Terraform: Allow to omit resource keys defined in a config based on the value

[jurajseffer]

Terraform Version

Terraform v0.7.11

Affected Resource(s)

All.

Terraform Configuration Files

resource "aws_elasticache_replication_group" "stack_redis" {
  ...
  node_type = "cache.t2.micro"
  snapshot_window = "${var.elasticache_snapshot_window}"
  ...
}

Debug Output

* aws_elasticache_replication_group.stack_redis: Error creating Elasticache Replication Group: InvalidParameterCombination: The requested configuration does not support snapshotting. Snapshot window parameter should not be specified.

Since just defining the snapshot_window parameter will cause AWS CLI to complain it should not be defined at all as I'm attempting to enable snapshotting for a t2.micro instance, it would be nice if I could pass in a value or somehow define the key to be omitted, conditionally (note the node type value is dynamic in my case). E.g.

resource "aws_elasticache_replication_group" "stack_redis" {
  ...
  snapshot_window = "${omit(var.elasticache_snapshot_window)}"
  ...
}

Something like that in case the value is empty - whatever that must be defined as, e.g:

elasticache_snapshot_window = null

[apparentlymart]

Hi @jurajseffer,

For string attributes Terraform doesn't really distinguish between the empty string and null, so it's possible that setting this value to the empty string would do what you need here. For example:

variable "instance_type" {
}

variable "instance_type_snapshot_window" {
    type = "map"
    default = {
        "cache.t2.micro": "",
        "cache.m3.large": "05:00-09:00"
    }
}

resource "aws_elasticache_replication_group" "stack_redis" {
    node_type = "${var.instance_type}"    
    snapshot_window = "${lookup(var.instance_type_snapshot_window, var.instance_type)}"
    // ...
}

In some future version of Terraform it's likely that the interpolation syntax will gain some support for boolean logic so we can more cleanly deal with conditionals, but for now looking up values in a map is the usual pattern for doing things conditionally.

[jurajseffer]

@apparentlymart Thanks for that. I tried setting it to an empty string and passing that in but that didn't work either - same message came back from aws.

[mitchellh]

Tagging this as an AWS bug, we should just detect empty string in this case and replace it.

I'm open to returning an "empty" type in the future from HIL though @apparentlymart

[ijin]

affects me too...I wonder if there's a workaround.

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