Errror: Value of count cannot be computed

[awhamann]

Issue

Unable to dynamically allocate one or more route table, routes or route associations per subnets. Attempts to do so returns the error:
aws_route_table.route_table: aws_route_table.route_table: value of 'count' cannot be computed

Terraform Version

Terraform v0.9.7

Affected Resource(s)

• aws_route_table
• aws_route
• aws_route_table_association

This may be a core issue as I have seen the behavior with other resources from providers other than AWS.

Terraform Configuration Files

main.tf

# Configure the AWS Provider
provider "aws" {
  access_key = <redacted>
  secret_key = <redacted>
  region     = "us-east-2"
}

variable addressSpace {
   type = "string"
   default = "10.10.11.0/24"
}

variable tags {
   type = "map"
   default = {
      Name = "awhOtherEnv"
      environment = "Anthony's other test environment."
   }
}

variable subnet-cnt {
   type = "string"
   default = "2"
}

resource "aws_vpc" "vpc" {
  cidr_block = "${var.addressSpace}"
  instance_tenancy = "default"
  tags = "${merge(var.tags, map("Name", format("%s-vpc", var.tags["Name"])))}"
}

resource "aws_internet_gateway" "internet-gateway" {
   vpc_id = "${aws_vpc.vpc.id}"
   tags = "${merge(var.tags, map("Name", format("%s-gw", var.tags["Name"])))}"
}

resource "aws_subnet" "subnet" {
   count = "${var.subnet-cnt}"
   vpc_id = "${aws_vpc.vpc.id}"
   cidr_block = "${cidrsubnet(aws_vpc.vpc.cidr_block, ceil(log(var.subnet-cnt,2)), count.index)}"
   tags = "${merge(var.tags, map("Name", format("%s-snet%02d", var.tags["Name"], count.index)))}"
   depends_on = [ "aws_vpc.vpc" ]
}

resource "aws_route_table" "route_table" {
   count = "${length(aws_subnet.subnet.*.id)}" # "${var.subnet-cnt}" #
   vpc_id = "${aws_vpc.vpc.id}"
   tags = "${merge(var.tags, map("Name", format("%s-rtbl%02d", var.tags["Name"], count.index)))}"
   depends_on = [ "aws_subnet.subnet", "aws_vpc.vpc" ]
}

resource "aws_route" "route" {
   count = "${length(aws_route_table.route_table.*.id)}"
   route_table_id = "${element(aws_route_table.route_table.*.id, count.index)}"
   destination_cidr_block = "0.0.0.0/0"
   gateway_id = "${aws_internet_gateway.internet-gateway.id}"
   depends_on = [ "aws_subnet.subnet", "aws_vpc.vpc" ]
}

resource "aws_route_table_association" "route_assn" {
   count = "${length(aws_route_table.route_table.*.id)}"
   route_table_id = "${element(aws_route_table.route_table.*.id, count.index)}"
   subnet_id = "${element(aws_subnet.subnet.*.id, count.index)}"
   depends_on = [ "aws_subnet.subnet", "aws_vpc.vpc" ]
}

Debug Output

terraform.log

Expected Behavior

What should have happened?

A route table should be have been created with one route associated with each subnet.

Actual Behavior

What actually happened?

Error running plan: 1 error(s) occurred:

• aws_route_table.route_table: aws_route_table.route_table: value of 'count' cannot be computed

Steps to Reproduce

1. terraform plan

References

#12570

[apparentlymart]

Hi @awhamann! Sorry this isn't behaving as expected.

Currently Terraform isn't able to automatically deal with this situation. Although in principle it should be possible to know the length of the subnets list even though the resources haven't been created yet, currently Terraform treats the entire list (including its length) as unknown until the resources have been created.

To work around this, it's necessary to use -target to create the subnet resources first:

$ terraform plan -target=aws_subnet.subnet -out=tfplan
...
$ terraform apply tfplan
...

In future we hope to make this work as expected through improvements to the configuration language. This issue is already captured in #14677, so I'm going to close this issue just to keep the discussion consolidated over there. Thanks for filing this issue.

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.