How to use timestamp() and merge() functions with tags

Problem

When using the timestamp() function directly within a resource's tags argument, it can overwrite other tags, such as default tags applied by a provider or module. This happens because assigning a new map to tags replaces the existing map entirely rather than merging with it.

Solution

To dynamically add a timestamp to tags without overwriting existing tags, you can define the timestamp and other dynamic values in a locals block and then use the merge() function to combine them with other tag variables.

This approach ensures that all sets of tags are correctly combined into a single map.

Procedure

This example demonstrates how to use locals with the timestamp() and merge() functions to combine a base set of tags with dynamically generated tags.

1. Define Local Values: In a locals block, create a map for your dynamic tags. This example includes LastModifiedTime using timestamp() and LastModifiedBy using a data source.
2. Merge Tags: Use the merge() function to combine the dynamic tags from local.computed_tags with your base tags from var.extra_tags.
3. Apply Merged Tags: You can then assign local.merged_tags to the tags argument of any resource.

This is the complete example configuration.

provider "aws" {
  region = "us-west-2"
}

variable "extra_tags" {
  type    = map(any)
  default = {
    Enviroment = "tags-test-env"
  }
}

data "aws_caller_identity" "current" {}

locals {
  computed_tags = {
    LastModifiedTime = timestamp()
    LastModifiedBy   = data.aws_caller_identity.current.arn
  }

  merged_tags = merge(local.computed_tags, var.extra_tags)
}

output "merged_tags_output" {
  value = local.merged_tags
}

Example Output

After applying this configuration, the output shows that the tags have been successfully merged.

Changes to Outputs:
  + merged_tags_output = {
      + "Enviroment"       = "tags-test-env"
      + "LastModifiedBy"   = "arn:aws:sts::ACCOUNT_ID:assumed-role/ROLE_NAME/USERNAME"
      + "LastModifiedTime" = "2023-10-27T10:00:00Z"
    }

Additional Information

For more details on the functions and concepts used in this guide, refer to the official Terraform documentation.

• Local Values
• timestamp Function
• merge Function