Terraform: Referencing attribute on resource created with count

[FransUrbo]

I've created four RDS instances (one master and three slaves in the three AZs) and am now trying to setup a ELB in front of the RDS instances.

I've used aws_elb to setup the ELB and am now trying to attach the RDS instances to that using aws_elb_attachment.

But I'm not sure how to do that using count.

# Attach all the RDS instances to the ELB.
resource "aws_elb_attachment" "main-rds-fc2" {
  count                         = 3 # !! _MUST_ be the same as 'aws_db_instance.main_fc2.count'  !!

  elb                           = "${aws_elb.main-rds-fc2.id}"
  instance                      = "${aws_db_instance.main_fc2.${count.index}.id}"
#  instance                      = "${format("aws_db_instance.main_fc2.%d.id", count.index)}"
}

The first example of instances gives me the error:

Error reading config for aws_elb_attachment[main-rds-fc2]: parse error at 1:28: expected "}" but found invalid sequence "$"

(I had expected that one to work!)

The second (commented out) looks like it would work:

module.core-main.aws_elb_attachment.main-rds-fc2.0
  elb:      "core-main-rds-fc2"
  instance: "aws_db_instance.main_fc2.0.id"

but gives me:

* aws_elb_attachment.main-rds-fc2.2: Failure registering instances with ELB: InvalidInstance: InvalidInstance found in [aws_db_instance.main_fc2.2.id]. Invalid id: "aws_db_instance.main_fc2.2.id" (Service: AmazonEC2; Status Code: 400; Error Code: InvalidInstanceID.Malformed; Request ID: 06cabc95-8f0c-484b-8f32-5451fa0c9ef7)

which is kind'a obvious - it's not a variable/reference, but a string.

Trying instance = "${format("${aws_db_instance.main_fc2.%d.id}", count.index)}" or instance = "${${format("aws_db_instance.main_fc2.%d.id", count.index)}}" both give me errors

Error downloading modules: module core-main: Error loading .terraform/modules/ce3ac1d445a47c76df64ef224dfadb09/loadbalancer-rds.tf: Error reading config for aws_elb_attachment[main-rds-fc2]: d.id: resource variables must be three parts: TYPE.NAME.ATTR in:

and

Error downloading modules: module core-main: Error loading .terraform/modules/ce3ac1d445a47c76df64ef224dfadb09/loadbalancer-rds.tf: Error reading config for aws_elb_attachment[main-rds-fc2]: parse error at 1:3: expected expression but found invalid sequence "$"

respectively.

Trying to create three separate resources:

resource "aws_elb_attachment" "main-rds-fc2.0" {
  elb                           = "${aws_elb.main-rds-fc2.id}"
  instance                      = "${aws_db_instance.main_fc2.0.id}"
}

instead gives me:

Error configuring: Unexpected value for InstanceType field: "0"

which, I guess is obvious because it thinks the third option/part is the attribute to get. But that's what's stated in the state file.

[FransUrbo]

Some more tries, I came up with:

  instance = "${format("$${aws_db_instance.main_fc2.%d.id}", count.index)}"

which seems to be the closest, but that also gave me

Error configuring: Unexpected value for InstanceType field: "0"

So I'm guessing it's more a question on how to retrieve an attribute from a counted resource.

[FransUrbo]

The last example of instance gives the plan:

+ module.core-main.aws_elb_attachment.main-rds-fc2.0
    elb:      "core-main-rds-fc2"
    instance: "${aws_db_instance.main_fc2.0.id}"

but again, there's FOUR parts, where the fourth is actually the attribute I want.

[ceqi]

@FransUrbo Have you tried this?
instance: "${aws_db_instance.mainfc2.*.id[0]}"

it returns first element from the 'id' list.
works for me.

[FransUrbo]

Ah, no never tried that! I'll try to remember that next time. I've scratched the ELB. Apparently, it's not possible to put a ELB in front of a RDS anyway (according to the AWS docs), so I'll have to resort to using round-robin with Route53.

But if that example should work, then how about changing the labels of this issue to documentation and update the docs about interpolation at https://www.terraform.io/docs/configuration/interpolation.html ?

[ramkarthik-sys]

I have come cross the similar issue when I am attaching my ASG EC2 instances to Internal ELB.

resource "aws_elb_attachment" "internal_elb_attach_to_proxy_asg" {
elb = "${aws_elb.proxy_elb.id}" // ELB ID
instance = "${aws_autoscaling_group.proxy_asg.id}" // Attaching ASG to the ELB
}

Error

Failure registering instances with ELB: InvalidInstance: InvalidInstance found in xxxxxx-ASG. Status Code: 400; Error Code: InvalidInstanceID.Malformed; Request ID:

Does anyone know how we fix this ?

[apparentlymart]

Agreed that we should have better docs for this.

The _Using Templates with Count_ section covers this pattern, but it makes it sounds like it's something unique to templates when in fact it's a general pattern that can be applied to any set of inter-related resources.

[ramkarthik-sys]

I have managed to fix this by using asg_attachment data source where we have to pass elb details to an ASG.

[farman022]

@FransUrbo Use the below syntax.
instance = "${aws_db_instance.main_fc2.*.id[count.index]}"

[mildwonkey]

I am going to close this issue due to inactivity. Our documentation has had many updates since this was opened!

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Thanks!

[hashibot[bot]]

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