General conventions

1. Use _ (underscore) instead of - (dash) in all: resource names, data source names, variable names, outputs.

   • Beware that actual cloud resources have many hidden restrictions in their naming conventions. Some cannot contain dashes, some must be camel cased. These conventions refer to Terraform names themselves.

2. Only use lowercase letters and numbers.

Resource and data source arguments

1. Do not repeat resource type in resource name (not partially, nor completely):

   • Good: resource "aws_route_table" "public" {}

   • Bad: resource "aws_route_table" "public_route_table" {}

   • Bad: resource "aws_route_table" "public_aws_route_table" {}

2. Resource name should be named this if there is no more descriptive and general name available, or if resource module creates single resource of this type (eg, there is single resource of type aws_nat_gateway and multiple resources of typeaws_route_table, so aws_nat_gateway should be named this and aws_route_table should have more descriptive names - like private, public, database).

3. Always use singular nouns for names.

4. Use - inside arguments values and in places where value will be exposed to a human (eg, inside DNS name of RDS instance).

5. Include count argument inside resource blocks as the first argument at the top and separate by newline after it. See example.

6. Include tags argument, if supported by resource as the last real argument, following by depends_on and lifecycle, if necessary. All of these should be separated by single empty line. See example.

7. When using condition in count argument use boolean value, if it makes sense, otherwise use length or other interpolation. See example.

8. To make inverted conditions don't introduce another variable unless really necessary, use 1 - boolean value instead. For example, count = "${1 - var.create_public_subnets}"

Code examples of resource

Usage of count

main.tf
resource "aws_route_table" "public" {
  count  = "2"
​
  vpc_id = "vpc-12345678"
  # ... remaining arguments omitted
}

main.tf
resource "aws_route_table" "public" {
  vpc_id = "vpc-12345678"
  count  = "2"
​
  # ... remaining arguments omitted
}

Placement of tags

main.tf
resource "aws_nat_gateway" "this" {
  count         = "1"
​
  allocation_id = "..."
  subnet_id     = "..."
​
  tags = {
    Name = "..."
  }
​
  depends_on = ["aws_internet_gateway.this"]
​
  lifecycle {
    create_before_destroy = true
  }
}

main.tf
resource "aws_nat_gateway" "this" {
  count = "1"
​
  tags = "..."
​
  depends_on = ["aws_internet_gateway.this"]
​
  lifecycle {
    create_before_destroy = true
  }
​
  allocation_id = "..."
  subnet_id     = "..."
}

Conditions in count

Variables

1. Don't reinvent the wheel in resource modules - use the same variable names, description and default as defined in "Argument Reference" section for the resource you are working on.

2. Omit type = "list" declaration if there is default = [] also.

3. Omit type = "map" declaration if there is default = {} also.

4. Use plural form in name of variables of type list and map.

5. When defining variables order the keys: description , type, default .

6. Always include description for all variables even if you think it is obvious.

Outputs

Name for the outputs is important to make them consistent and understandable outside of its scope (when user is using a module it should be obvious what type and attribute of the value is returned).

1. The general recommendation for the names of outputs is that it should be descriptive for the value it contains and be less free-form than you would normally want.

2. Good structure for names of output looks like {name}_{type}_{attribute} , where:

   1. {name} is a resource or data source name without provider prefix. {name} for aws_subnet is subnet, foraws_vpc it is vpc.

   2. {type} is a type of a resource sources

   3. {attribute} is an attribute returned by the output

   4. ​See examples.

3. If output is returning a value with interpolation functions and multiple resources, the {name} and {type} there should be as generic as possible (this is often the most generic and should be preferred). See example.

4. If the returned value is a list it should have plural name. See example.

5. Always include description for all outputs even if you think it is obvious.

Code examples of output

Return at most one ID of security group:

outputs.tf
output "this_security_group_id" {
  description = "The ID of the security group"
  value       = "${element(concat(coalescelist(aws_security_group.this.*.id, aws_security_group.this_name_prefix.*.id), list("")), 0)}"
}

When there are multiple resources of the same type, this should be preferred and it should be part of name in output, also another_security_group_id should be named web_security_group_id:

outputs.tf
output "security_group_id" {
  description = "The ID of the security group"
  value       = "${element(concat(coalescelist(aws_security_group.this.*.id, aws_security_group.web.*.id), list("")), 0)}"
}
​
output "another_security_group_id" {
  description = "The ID of web security group"
  value       = "${element(concat(aws_security_group.web.*.id, list("")), 0)}"
}

Use plural name if the returning value is a list

outputs.tf
output "this_rds_cluster_instance_endpoints" {
  description = "A list of all cluster instance endpoints"
  value       = ["${aws_rds_cluster_instance.this.*.endpoint}"]
}

Conditions in output

There are two resources of type aws_db_instance with names this and this_mssql where at most one resource can be created at the same time.

outputs.tf
output "this_db_instance_id" {
  description = "The RDS instance ID"
  value       = "${element(concat(coalescelist(aws_db_instance.this_mssql.*.id, aws_db_instance.this.*.id), list("")), 0)}"
}

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