What does keyword `count` mean in data source? - amazon-web-services

Question

when reading open-source terraform module chgangaraju/terraform-aws-cloudfront-s3-website, I found they use count in Data source. But I didn't find any document about count.
what does it mean in this place?
data "aws_acm_certificate" "acm_cert" {
count = var.use_default_domain ? 0 : 1
domain = coalesce(var.acm_certificate_domain, "*.${var.hosted_zone}")
provider = aws.aws_cloudfront
//CloudFront uses certificates from US-EAST-1 region only
statuses = [
"ISSUED",
]
}

Answer 1

It means same as for resource. The pattern uses that you have is an Conditional Expression:
count = var.use_default_domain ? 0 : 1
The expression with count are often used for optional resources or data sources. Specifically, in your case, if you set use_default_domain to false, the CloudFront distro created in this TF script will be created with your own, custom domain and SSL certificate. For this to happen, the data source acm_cert will fetch information about SSL certificate from ACM for your acm_certificate_domain.
In contrast, when use_default_domain is true, you are going to use default domain and SSL certificate from CloudFront. For that you don't need to have any SSL certificate in ACM. Subsequently, TF will not fetch it.
Technically, if use_default_domain is true, then count is 0 and the data "aws_acm_certificate" "acm_cert" is not executed. But, if count is 1 (when use_default_domain is false), the data source will run and try to fetch information about your custom SSL certificate.

Answer 2

A data source is a query, which is used for getting data from the outside world and making it available to your Terraform configuration
terraform example code
provider "aws" {
region = "us-west-2"
}
data "aws_ami" "example" {
count = 2 // look at this line!
most_recent = true
owners = ["self"]
}
output "amis" {
value = "${data.aws_ami.example.*.id}"
}
when you specify the count in the data source block, it's similar to SQL limit. This can be thought of as equivalent to the following SQL query:
select * from data_source
where owners in ('self')
order by most_recent desc
limit 2; /** <= look at this line! **/
Response
The data source returns two records and sends them to the output.
You will see the following results in your terminal.
amis = [
"ami-0345c0186ced78ce6",
"ami-0345c0186ced78ce6",
]
Terraform Documentation: Multiple Resource Instances
Data resources support count and for_each meta-arguments as defined for managed resources, with the same syntax and behavior.
As with managed resources, when count or for_each is present it is important to distinguish the resource itself from the multiple resource instances it creates. Each instance will separately read from its data source with its own variant of the constraint arguments, producing an indexed result.
Related Question
How can I output a data source that uses count?