Templating all the things!

Over the course of the last months I’ve grown to love Go’s built-in templating language and started to apply it to all sorts of problems: First to more complex ConcourceCI pipelines and recently also to docker-compose files. The latter has ended up in a little CLI-tool called tpl, which simply takes a template, various data points from the world around it and writes the rendering to stdout. It is basically an extension to what I wrote about in “Creating test-requests from Vault” a couple of days ago.

The initial use-case was that I had a Vault instance running on my local machine and wanted to expose its secrets to containers within Docker. For that I needed to provide an external IP address to docker stack:

version: "3"
services:
  core-service:
    external_hosts:
    - "vault:{{ .Network.ExternalIP }}"
    ...

That’s the template I wanted to render to docker-compose. tpl to the rescue!

$ tpl docker-compose.yml.tpl | docker stack deploy --compose-file - project

(Just make sure to execute docker stack deploy in the same folder as the docker-compose.yml.tpl file is located in order to make volumes mounts find their folders.)

I also have tons of applications that could benefit from taking their credentials from a secure store. These apps shouldn’t have to know about Vault, though, but, again, should be able to read their configuration from stdin:

$ cat app.yml.tpl
credentials:
  username: {{ vault-secret "secrets/app/creds" "username" }}
  password: {{ vault-secret "secrets/app/creds" "password" }}

$ tpl app.yml.tpl | app --config -

If you want to give it a spin, you can find the code on Github but also on brew if you’re on macOS:

$ brew tap zerok/main https://github.com/zerok/homebrew-tap
$ brew install zerok/main/tpl