aws_lambda_event_source_mapping, attribute queues is misleading

[Tuzosdaniel12]

Description

Small observation after spending a day trying to understand validation failure when creating aws_lambda_event_source_mapping. After going back and carefully reading the docs I found that issue came from queues attribute.

At first glance one can assumed that queues = ["example"] will accept an array of strings with different queue

In result I created my resource in the following format.

resource "aws_lambda_event_source_mapping" "mq_triggers" {
  batch_size       = 10
  event_source_arn = var.broker_arn
  enabled          = true
  function_name    = aws_lambda_function.message_handler.arn
  queues           = ["START", "STOP", "STATUS"]

  source_access_configuration {
    type = "BASIC_AUTH"
    uri  = aws_secretsmanager_secret.broker_credentials.arn
  }
}

Which resulted in the following error.

Error: error creating Lambda Event Source Mapping (arn:aws:mq:....): ValidationException: 
	status code: 400

After banging my head I reread the docs and I soon understood that I could only send in a single string in the array.

queues - (Optional) The name of the Amazon MQ broker destination queue to consume. Only available for MQ sources. A single queue name must be specified

So I changed my code to do each queue in a loop.

resource "aws_lambda_event_source_mapping" "mq_triggers" {
  for_each = toset(local.queues)
  batch_size       = 10
  event_source_arn = var.broker_arn
  enabled          = true
  function_name    = aws_lambda_function.message_handler.arn
  queues           = [each.key]

  source_access_configuration {
    type = "BASIC_AUTH"
    uri  = aws_secretsmanager_secret_version.broker_credentials_secret.arn
  }
}

Is this the intended behavior? If yes, should the attribute name change to queues = "example"since is only accepts single value?

References

https://registry.terraform.io/providers/hashicorp/aws/latest/docs/resources/lambda_event_source_mapping

Would you like to implement a fix?

None

[github-actions]

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[justinretzolk]

Hey @Tuzosdaniel12 👋 Thank you for taking the time to raise this! The aws_lambda_event_source_mapping resource uses the lambda.CreateEventSourceMapping function from the AWS Go SDK. I've linked to lambda.CreateEventSourceMappingInput, which is the Go type used as an input to said function, since it's a bit more relevant in this case. The input for this function has a Queues field, which correlates to the queues argument for the resource in Terraform. To save you a click, the field has the following definition:

// (MQ) The name of the Amazon MQ broker destination queue to consume.
Queues []*string min:"1" type:"list"

In this case, the AWS Provider has followed the design principle of closely matching the underlying API, which is why the argument is a list type, despite my agreeing with you that it's a bit of an odd case (though not entirely unique in the context of the AWS API), and definitely see how it could cause confusion.

With that in mind, I would say that this is indeed expected behavior, and that the argument should be left as is.

[justinretzolk]

As a follow up here, I noticed that neither the lambda.CreateEventSourceMapping, nor the argument's schema definition require that there is only a single element. While the documentation that you linked hints at it, it doesn't feel like strong enough wording when the API documentation for the function clearly indicates that the list has a fixed size of 1:

Array Members: Fixed number of 1 item.

I'd like to raise a pull request to make this more clear. Thank you again for taking the time to raise this and call it out!

[github-actions]

This functionality has been released in v5.6.0 of the Terraform AWS Provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!

[Tuzosdaniel12]

Thank you for the support!

[github-actions]

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