The given key does not identify an element in this collection value: a number is required.

[kalaibarak]

Terraform Enterprise edition 0.13.7
provider aws version 3.37.0
data.tf

data "aws_instance" "instance_name" {
  for_each      = toset(data.aws_instances.instance_cloudwatch.ids)
  instance_id   = each.value
 }
data "aws_instances" "instance_cloudwatch" {

  instance_tags = {
    Type        = var.type
  }
}
data "aws_ebs_volumes" "cw_volumes" {
  for_each    = toset(flatten(data.aws_instances.instance_cloudwatch.ids))
}

The resource section

resource "aws_cloudwatch_metric_alarm" "iops" {
  for_each                  = toset(data.aws_instances.instance_cloudwatch.ids)
  not related lines
  metric {
      metric_name = "VolumeWriteOps"
      namespace   = "AWS/EBS"
      period      = "60"
      stat        = "Sum"
      dimensions = {
        InstanceId   = each.value
        VolumeId     = toset(flatten(values(data.aws_ebs_volumes.cw_volumes)[each.key].*))
      }
    }  

error
on modules/notification/iops.tf line 34, in resource "aws_cloudwatch_metric_alarm" "iops":
34: VolumeId = toset(flatten(values(data.aws_ebs_volumes.cw_volumes)[each.key].*))
|----------------
| data.aws_ebs_volumes.cw_volumes is object with 2 attributes
| each.key is "i-078c2050de4851e93"

The given key does not identify an element in this collection value: a number
is required.

[justinretzolk]

Hey @kalaibarak 👋 Thank you for taking the time to raise this! There are a couple of things that I noticed when attempting to reproduce this.

The first is the reason that you're receiving the error you mentioned. This is coming from the VolumeId argument that you're passing. Since you're specifying values(data.aws_ebs_volumes.cw_volumes), the keys are removed, as shown in the following example output from my test:

Configuration:

output "example" {
  value = values(data.aws_ebs_volumes.cw_volumes)
}

Output:

  + example = [
      + {
          + filter = null
          + id     = "us-east-1"
          + ids    = [
              + "vol-0e6XXXXXXXXXX",
             ...omitted for brevity
            ]
          + tags   = null
        },
      + {
          + filter = null
          + id     = "us-east-1"
          + ids    = [
              + "vol-0e66ca09c26587569",
             ...omitted for brevity
            ]
          + tags   = null
        },
    ]

This means that when you then try to index with [each.key], there's no key to index off of, which is why the error is thrown. This can be mitigated by updating your configuration as follows:

        VolumeId     = toset(flatten(data.aws_ebs_volumes.cw_volumes[each.key].ids))

Updating my example (I'm passing an instance ID directly, just for the sake of making the reproduction easier to read), yields this change to the output:

Configuration:

output "example" {
  value = toset(flatten(data.aws_ebs_volumes.volumes["i-0b1edXXXXXXX"].ids)) // ID obfuscated for security's sake
}

Output:

  + example = [
      + "vol-0e6XXXXXXXX",
      ...omitted for brevity
    ]

Unfortunately, that won't entirely solve your issue, as there were two additional things that came up when I got past the initial issue that you reported:

1. The VolumeId argument expects a string rather than a list of strings
2. As far as I can tell, the AWS/EBS namespace supports the VolumeId dimension, but does not support InstanceId. This could be a misunderstanding on my part, but may be something that you run into, so I figured it was worth noting.

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