Terraform: Variable cannot contain interpolation

[sidhurana]

I am trying to create status check cloudwatch alarm that will be applicable to all instances :

In variables file :

variable "hostnames" {
default = {
"0" = "www-l01"
"1" = "www-l02"
"2" = "aps-l01"
"3" = "aps-l02"
}
}

variable "instanceid" {
default = {
"0" = "${aws_instance.www-l01.id}"
"1" = "${aws_instance.www-l02.id}"
"2" = "${aws_instance.aps-l01.id}"
"3" = "${aws_instance.aps-l01.id}"
}
}

In cloudwatch file :-

resource "aws_cloudwatch_metric_alarm" "status_check" {
count = 2
alarm_name = "${lookup(var.hostnames, count.index)}"
comparison_operator = "GreaterThanOrEqualToThreshold"
evaluation_periods = "2"
metric_name = "StatusCheckFailed"
namespace = "AWS/EC2"
statistic = "SampleCount"
threshold = "2"
period = "60"
alarm_description = "Status Check of Instance"
insufficient_data_actions = []
dimensions {
InstanceId = "${lookup(var.instanceid, count.index)}"
}
alarm_actions = ["arn:aws:automate:eu-west-1:ec2:recover"]
}

While executing it gave me below error :-

module root: 1 error(s) occurred:

• Variable 'instanceid': cannot contain interpolations

Any idea about best way to resolve this ?

[pasali]

@sidhurana as far as I know terraform does not support interpolations in variables see #4397

[sidhurana]

@pasali : Thanks , Do you know any other way to achieve this ?

[pasali]

@sidhurana, i did not quite understand what you are trying to achieve, but i assume you want to get all instance ids So you can do something like that:

dimensions {
  InstanceId = "${aws_instance.your-resource-name.*.id}"
}

let me know if this works for you.

[martin-flaregames]

@sidhurana extending @pasali proposal, you can concatenate two lists into one.

https://www.terraform.io/docs/configuration/interpolation.html#concat_list1_list2_

dimensions {
  InstanceId = "${concat(aws_instance.www.*.id, aws_instance.aps.*.id)}"
}

in order to make this work you must generate your instances using the "count" parameter and not by declaring them separately.

Another alternative would be to create a list element by element using https://www.terraform.io/docs/configuration/interpolation.html#list_items_

dimensions {
  InstanceId = "${list(aws_instance.www-l01.id, aws_instance.www-l02.id, aws_instance.aps-l01.id, aws_instance.aps-l01.id)}"
}

the same way you can create a map like this
https://www.terraform.io/docs/configuration/interpolation.html#map_key_value_

dimensions {
  InstanceId = "${map("0", aws_instance.www-l01.id, "1", aws_instance.www-l02.id, "2", aws_instance.aps-l01.id, "3", aws_instance.aps-l01.id)}"
}

[sidhurana]

@pasali : Thanks. yes, I am trying to get all instance ID's so i can pass it to cloudwatch resource and that will create cloudwatch alarm for all instances all at once.
@martin-flaregames : Thanks for examples and reference pages.

I am getting below error while trying above map and list examples in InstanceId:

• dimensions: 1 error(s) decoding:
• '[InstanceId]' expected type 'string', got unconvertible type 'map[string]interface {}'
• dimensions: 1 error(s) decoding:

[martin-flaregames]

Well the examples just show you how to create a list or a map outside a variable declaration, but the "IsntanceId" entry in "dimensions" expects only a single string. So you still need to iterate over the items using the count property of aws_cloudwatch_metric_alarm.

Btw, you do not need a map for this, you can use element instead of lookup when using lists.
https://www.terraform.io/docs/configuration/interpolation.html#element_list_index_

Possibly the best alternative would be to create a module accepting a list of instance IDs as parameter. I could build such an example for you, but it will take time.

[pasali]

@sidhurana this should do the trick

dimensions {
  InstanceId = "${element(aws_instance.your-resource-name.*.id, count.index)}"
}

[sidhurana]

@martin-flaregames : Thanks for your input.

@pasali : Thanks.. I have spawned instances by declaring them separately. I think it will work perfectly in case all instances created by count parameter. So, in my case i have different resource names for all instances.

Please correct me if I am wrong.

[martin-flaregames]

@sidhurana try combining @pasali solution with the previously proposed list interpolation.

```
dimensions {
InstanceId = "${element(list(aws_instance.www-l01.id, aws_instance.www-l02.id, aws_instance.aps-l01.id, aws_instance.aps-l01.id), count.index)}"
}
````

[pasali]

@sidhurana you can try what @martin-flaregames suggested. But personally i would use count parameter to create instances. If you use count parameter, your resource declarations will be more dynamic.

[martin-flaregames]

completely agree with @pasali, @sidhurana you should strive to create the instances using count. If for some reason you need two different aws_instance setups then take a look at the previous example with concat. You can try my previous suggestion to see if things are working as expected but switching to a count based creation of aws_instance will provide you a way of changing the number of instances without worrying about managing the list manually.

[sidhurana]

@martin-flaregames: It works perfectly.
@pasali : Yeah, I agreed. I will try to change my terraform config.

[martin-flaregames]

@sidhurana you may proceed to close the issue if you have no further questions or remarks :)

And please notice that there may be a bug in your original list: aws_instance.aps-l01.id is listed twice. You may have intended to say aws_instance.aps-l02.id. This is another argument for using count ;)

[mitchellh]

I'm closing it as well since we have issues to cover the feature request and this issue isn't a bug. Thanks everyone for helping!

[sidhurana]

@martin-flaregames :Yeah , that was typo..I agree that using count will avoid such errors and will shorten code as well... Thanks all for helping.

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