VPC flow log churn using Terraform 0.13

[acdha]

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Terraform CLI and Terraform AWS Provider Version

Terraform v0.13.2
+ provider registry.terraform.io/-/aws v3.5.0
+ provider registry.terraform.io/hashicorp/aws v3.5.0
+ provider registry.terraform.io/hashicorp/http v1.2.0

Affected Resource(s)

• aws_flow_log
• aws_cloudwatch_log_group

Terraform Configuration Files

data "aws_cloudwatch_log_group" "VPCFlowLogs" {
  name = "/Security/VPCFlowLogs"
}

data "aws_iam_role" "VPCFlowLogger" {
  name = "VPCFlowLogger"
}

resource "aws_flow_log" "vpc_flow_logs" {
  vpc_id          = module.vpc.vpc_id
  traffic_type    = "ALL"
  log_destination = data.aws_cloudwatch_log_group.VPCFlowLogs.arn
  iam_role_arn    = data.aws_iam_role.VPCFlowLogger.arn
}

Expected Behavior

1. Terraform would update the flog log once and not attempt to recreate it on every run

Actual Behavior

 ~ log_destination          = "arn:aws:logs:us-east-1:…:log-group:/Security/VPCFlowLogs" -> "arn:aws:logs:us-east-1:…:log-group:/Security/VPCFlowLogs:*" # forces replacement

Steps to Reproduce

1. terraform apply

Important Factoids

This is new in Terraform 0.13 and did not happen with 0.12.29 and the AWS provider 3.20

References

I was not expecting to see this with #14214 having shipped in 3.0.0

[anGie44]

Hi @acdha, thank you for creating this issue. I believe the diff occurs b/c #14214 removed the trailing suffix in the cloudwatch_log_group resource, but not in the data-source and behind the scenes, the aws_flow_log resource automatically trims the configured log_destination value's :* suffix as seen here. In the meantime I would recommend using a replace method like described here #14214 (comment) to handle the perpetual diff.

[acdha]

That is exactly what I did and it’s working well

[anGie44]

just a follow-up question @acdha: did the workaround not behave as expected in Terraform 0.13 vs. 0.12?

[anGie44]

Note to future self (and others): to have the aws_cloudwatch_log_group data source behave on-par with the resource's ARN handling, this would need to be handled in the next major release as it introduces a breaking-change.

[acdha]

It's definitely not hard to work around so I wonder whether this could be perhaps addressed by simply updating the documentation (it seems like more trouble than it'd be worth to add something like an accessor which trims it).

[anGie44]

Sure thing @acdha! If you or someone who comes across this issue wants to submit a PR with the documentation update we'll be happy to review it 😄

I'm going to leave this issue open in the meantime as it can still be addressed in the data-source code but further down the line in the next major release 👍

[github-actions]

This functionality has been released in v4.0.0 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!

[github-actions]

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