn = aws_lb_target_group.green.arn
      }
    }
  }
  

References

No response

Would you like to implement the enhancement?

No

[github-actions]

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[muru]

I tested this change locally (adding Computed: true to the weight attribute):

diff --git a/internal/service/elbv2/listener_rule.go b/internal/service/elbv2/listener_rule.go
index 9f26aca546..db9d2ba169 100644
--- a/internal/service/elbv2/listener_rule.go
+++ b/internal/service/elbv2/listener_rule.go
@@ -254,8 +254,9 @@ func resourceListenerRule() *schema.Resource {
                                                                                                names.AttrWeight: {
                                                                                                        Type:         schema.TypeInt,
                                                                                                        ValidateFunc: validation.IntBetween(0, 999),
-                                                                                                       Default:      1,
+                                                                                                       Default:      nil,
                                                                                                        Optional:     true,
+                                                                                                       Computed:     true,
                                                                                                },
                                                                                        },
                                                                                },

It worked well enough for me if I drop the weight altogether for blue-green listener rules. This forces a change of the default to nil from 1:

│ resource aws_alb_listener_rule: weight: Default must be nil if computed

Usually I'd think that a change in default is a compatibility break, but it seems the API considers the weight to be optional, so if the API default is also 1, this might not be a breaking change? (Though it also says the target group ARN is optional as well, which is confusing.)

Perhaps instead of declaring the default as 1 in the schema, it can be set in the code when creating?