When you throw in the towel on declarative

When you throw in the towel on declarative

I’ve talked a lot recently about the declarative versus imperative viewpoints. Its the Lilliput vs Blefuscu of our time. Its the vi versus emacs saga.

Today I ran into a scenario that I just threw in the towel on. I had a largish yaml file (~300 lines) that is actually a config to a container (e.g. its not Kubernetes yaml).

I’m using kustomize which means i cannot use the tricks I would in helm with ‘tpl’ and {{ }} (those are imperative or templating!).

I need to change one line in it per environment (a hostname). And I really didn’t feel good about copying the file into multiple output directories.

After an hour, I threw in the towel. The declarative police will come and get me, but I present to you my solution. Open sourced for your pleasure.

See my new container ‘envsubst‘. It auto-builds into dockerhub for you lazy folks. You are one

docker pull agilicus/envsubst

away from this big pile of perfection.

It’s simple. This container takes arguments in the form input:output. `input` will be passed through envsubst and redirected to `output`, making directories as needed.

`docker run --rm -it CONTAINER /etc/passwd:/dev/stdout`

So e.g.:

`docker run --rm -it CONTAINER /config/ifile:/etc/config/dir/ofile`

will take `ifile`, run through `envsubst`, `mkdir -p /etc/config/dir`, and place the output in `/etc/config/dir/ofile`

Now, from Kubernetes, I make an ’emptyDir: {}’. I mount it (read-only) in my ultimate container, and read-write in this new one (as an initContainer). I pass ‘args’ as the list of files above. Presto. All environment variables are expanded. Into that big grotty yaml file that started this problem i place a ${THE_URL}. And I’m done.

Am I proud of this? Well, I don’t have a lot of skin in the declarative vs imperative game. So. Um. I’m done with it.


1 Comment on “When you throw in the towel on declarative

  1. I’m sure someone has once said the universe is containers all the way down…

    Here I might have probably just abused sed in the init script somehow:
    sed -e “s//${VAR1}/g” -e “s//${VAR2}/g” -i”” filename

    But that’s the great thing about religious arguments: we all all get to be wrong.

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