terraform: lambda invocation during destroy - aws-lambda

Question

I have lambda invocation in our terraform-built environment:
data "aws_lambda_invocation" "this" {
count = var.invocation == "true" ? 1 : 0
function_name = aws_lambda_function.this.function_name
input = <<JSON
{
"Name": "Invocation"
}
JSON
}
The problem: the function is invoked not only during creation ("apply") but deletion ("destroy") too. How to invoke it during creation only? I thought about checking environment variables in the lambda (perhaps TF adds name of the process here or something like that) but I hope there's a better way.

Answer 1

Worth checking if you can use the -var 'lambda_xxx=execute' option while running the terraform command to check if the lambda code needs to be executed or not terraform docs
Using that variable lambda_xxx passed in via the command line while executing the command, you can check in the terraform code whether you want to run the lambda code or not.
Below code creates a waf only if the count is 1
resource "aws_waf_rule" "wafrule" {
depends_on = ["aws_waf_ipset.ipset"]
name = "${var.environment}-WAFRule"
metric_name = "${replace(var.environment, "-", "")}WAFRule"
count = "${var.is_waf_enabled == "true" ? 1 : 0}"
predicates {
data_id = "${aws_waf_ipset.ipset.id}"
negated = false
type = "IPMatch"
}
}
Variable declared in variables.tf file
variable "is_waf_enabled" {
type = "string"
default = "false"
description = "String value to indicate if WAF/API KEY is turned on or off (true/any_value)"
}
When you run the command any value other than true is considered false as we are just checking for string true.
Similarly you can do this for your lambda.

Answer 2

There are better alternative solutions for this problem now, which weren't available at the time the question was asked.
Lambda Invocation Resource in AWS provider: https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/lambda_invocation
Lambda Based Resource in LambdaBased provider: https://registry.terraform.io/providers/thetradedesk/lambdabased/latest/docs/resources/lambdabased_resource
With the disclaimer that I'm the developer of the latter one: If the underlying problem is managing a resource through lambda functions, the lambda based resource has some good features tailored specifically to accomplish that with the obvious drawback of adding another provider dependency.