Terraform variables and count = 0

Question

We use the same terraform definitions across all environments. So far that worked well, but now I face a problem I couldn't solve yet. I have a RDS and ElastiCache for a service that I don't need in the demo env that I'm setting up right now, so I set the count to 0. For the other environments, I need to expose them via an output var:

resource "aws_elasticache_cluster" "cc_redis" {
  cluster_id = "cc-${var.env}"
  engine = "redis"
  node_type = "cache.t2.small"
  security_group_ids = ["..."]
  count = "${var.env == "demo" ? 0 : 1}"
}

output "cc_redis_host" {
  value = "${aws_elasticache_cluster.cc_redis.cache_nodes.0.address}"
}

Now I'm getting this error:

output.cc_redis_host: Resource 'aws_elasticache_cluster.cc_redis' not found
for variable 'aws_elasticache_cluster.cc_redis.cache_nodes.0.address'

I don't mind (much) having a useless variable set, but I can't get it to work in the first place. A simple conditional doesn't resolve this, since terraform evaluates the false side of conditionals even though it's not used. I found this hack but couldn't get it to work either.

Answer 1

Try this:

output "cc_redis" {
  value = "${element(concat(aws_elasticache_cluster.cc_redis.*.cache_nodes.0.address, list("")), 0)}"
}

TF doesn't seem to care that the count may be 0 if you wildcard higher up the chain.

This may output more than you want, but you can parse out what you need from it.