Terraform multiple cloudwatch events trigger same lambda function - amazon-web-services

Question

I have setup the following in Terraform. So two event rules, start_event at 8am and stop_event at 6pm.
# Create cloudwatch event rules
resource "aws_cloudwatch_event_rule" "stop_ec2_event_rule" {
name = "stop-ec2-event-rule"
description = "Stop EC2 instance at a specified time each day"
schedule_expression = var.cloudwatch_schedule_stop
}
resource "aws_cloudwatch_event_rule" "start_ec2_event_rule" {
name = "start-ec2-event-rule"
description = "Start EC2 instance at a specified time each day"
schedule_expression = var.cloudwatch_schedule_start
}
Each event passes an action to the lambda
resource "aws_cloudwatch_event_target" "stop_ec2_event_rule_target" {
rule = aws_cloudwatch_event_rule.stop_ec2_event_rule.name
target_id = "TriggerLambdaFunction"
arn = aws_lambda_function.lambda_rscheduler.arn
input = "{\"environment\":\"${var.environment}\", \"action\":\"stop\"}"
}
resource "aws_cloudwatch_event_target" "start_ec2_event_rule_target" {
rule = aws_cloudwatch_event_rule.start_ec2_event_rule.name
target_id = "TriggerLambdaFunction"
arn = aws_lambda_function.lambda_rscheduler.arn
input = "{\"environment\":\"${var.environment}\", \"action\":\"start\"}"
}
This works
resource "aws_lambda_permission" "allow_cloudwatch" {
statement_id = "AllowExecutionFromCloudWatch"
action = "lambda:InvokeFunction"
function_name = aws_lambda_function.lambda_rscheduler.function_name
principal = "events.amazonaws.com"
source_arn = aws_cloudwatch_event_rule.stop_ec2_event_rule.arn
This issue I'm facing is that I cannot get Terraform to associate the start_event with the lambda function. I go into the AWS console and I can manually add the CloudWatch start_event trigger to the lambda function.
If I have the start_event resources
resource "aws_lambda_permission" "allow_cloudwatch" {
statement_id = "AllowExecutionFromCloudWatch"
action = "lambda:InvokeFunction"
function_name = aws_lambda_function.lambda_rscheduler.function_name
principal = "events.amazonaws.com"
source_arn = aws_cloudwatch_event_rule.start_ec2_event_rule.arn
It will complain that the statement_id is duplicated.
I needed something like the terraform aws_lambda_event_source_mapping but that only allows Lambda functions to get events from Kinesis, DynamoDB and SQS; and not a CloudWatch event.
How can I tell terraform to associate multiple cloudwatch events to the same lambda function; when I can manually do it from the AWS console?

Answer 1

statement_id is not compulsory, so you can safely omit it from your aws_lambda_permission and terraform will unique generate id for you automatically. You can also use count or for_each to save you some typing for aws_lambda_permission.
For example, using for_each you could define aws_lambda_permission to be:
resource "aws_lambda_permission" "allow_cloudwatch" {
for_each = {for idx, v in [
aws_cloudwatch_event_rule.stop_ec2_event_rule,
aws_cloudwatch_event_rule.start_ec2_event_rule
]: idx => v}
action = "lambda:InvokeFunction"
function_name = aws_lambda_function.lambda_rscheduler.function_name
principal = "events.amazonaws.com"
source_arn = each.value.arn
}
Analogical versions could be written for aws_cloudwatch_event_rule and aws_cloudwatch_event_target so that your code is based on for_each or count without the copy-and-paste repetitions.