Allow parametrization for unregistration of all targets in target groups when attaching instance/ip to target group

[obourdon]

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Description

Having looked at the aws/resource_aws_lb_target_group_attachment.go code and considering the fact that by default, AWS/terraform-aws-provider (don't know which one) assigns all spawned instances in an Autoscaling group to all listeners in target group, it might be interesting to add an extra argument which, if specified to non-default false value, cleans up all existing attached targets prior to register the give one. Currently, without this argument the list of targets remain unchanged

New or Affected Resource(s)

• aws_lb_target_group_attachment

Potential Terraform Configuration

resource "aws_lb_target_group_attachment" "monitoring" {
    count            = "${var.monitoring_enable * var.cluster_size}"
    target_group_arn = "${element(aws_lb_target_group.monitoring.*.arn, count.index)}"
    target_id        = "${element(data.aws_instances.asg_instances.ids, count.index)}"
    port             = 12345
    replace_all_targets = true
 }

References

None

[ljluestc]

resource "aws_lb_target_group" "example" {
  name     = "example-target-group"
  port     = 80
  protocol = "HTTP"
  vpc_id   = "your-vpc-id"
}

# Deregister all existing targets
resource "aws_lb_target_group_attachment" "deregister" {
  target_group_arn = aws_lb_target_group.example.arn
  count            = length(data.aws_instances.asg_instances.ids)

  target_id = data.aws_instances.asg_instances.ids[count.index]
  port      = 80

  lifecycle {
    create_before_destroy = true
  }
}

# Register new targets
resource "aws_lb_target_group_attachment" "register" {
  target_group_arn = aws_lb_target_group.example.arn
  count            = var.monitoring_enable * var.cluster_size

  target_id = element(data.aws_instances.asg_instances.ids, count.index)
  port      = 80
}