I'm writing a Helm chart for a custom application that we'll need to bring up in different environments within my organization. This application has some pieces in Kubernetes (which is why I'm writing the Helm chart) and other pieces outside of K8S, more specifically various resources in AWS which I have codified with Terraform.

One of those resources is a Lambda function, which I have fronted with API Gateway. This means that when I run the Terraform in a new environment, it creates the Lambda function and attaches an API Gateway endpoint to it, with a brand new URL which AWS generates for that endpoint. I'm having Terraform record that URL as an output variable, and moreover I have a non-local backend configured so that Terraform is saving its state remotely.

What I want to do is tie them both together, directly from Helm. I want a way to run the Terraform so that it brings up my Lambda, and by doing so saves the generated API Gateway URL to its remote state file. Then when I install my Helm chart, I'd like it if Helm were smart enough to automatically pull from the Terraform remote state file to get the URL it needs of the API Gateway endpoint, to use as a variable within my chart.

Currently, I either have to copy and paste, or use Bash. I can get away with doing it with a bash script much like this one:

#!/bin/bash
terraform init
terraform plan -out=tfplan.out
terraform apply tfplan.out
export WEBHOOK_URL=$(terraform output webhook_url)
helm install ./mychart --set webhook.url="${WEBHOOK_URL}"

But using a Bash script to accomplish this is not ideal. It requires that I run it in the same directory as the Terraform files (because the output command must be called from that directory), and it doesn't account for different methods of authentication we might use. Moreover, other developers on the team might want to run Terraform and Helm directly and not have to rely on a custom bash script to do it for them. Since this bash script is effectively acting as an "operator," and since Helm already is kind of an operator itself, I'm wondering if there's some way I can do it entirely within Helm?

The Terraform remote state files are ultimately just JSON files. I happen to be using the Consul backend, but I could just as easily use the S3 backend or any other; at the end of the day Terraform will manifest its state as a JSON file somewhere, where (presumably) Helm could read it and pick out the specific output value. Except I'm not sure if Helm is powerful enough to do this. Looking over their documentation, I didn't really see anything outside of writing your normal values.yaml templates to specify defaults. Does Helm have any functions built into it around making REST requests for external JSON? Is this something that could be done?