amazon web services - terraform "element" and "concat" used together

Question

we have a module that builds a security proxy that hosts an elasticsearch site using terraform. In its code there is this;

elastic_search_endpoint = "${element(concat(module.es_cluster.elasticsearch_endpoint, list("")),0)}"

which as I understand, then goes and finds the es_cluster module and gets the elasticsearch endpoint that was outputted from that. This then allows the proxy to have this endpoint available so it can run elasticsearch.

But I don't actually understand what this piece of code is doing and why the 'element' and 'concat' functions are there. Why can't it just be like this?

elastic_search_endpoint = "${module.es_cluster.elasticsearch_endpoint}"

Answer 1

Let's break this up and see what each part does.

It's not shown in the example, but I'm going to assume that module.es_cluster.elasticsearch_endpoint is an output value that is a list of eitehr zero or one ElasticSearch endpoints, presumably because that module allows disabling the generation of an ElasticSearch endpoint.

If so, that means that module.es_cluster.elasticsearch_endpoint would either be [] (empty list) or ["es.example.com"].

Let's consider the case where it's a one-element list first: concat(module.es_cluster.elasticsearch_endpoint, list("")) in that case will produce the list ["es.example.com", ""]. Then element(..., 0) will take the first element, giving "es.example.com" as the final result.

In the empty-list case, concat(module.es_cluster.elasticsearch_endpoint, list("")) produces the list [""]. Then element(..., 0) will take the first element, giving "" as the final result.

Given all of this, it seems like the intent of this expression is to either return the one ElasticSearch endpoint, if available, or to return an empty string as a placeholder if not.

---

I expect this is written this specific way because it was targeting an earlier version of the Terraform language which had fewer features. A different way to write this expression in current Terraform (v0.14 is current as of my writing this) would be:

  elastic_search_endpoint = (
    length(module.es_cluster.elasticsearch_endpoint) > 0 ? module.es_cluster.elasticsearch_endpoint : ""
  )

It's awkward that this includes the full output reference twice though. That might be justification for using the concat approach even in modern Terraform, although arguably the intent wouldn't be so clear to a future reader:

  elastic_search_endpoint = (
    concat(module.es_cluster.elasticsearch_endpoint, "")[0]
  )

Modern Terraform also includes the possibility of null values, so if I were writing a module like yours today I'd probably prefer to return a null rather than an empty string, in order to be clearer that it's representing the absense of a value:

  elastic_search_endpoint = (
    length(module.es_cluster.elasticsearch_endpoint) > 0 ? module.es_cluster.elasticsearch_endpoint : null
  )

  elastic_search_endpoint = (
    concat(module.es_cluster.elasticsearch_endpoint, null)[0]
  )