terraform environments and updating a single resource - aws-lambda

Question

I am using Terraform for continuous deployments of lambda functions. The lambda module creates the function and initial aliases [DEV,QA,PROD]. When a change is made the source_code_hash is updated and Terraform updates the code. The challenge is when I want to update the alias from DEV to QA it updates the entire stack. The code is below. Your help is appreciated.
$ cat main.tf
module "sample" {
source = "./lambda"
name = "sample"
runtime = "nodejs6.10"
role = "${aws_iam_role.iam_role_for_lambda.arn}"
filename = "../Archive.zip"
source_code_hash = "${base64sha256(file("../Archive.zip"))}"
source_dir = "../sample"
alias = "${var.env_name}"
}
$ cat module/main.tf
resource "aws_lambda_function" "lambda" {
filename = "${var.filename}"
function_name = "${var.name}"
role = "${var.role}"
handler = "${var.name}.${var.handler}"
runtime = "${var.runtime}"
source_code_hash = "${data.archive_file.lambda_zip.output_base64sha256}"
publish = "true"
}
resource "aws_lambda_alias" "lambda_alias" {
count = "2"
name = "${element(var.alias, count.index)}"
#name = "${var.alias}"
description = "${var.name}"
function_name = "${aws_lambda_function.lambda.arn}"
function_version = "${aws_lambda_function.lambda.version}"
}

Answer 1

You need different tfvars and tfstate files for different environments.
Suppose you use latest terraform version.
create an environment folder (for example, env) and set <env>-backend.tf and <env>.tfvars files for each environment
$ cat main.tf
terraform {
required_version = ">= 0.9.1"
backend "s3" {
encrypt = "true"
}
}
$ cat env/dev-backend.tf
bucket = "terraform-<change-to-s3-global-unique-id>"
key = "terraform/dev/terraform.tfstate"
kms_key_id = "xxxx-xxxx-xxxx-xxxx"
$ cat env/dev.tfvars
env_name = dev
Do the same for qa and prod environments.
So you should be fine to run below commands to get same lambda tf codes work in different environments
rm -rf .terraform
export env="dev"
terraform init -backend=true -backend-config=env/${env}-backend.tf
terraform plan -var-file=./env/${env}.tfvars